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There's been a lot of focus on 850mb temperatures over the last few days and will they/won't they support snowfall, so just wanted to create a post for any of the less experienced members who are probably left scratching their heads about the conflicting views amongst different members and want to know what to believe. Let's start with why everyone looks for the -6c 850mb (or hPa) line and it's a bit of a history lesson I'm afraid. I believe in terms of these (and other) forums, the magical figure of -6c was really coined as the "snow line" back in the bad old days of the late 90's and early 00's over on the old BBC Snowwatch forums, when things were so desperate that we were crying out for any sort of 2 day northerly toppler just to look forward to (which is why I do find it amusing that some members can't see the potential in the upcoming period, I think unfortunately December 2010 distorted the expectations of some). Anyway, because back in those days we were primarily looking for Northerly topplers for any sort of cold, given an almost complete lack of Easterlies between 1996-2005, the 850mb temperature needed to be significantly below 0c for the airmass to be conducive for snow falling (this is a bit of an oversimplification, but let's keep it light!). Why? Well in the lower part of the troposphere we generally expect that as we travel from the clouds to the surface of the earth, the temperature increases. However we also have to remember that cold air falls, and so what we expect is the colder air from the 850mb level to fall down closer to the surface of the earth over time. The rate of this temperature increase (or "thermal gradient") varies hugely due to an overwhelming number of factors, however if we consider this in a simplified form again, the answer lies, for us at least, in the oceans. Because from a North or North-Westerly airflow the air is travelling a long distance over a generally warm Atlantic ocean, there is more likelihood of warmer pockets of air close to the surface of any airflow from the N/NW cancelling out some of the colder air dropping from aloft than there would be from say the East, where the air is travelling a far shorter distance over the North Sea and so is less likely to include these warmer pockets of air. These warmer pockets of air can affect many of the parameters that we won't go into in this post, but the two that are worth calling out as they are mentioned a lot are the 2m temperature and the 2m dew point. So if we try and summarise the above two paragraphs, in a N/NWly airflow the air is travelling over warmer oceans and so warmer pockets of air can be expected to be found closer to the surface. To override these warmer pockets of air we need the cold air aloft (at the 850mb level) which falls down towards the surface of the earth to be cold enough to override these warmer pockets of air and make the air from cloud to surface cold enough throughout to support all of our lovely snowflakes falling from the clouds remaining as snowflakes rather than falling through a warmer pocket (or layer) of air and melting into horrible rain, which from experience is much more difficult to make snowballs from. I think the primary reason we chose the -6c line was that back in those bad old days where Wetterzentrale was the choice of most members to consume their daily dose of GFS from, the dashed isotherm line which indicated the 850mb temperatures were spaced out at approximately every 5c (although for some reason I seem to remember Wettzentrale often showing -6c instead of -5c as one of the dashed isotherm lines, correct me if I'm wrong if anyone can remember), and so we would often look for the -5/-6c isotherm on the 850mb temperature charts as our guaranteed snow line, as it allowed for enough headroom for some warmer pockets of air at the surface to be overridden by the falling colder air from above. At this time I think it's fair to say that none of our collective weather knowledge was anything like it was today, and so looking for something simple like a single "snow line" helped us in our search for snow. So, on to the next part, what exactly is the 850mb (or hPa - they both essentially mean the same thing) temperature chart. Again I will keep this as simple as I can, so apologies to anyone offended by the oversimplified statement I may be about to make. Well to answer the first part, it's exactly what it says - it is the temperature of the air at the point in the atmosphere where the air pressure is equal to 850 millibars (mb) or hectopascals (hPa). But the exact height above the surface of the earth of where pressure is equal to 850mb can change, and that is what all of the pretty colours on the "height" charts we see are. So we've been talking about the 850mb level of the atmosphere, let's take a jump to the other common charts we see - the 500mb charts, such as the one below: The colours here represent how high above the earth the 500mb pressure level is. The more towards blue/purple the colour is, the lower the height (or closer to the surface of the earth) the 500mb pressure level is, and vice versa for the greens/yellows/oranges. This also has a knock on effect on how high/low the 850mb pressure level below it is. So what we are saying in the context of our upcoming cold spell is that under that huge low pressure system, the height above the surface of the earth of both the 500mb and the 850mb pressure level is lower. What this means is that the colder air aloft, at the 850mb level, does not have as far to fall down towards the surface, and therefore more colder air is able to come down and help override any warmer pockets/layers of air towards the surface. This means that we don't necessarily need the 850mb temperature to be below -6c to support snow falling to the surface. We also then need to bear in mind that as that low pressure falls further South, and we change our feed of air from being from the North/North-West across the Atlantic ocean to instead coming from the East from the near continent, fewer of these warmer pockets of air are likely to exist. Just to give you an example of this from the most recent 6z run, here are two 850mb temperature charts, the first one at +138 hours, and the other at +168 hours: It would stand to reason that if it was as simple as colder 850mb temperature = colder surface temperature, then we would expect to see a lower 2m dew point temperature in the corresponding +168 chart right? Well see for yourself: You can clearly see that despite the 850mb temperature being 1-2c warmer at +138 than at +168, the dew point temperature is 1-2c lower at +138c. You can again see the reason for this by looking at the 500mb height charts: We can see here that the lower heights at +138 aid the cooler 2m dew point temperatures. So in summary for what is a very long post, you do not require -6c or colder 850mb temperatures, even in a North-Westerly airflow sometimes, in order to guarantee all of the relevant parameters for snow being the right side of marginal, it is significantly more complex than that, and that is why the upcoming period could well deliver a lot of surprises - the word could being crucial in there!

A first look at how to use 500mb anomaly charts to help try and predict the weather These are notes from about 8 years ago. I have tried to check through to avoid any mistakes in the original text! First off what are they, well as they say the heading on the tin explains. They are at 500mb, roughly 18,000ft, and show the predicted contour lines, green in the case of NOAA charts and black (by and large) for ECMWF-GFS charts. The dashed red or blue lines, on the NOAA charts, with numbers on them are the predicted anomalies, hence the title, of heights in the areas shown by the dashed lines. A word of warning, the anomaly lines are, in my view, best ignored in any attempt to use these charts to suggest what the upper air pattern at 500mb may look like in the time scale they show. There are few charts that show what the ACTUAL height is for the time scale given for the anomaly charts so knowing what the anomaly may be at T+00 let alone at T+240 or more is even more difficult to work out. With care and experience they can be used to help ‘see’ what is being predicted but do be careful. On the NOAA charts the anomalies are indicated by dashed red (positive) or blue (negative) heights compared to the long term average for the dates shown. Why 500mb? Well in the days prior to computer models the 500mb height was the main ‘tool’ used by forecasters to try and get an idea of what the upper air might look like 24 hours ahead and thus what the surface pattern might be. Today with hugely complex weather computers all these and many more calculations at a large number of levels from the surface to way out into the upper atmosphere are routinely done by computers. They still follow the same basic rules forecasters of old (like me!) used 40-50 years ago. Largely very complex mathematics to solve the Laws of Thermodynamics. That said the idea of using the 500mb level is still very useful to a forecaster, hence the charts we are looking at. Predicting the upper air, difficult though that is, is much easier than that at the surface. Much of this has to do with the complication caused by moisture (water/ice vapour). By 18000ft a lot of this is no longer available at least over the northern hemisphere and especially so in winter as temperatures at all levels are much lower than in the summer. This does enable the models to fairly accurately deal with the expected airflow at this height. Of course these charts, just like any forecast chart, are far from infallible. But they do, from tests over a 3 year period I have carried out, lead me to believe that in over 70% of occasions they predict fairly accurately the 500mb flow. By that I am referring to the major wave pattern in the atmosphere. Around the globe, north and south, long wave patterns, sometimes referred to as Rossby waves are permanently in the atmosphere. In both hemispheres the general flow is from west to east, not so around the equator where an easterly is often the direction. Within this broad westerly flow are the ridges and troughs that largely govern how the weather is at the surface. Thus if we can get the upper wind direction and pattern near enough correct then it does give a forecaster a fighting chance of getting the weather at the surface reasonably accurate. Unlike some comments you will see during the winter about ‘short wave’ developments ‘scuppering things’ they cannot develop at the surface unless the upper air pattern allows them not the other way round. We then get into the discussion as to which comes first, surface or upper air. Hugely complex and with no easy answer but the upper air does govern the surface on MOST occasions and not the other way round. Trust me. What are ridges and troughs? As a VERY rough guide where the ridges are generally the weather is more settled and where the troughs are is often unsettled. Bends in the contour charts is a very simple way, the ridge is the bend at the top, see below, and the trough is the bend at the bottom is as simple as I can make it. Sorry if the diagram is not very clear, not very good at paint and insert! (2012 period) So I hope that shows as simply as possible what I mean. In very simple terms the ridges and troughs are caused by differences in heating north to south in the hemispheres and the mountains chains which occur over the world surface. Do NOT get too hung up on the detailed explanation for the time being. To try and use them they need to be consistent run to run with themselves and if comparing one type of chart (say NOAA) with another set (ECMWF-GFS) then consistent with one another. I take a run of 3 days minimum as being consistent with a 70-75% probability that the pattern predicted will occur in the 6-10 day period. With NOAA we get charts for 8-14, so similar with itself. At this range then about 65% probability of accuracy. The charts can be used to predict major pattern changes in the wave lengths, or a continuation of the current pattern. Accuracy seems about the same for either. It is 8-10 years since I ran a 12 month check on their accuracy (chiefly for NOAA) but I have not detected any noticeable change either way recently. They can be used to give a fair idea, with a lot of practice, to decide where the main centres may occur at the surface. The type of air mass can also be predicted from the general flow being shown. What they do not show is day to day surface variations or indeed surface detail. 500mb anomaly charts and are they useful for forecast guidance? These are examples from way back in 2013. They show that these charts can be used to predict marked changes in weather patterns. Be that from blocked (meridional) to unsettled and vice verca. Why not show examples from recent weather. Good question but my ability to explain recently has become not very good, words I want to use, phrases, names etc, so these examples from 7 or 8 years ago should be better explained than any attempt by me now. First of all you will have to take my word to some extent on what was showing on these charts in the prior days and whether the ones in the example are similar or not as it would take up too much space showing charts for the past 4 or 5 days. So just the ones that I feel are relevant for what I am showing. So again showing the NOAA chart I showed in part 1 from last evening (Wednesday 7 February valid, depending whether we take the 6-10 valid for 12-16 or the 8-14 valid for 14-20 February). I must first make the comment that close scrutiny over many months, even several years, shows that the 8-14 is often a less ‘gung-ho’ if that is the correct term than the 6-10. The 6-10 will quite often show a stronger flow, more meridional flows and larger anomalies than the 8-14. Whether these differences are real or not I have never done any research on them. The major thing is to see between the two how any changes are being predicted and do they look realistic over several days. I then compare them to the ECMWF outputs valid for the 10 days ahead, and latterly the NAEFS outputs. ECMWF-GFS is output using 00 and 12z data as is the NAEFS, NOAA do one only (mid evening our time). Back to the 6-10 and 8-14 NOAA; 6-10 on the left The 8-14 shows a bit less 500mb flow in the Iceland area than the 6-10 but not by much and it also shows the trough from that area as being a little less marked, a smoother flow almost. Beyond that the major pattern seems pretty similar in all aspects. So what does the ECMWF-GFS from that same morning show? The charts are actually both means from T+168 – T+240 so they are for the same period as the 6-10 day NOAA outlook, ending 17 February. To be totally coincident it would need the issue from the previous morning but from what I recall it makes little difference in this case. We now have two interpretations for the same time from differing models which can be both frustrating and useful, probably in about equal measure! ECMWF on the left shows a fairly similar pattern to NOAA in the area of Iceland and not that different in the UK area for the predicted 500mb flow. Let’s leave the anomaly parts out for the moment. GFS has the ridge more pronounced further ENE than both the other models but otherwise suggests a similar suggestion of ridging towards Greenland and not too different for where the European trough is shown. Indeed the eastern US trough and the western US pattern are again not identical but fairly similar in positions, with GFS favouring a more pronounced ridge in the far west possibly there more a touch towards the NOAA idea? So we have considerable similarity on the 500mb patterns on all 3 models. How have they changed over the past few days? This will give us an idea of is the pattern consistent and when did this start for some idea on when the pattern shown may be said to be going to show on actual upper air charts? Just to show how they do change let’s look at what was predicted on 19 January for 6-10 days ahead, that is out to about 2 February. I am sure most of you remember what the UK weather was generally at that time of issue? Cold, frosts and snow in places with a good deal of discussion about how long it might last. As you can see from the predicted pattern the change to a more unsettled westerly was showing up on the 500mb anomaly charts so that would come as no surprise to those who use them. Just briefly turning to the anomalies; note the reasonable sized –ve anomaly west of the UK. This would suggest that the main upper low, within the overall westerly pattern would probably be bringing upper winds from a fairly SW’ly direction, thus at the surface somewhat milder air might well be expected over the UK. How about what the height for southern UK on the prediction? Over the Channel area heights of about 546DM so nothing much out of the ordinary. (That comes from working for very many years and having a very rough idea of what 500mb heights translate to surface values at different times of the year). Certainly not cold but not unusually mild. Looking at my own weather station data for the end of January, say 25th onwards and maximum temperatures were, 2.3, 9.4, 9.4, 10.2, 14.1 11.4, 9.9 degrees C with my average maximum for the end of January around 6C. So the expected change to milder weather was well indicated but the actual degree of mildness was less well predicted. I think this probably was fairly similar across much of the country? So when did these charts start to suggest another change to more blocked type and thus colder? This was part of my post into the forum on, I think 19 January before I went on holiday. So not 100% for sure but it certainly suggested that the very cold and quite snowy spell was not going to last. I hope you will believe me when I say I am not manipulating things to fit but as they were. On to the first charts that started to suggest another return to a more blocked type. This occurred whilst I was on holiday so I am not able to show the actual charts but from notes I made from my hotel each evening after looking at the outputs, something along the lines, again from my general notes that I keep on my pc Up to about 25 jan all 3 showed less cold wly type, ridge over Europe=prob wsw/sw flow even but then slowly the 3 changed to a suggestion of a blocking type in the 10 day time frame, not Greenland but Azores ridging north turning flow n of west over uk on all e with a trough about 10-15 east of uk So from 25 jan=4 feb poss start colder type, it could be as early as 2-3 days before this Or 30th=9th for it to start More prob 4th and then 6-10 days perhaps at this range, could be longer? Below is noaa from sun 2 feb 6-10 and 8-14 The chart above is the first one I saw at home so I was able to copy this. Hopefully it is quite obvious that the pattern and consistency I mentioned in rough notes on holiday continued and the pattern on the last chart is quite different from that of 19 January. Please believe me when I say that all 3 of the anomaly charts had shown this change and a similar consistency. So I stuck my neck out for the start of another cold spell on 25 January to start around 4 February. Got the date wrong but the change in pattern did occur. So back to what we have in the past 24 hours from all 3 and also a look at what NAEFS showed this morning out to T+240. NOAA 6-10 on left and ECMWF-GFS on the right with NAEFS below. NAEFS at 240 hours Remember that NAEFS shows anomaly height differences only To me all 4 versions are not that far out from one another-you may disagree of course. To my eyes and understanding having followed this kind of chart for 3-4 years I would say that I would side with the UK Met 6-15 day outlook. There is no sign, yet (!), of any major shift in the upper air pattern. They all to me also indicate again the possibility with heights being higher NW of the country than NE, (this does not mean the surface high will not be NE of the UK) that there continues to be the risk of disturbances moving down from the NW in a SE direction. Looking at the jet stream predictions over a similar time scale is another factor that tends to support this idea. Just where the surface high will spend much of its time is difficult to be firm about. Certainly not south of 50N. Somewhere north of 55N and probably west of rather than east of the meridian. But I would suggest a 30% probability that we may see it east, more like NE of the meridian for some of the time. Overall no mild weather for at least two weeks with the details as always from model consensus in the T+24 to T+144 time frames. I have not spent much time on the anomaly part of this for a very good reason. I would rather newcomers to this idea of upper air charts become familiar with and have a good understanding of what the actual contour lines can tell us before venturing into the anomaly side of things. I hope you enjoyed reading this and equally that you may be persuaded that this aspect of meteorology to us on the web with limited data access can give a good insight into general weather patterns even on the surface.

A revised version of the earlier "Winter Snow Setups/Non-Snow Setups" topics, this goes through the range of winter setups we can get. As in summer, the main determining factors in what sort of winter weather we get are the positioning and strength of the jet stream. A strong jet stream means that depressions will frequently move from west to east, giving a "zonal" pattern over the UK. Because the Atlantic is relatively warm and moist, assisted by the warm North Atlantic Drift, zonal types often tend to be mild- but not always. A weak jet means lows track less frequently from west to east and blocking highs can form more readily. Whether we get cold wintry weather depends on the positioning of the high. Zonal, northerly tracking jet When low pressure systems track well to the north of Scotland, we usually end up with high pressure close by to the south, and a mild moist tropical maritime airmass covering the north. It is usually dry and mild in the south with a fair amount of sunshine, but cloudier and wetter in the north and west. Bartlett/Euro High An extension of the northerly tracking jet scenario, this setup sees the Azores High displaced over to Europe, keeping Britain in a persistent tropical maritime south-westerly regime. This setup brings Britain's warmest winter temperatures. This setup is often associated with large rainfall totals in the Scottish Highlands. Broadly speaking it tends to be dry and sunny wherever the high covers, northern and western Scotland tend to be dull and wet, and intervening areas often end up fairly dry but cloudy. Zonal, jet tracking over and to the north of Scotland This is the most common "zonal" winter pattern with low pressure systems regularly moving from west to east, bands of rain moving east at intervals, with brighter showery polar maritime air in between the rain belts. If the lows track from SW to NE then southern and eastern areas often spend a lot of time in "warm sector" mild moist tropical maritime air. It tends to be wet everywhere, sunny in the east and dull in the west. If the lows track more from NW to SE much of Britain spends a lot of time in polar maritime air, giving sunshine and showers. It tends to be wet but sunny in most regions, especially sunny in the east and especially wet in the west. On rare occasions, if there is an influx of Arctic air or cold pools from Canada/Alaska into the mid-Atlantic, we get so-called "cold zonality" with widespread snowfalls, especially in northern and western regions. An extreme case of this occurred in January 1984. Zonal, jet tracking right over Britain This pattern tends to be very wet as the lows track straight over the British Isles. Temperatures tend to be close to normal but with a bias towards milder conditions in the south, and cold polar incursions often reaching the north, giving snow for Scotland and northern England. Zonal, southerly tracking jet This pattern is not very common but when it does happen it can usher in prolonged spells of cold snowy weather for the British Isles. Low pressure stays to the south, sometimes bringing fronts into southern areas which can bring snow as the milder air meets cold polar air to the north. Otherwise, high pressure oscillates between Greenland and Scandinavia bringing repeated bursts of northerly and easterly winds. Blocked, high pressure over Britain When a winter anticyclone settles over Britain the weather tends to be dry, but the weak winter sun is ineffective at burning away low cloud. Thus sunshine amounts can vary considerably depending on how much cloud is trapped within the high- in general an input of moist tropical maritime air, or an easterly drift from the moist North Sea, may result in days on end of "anticyclonic gloom" with low cloud and mist and no chance of any sunshine. Alternatively, a clear anticyclone may bring frosty foggy nights and sunny days, as happened in December 2001 (below). Blocked, high pressure to the east When high pressure is well out to the east, this allows Atlantic lows to come towards the British Isles but they stall to the west, which tends to give rise to mild, rather cloudy southerly regimes. The equivalent of the summertime "tropical continental" southerly type which dominated the month of July 2006 rarely occurs in winter, but it does crop up occasionally. Depending on the amount of cloud circulating around the high's western periphery, a prolonged spell of anticyclonic gloom may ensue (as happened in February 1993 and early December 2004), or it may be warm and sunny by day but with cool nights, as happened in February 2008 (below). Unfortunately, with highs both over and to the east of Britain there is no synoptic way of determining how cloudy the highs will be- satellite imagery, atmospheric profiles etc. are your best bet for guidance. Blocked, strong Azores or mid-Atlantic high This kind of blocked pattern results in mostly mild weather as north-westerly winds suck up mild air from the Azores and around the high's periphery to the UK. Sometimes as a low moves out into Scandinavia it may introduce a brief burst of cold northerly or north-westerly winds with some snow showers, but these blasts usually tend to be short-lived. Blocked, Scandinavian high The Scandinavian High is often regarded as the "holy grail" by many cold/snow lovers, because it directs cold continental air across from the east. However, the Scandinavian High is really more of a building block towards an easterly- if the high is kept too far east the continental air may well stay away to the east. If an easterly does reach Britain then it will pick up moisture over the North Sea, and the resulting weather is largely dependent on the upper air temperatures, and the 850hPa temperature is often used as a guide. If the upper air is relatively mild (typically above -5C), the air will be stable and the moisture will give rise to layers of stratocumulus and persistent dull dry weather. However, if the upper air is cold (typically below -5C, preferably -10C or below) then the air will be unstable, and will give rise to heavy, often prolonged showers, especially but not exclusively for eastern areas. This setup brings much of England and Wales its coldest weather, and can produce significant snowfalls as happened in February 1991 (below). Northerly type Northerlies are another major source of snow events, brought about by high pressure to the west, and low pressure over Scandinavia or the North Sea. However, northerlies too have a major "stumbling block" if it's widespread snow you're after. Unless there is a southerly tracking jet stream, or a strong anticyclone over Greenland (preferably both), we tend to get brief "topplers" with just 36-48 hours of northerly winds, a few wintry showers for exposed coasts, and then milder weather pushes in. However, if a block can hold to our north-west for long enough for the northerly to sustain for upwards of a few days, then we will often see troughs form in the airflow bringing snow showers well inland. A large area of high pressure over Greenland, extending towards Iceland, will usually keep the British Isles affected by repeated bursts of polar air from the north. The "polar low", a low that forms in cold northerly airstreams and tracks south, is a particularly prominent source of snowfalls in a northerly regime. Although it is usually northern and eastern areas that see the most snow in a northerly regime, western areas can see the largest amount when pressure is low to the north, resulting in the Arctic air being sent south through the east Atlantic and around to Britain from the west or north-west (similar to the "cold zonality" described earlier, but via a northerly regime). Continuing the Christmas theme, this brought many western areas a white Christmas in 2004. Frontal battlegrounds Finally, when pressure is high to the north or east bringing cold polar and/or continental air towards Britain, and this cold air meets Atlantic systems coming in from the south-west, causing the systems to stall, this can lead to prolonged outbreaks of snow. For example many western areas were heavily hit during early February 1996 from this kind of setup.

Hi all I promised to give this guide, so here it is. Its not complete and it is for those who are new to trying to work out this very difficult question. What to look for to get snow at sea level. (1) in showers (2) frontal weather (1) In showers 1) Dry bulb temperature below 5C, often 3C is a better mark 2) Dewpoint at or below zero 3) wet bulb temperature, if you have a weather station, no more than about 2C 4) 1000-500mb thickness (DAM) less than 522dm, lower if you are on the coast, but as high as 540dm it is possible in a heavy shower, but unlikely. 5) 850mb temperature of -7C or below, -5C it can occur but not often. 6) on the 850mb chart if the value shown on the contour line is below about 1290dm or 1300dm, then there is a high chance that ppn will be of snow. (The Met Office use 1293dm for a 50% and 1281dm as a 90% chance of snow) 7) zero degree isotherm or freezing level of 1,000ft or less to give a 50% or higher chance of snow at sea level. At 2,000 ft above sea level the chance is reduced considerably. (2) Frontal Weather Two types of front (1) warm front (2) cold front In (1) warmer air is flowing over the top of cold air, and in the case of (2) cold air is undercutting the warm air ahead of it. (1) is, I suppose the classic heavy snow situation which with strong winds can give blizzard conditions, even on relatively low ground, assuming all the factors are in its favour. This assumes they are, namely that very cold air lies near the surface but is not being moved away by an approaching frontal system. This stagnates and eventually retreats away again. In this instance then the above requirements need to be met along with (a) If prolonged and fairly heavy ppn occurs then what we call the wet bulb temperature will start to lower and this can enhance the probability of snow falling. In this instance if the wet bulb temperature is at 3C or below then the ppn can turn to snow from rain. Also, and this applies to showery conditions also, if the wet bulb freezing level(not easy to find on any chart!) is 2,000 ft or below, then ppn can readily turn to snow. In light ppn then, often, regardless of any of the above factors being favourable drizzle or light rain will fall not snow. (2) In this instance then if the air is not all that mild in the so called warm air and the cold air is very cold, with near negative values close behind the front, then even moderate ppn will readily turn to snow as the cold air undercuts the mild air. Use the values above for a guide. This has only touched the surface of trying to forecast will it snow or not but I hope gives, the less experienced, a guide of what to look for. On the topic of wet bulb temperature. I have still not worked out how to give Paul sufficient data that they can set up an algorithm to produce charts of wet bulb potential temperature and also wet bulb freezing level charts. If anyone knows how to do this please pm me or Paul as it would be another big step for Net Weather to be able to produce such charts from the GFS run. I hope you have lots of opportunity this winter to try this guide out. Footnote by Blessed Weather: This article was written by John Holmes, retired Met Office professional, in 2015.

There was already a guide written by me about UK thunderstorm set-ups, but it was done some 10+ years ago now and I've felt for a while that it needed a re-vamp and updating to make a more comprehensive guide to the processes that produce the various types of thunderstorms we see in the UK. So here it is ... the Netweather guide to thunderstorms in the British Isles .... 15 pages long: Thunderstorms in the British Isles.pdf

The MJO is a major contributor to the global weather patterns, so for those who want to understand a little bit more about it here is a brief overview of my current understanding of the MJO and Rossby and Kelvin Waves. First lets talk a little bit about waves or more specifically Rossby and Kelvin waves. These can occur both in the Atmosphere and in the Ocean and it is important to be clear about the difference between the two. Oceanic Rossby waves take the form of slight height changes in the sea and more apparent changes in the depth of the thermocline. These can take months or years to cross an oceanic basin and have there orrigin in anomalous atmospheric pressure patterns.In the North Pacific, for instance, a Rossby wave, after the 10 years or so that it takes to cross the basin, can push the Kuroshio Current northwards and affect weather on the North America continent. This might have happened already in 1993, the culprit Rossby wave being an effect of the 1982-83 El Niño.The important thing about oceanic rossby waves are that they are slow and westward moving. Kelvin waves move faster and eastwards taking about 70 days to cross the Pacific. See in the link below how an easterly wind anomaly at the equator can produce these waves and in the subsequent link how they are reflected to ultimately produce a pattern which has similarities to the el nino,la nina pattern. Oceanic Rossby and Kelvin wave Thory The Evolution of Oceanic Kelvin and Rossby waves The point here for me is that strong MJO events have large impacts of weather patterns and probably contribute to la nina and el nino events. We should note however that el nino and la nina tend to closely follow the volumes of warm water (20C+) at the equator so the MJO does not have it all its own way. Warm Water Volume and ENSO We recognise Rossby waves in the atmosphere as the long waves in the jetstream but there are also Kelvin waves in the atmosphere which travel eastwards around the world typically taking 40-50 days which show up as a pressure anomally. These lesser known waves may actaully play a most important role is triggering the MJO cycle. Perhaps we should just note that gravity waves are a different phenomenon and although Atmospheric Rossby waves are thought of as planetary waves I prefer to use this term for those waves in the Stratosphere, Mesosphere and Ionosphere. So onto the MJO events which have there orrigins in enhanced convection over the tropical western Pacific which create a low pressure which radiates rapidly eastward as a dry equatorial Kelvin wave over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the sea level pressure anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle. So we have a trigger which circulates the world over a set period (typically 50days) with one event triggering the next.Here we should note that the MJO Phases do not coincide with this circulation but reflect the eastward migration of convection once convection has been triggered. Once convection fires at the start of the cycle you will get a Rossby wave response with pressure troughs to the north and south of the area of convection. The low pressure will bring colder air in to the west of the convection killing of convection while eastward moving warm air spreads the convection eastwards. Eastwards of the low pressure systems will be strong anticyclones (high pressure) which will give strongly easterly winds at the equator. These pressure systems affect the mid latitude jetstream and hence the pattern across the north Pacific, the US and to some extent the North Atlantic and the UK. MJO Phase 2 or 3 weather pattern response MJO Phase 6 or 7 weather pattern response At the moment cool waters in the central pacific due to la nina are tending to damp down the eastward movement of convection while anomalous highs ahead of the convection will be acting to enhance la nina and slowly move it Westwards. Phase 5 through 8 of the MJO can result in a high pressure anomaly towards Alaska and a deep trough down into the central US. There are some suggestions that this high pressure towards Alaska ridges into the arctic region causing a displacement of the stratospheric vortex forcing the arctic oscillation to trend negatively. Perhaps I will revisit this when I know a bit more.

I hope the article below will help to explain what the term means and how its value is arrived at. The term DAM is used at times but its correct term is 'thickness' between the two levels in the atmosphere. Remember although its often referred to at the 1000-500mb level it can be used between any two levels. For snow forecasting the other most often used is the 1000-850mb values. DAM heights or total thickness between two levels, usually the 1000mb and 500mb I hope this may help (!) to show how complex is the relationship but also how relatively easy it is, knowing the two heights, to calculate the ‘thickness’. This can be done for any two heights. The two most referred to, usually on Net Wx to do with the will it or won’t it snow, are the 1000-500mb and the 1000-850mb heights for ‘thicknesses. Fortunately this has all been done for us by Paul and Karl with the charts shown below! DAM is what refers to the 1000-500mb thickness chart. Its rather complex but there are several ways to work out its value. Below are some of the methods which might help = height (500 hPa surface) - height (1000 hPa surface) [ for those of you, like me, too old to catch up with all the changes the world brings, millibars = hPa!, so 500 hPa is exactly the same as 500 mb. ] h(500) = h(1000)+h'(thickness). Or from that h'(thickness)=h(500)-h(1000) Thickness can be calculated from the heights reported on a radio-sonde ascent, or a thermodynamic diagram can be used to add up the partial thicknesses over successive layers to achieve the net (total) thickness. An example of the former would be 500 hPa height = 5407 m 1000 hPa height = 23 m Thickness = 5407-23 = 5384 m (or 538 dam) Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m 1000 hPa height = - 13 m Thickness = 5524 -(-13) = 5537 m (or 554 dam) Note the example above when surface pressure is BELOW 1000mb. Roughly it is taken that 8mb is equivalent to 6DM when forecasters are manually drawing the various upper and surface charts. If we take the actual msl and 500mb chart from GFS/Extra for 06Z this morning, see below On the left is the surface isobar chart with the 500mb height; to its right is the ‘thickness’ chart Notice the differences in values between the left and right charts-obviously the surface values are identical but NOT the ‘thickness’ and 500mb values. Or to look at how the 00z ascent for Herstmanceux differs in its 500mb height and its 500mb ‘thickness’ In the basic data format the 500mb height was given as 500.0 5490 -22.9 -50.9; i.e. 5490DM; that of the 1000mb height was 1000.0 87 8.2 5.6 The ‘thickness’ is 1000 hPa to 500 hPa thickness: 5403.00 How is that arrived at, see the formula above 100mb height is 87 500mb height is 5490 Therefore 500mb ‘thickness’=5490-87=5403DM Additional information on atmospheric thickness and it's use is available on the NOAA National Weather Service website: https://www.weather.gov/source/zhu/ZHU_Training_Page/Miscellaneous/Heights_Thicknesses/thickness_temperature.htm John Holmes

The Arctic Oscillation Arctic Oscillation is an important lead on expected winter conditions in the Northern Hemisphere, loosely described as negative ( colder ) positive ( milder). The image below gives you a great contrast of a winter we will all easily recall with an extremely negative AO and a little further back a winter at the opposite end of the scale. When considering the overall forecast for Winter it is important to note any variables which provide clues as to which end of the scale the AO will tip towards, this in turn informs us of potential for blocking episodes and also the behaviour of the jet stream. Throughout the forecast elements indicative of the mean negative AO over winter are noted. Further description The Arctic Oscillation describes simultaneous, geographically 'choreographed' shifts in multiple features of the polar vortex: air pressure, temperature, and the location and strength of the jet stream. They all follow the hemisphere-wide oscillation of atmospheric mass back and forth between the Arctic and the middle latitudes, sort of like water sloshing in a bowl. L : Positive AO R:Negative AO At one extreme of the sloshing, there is lower-than-average air pressure over the Arctic and higher-than-average pressure over the mid-latitudes. The jet stream is farther north than average under these conditions, and it steers storms northward of their usual paths. The mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual. These are all characteristics of a strong, “well-behaved†polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index, which tracks relative pressure anomalies across the N. Hemisphere, will have large, positive values. At the other extreme, the conditions are reversed. Air pressure is higher than average over the Arctic and lower than average over the mid-latitudes. The jet stream shifts southward of its average latitude and can develop waves or “kinks,†with “troughs†that help steer frigid, polar air southward. These are all characteristics of a weak polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index will have large, negative values. Source : Climate.gov C.Kennedy, R Lindsay

Hi. I just wanted to create a post from an educational perspective and also be a point of reference regarding the importance of the GSDM and the AAM on the medium and long term. Hopefully, anyone who questions this should change their mind after reading the below. The reason it is worth doing this is because there is no better, clearer, example of the importance and connections between those upstream, Pacific developments and the downstream influence. I'll attempt to highlight and stitch this together using some of the usual plots and charts which tell the story so very well indeed and it is this which, hopefully, helps people to at least better understand the influences of the GSDM and what to look for moving forward. OVERVIEW: Firstly, we have just endured a very wet December, this comes as no surprise given what happened to the AAM in late November. Interestingly, seasonal models all pointed towards December being a +ve NAO month, in keeping with the long-term teleconnections with regard El Nino and a +IOD in early winter. The late Nov and early Dec cold spell can be linked back to the behaviour of the GSDM too, while what followed through much of December certainly can and what is happening now and looking ahead, most certainly is. Despite a weakened stratospheric polar vortex, it is the troposphere that is 'leading the dance' rather than the stratosphere for the most part even though it is lending a hand. The late Nov/early Dec blocked pattern has links to the temporary rise in the AAM in mid-November, with the usual lag in place. We can see this first evolution in the below plots... The second and more obvious evolution was then the more pronounced fall in AAM through late Nov as the MJO returned to the W Hem and westerly inertia was removed from the GSDM budget and, as usual, AAM fell along with a -FT (Frictional Torque) and -MT (Mountain Torque) event. The result was, as ever, propagation through the extra-tropics and then into the mid-latitudes of more westerly momentum, propped up by easterly momentum at circa 30-40N - Remember the inflated Azores high in the run-up to Christmas? - We can see how the +AAM anomalies, in particular, were distributed through the atmosphere within the usual window of approximately 14 days. No surprise we then saw an enhanced period of +AO and +NAO conditions just before mid-December onwards. Lastly, comes the 'big event' within the last week or so in terms of the pronounced rise in AAM. Once again, in complete contrast to what happened through mid-November, with a lag of course, we can see how the marked rise in AAM has allowed the propagation through the sub-tropics and into the mid-latitudes of more easterly inertia (-AAM) very well indeed, this is particularly obvious on the relative AAM transports plot, as highlighted below. Once again, no surprises at all and of which has been documented by the usual few on the group we are now seeing a complete reversal of that +AO and +NAO period with an array of mid-latitude and, eventually, high latitude blocking patterns. We can also see this visually on the zonal wind plots at 100hPa (top of the trop/bottom of the strat) too. While like in many areas of meteorology, there is never often a usual "a+b=c evolution", but this is about as close to that as you can get. We have certainly had help from the stratosphere here mind because a robust stratospheric polar vortex that is downwelling westerly winds into the troposphere can often be the dominant player, overriding what has occurred over the last month. There is likely to be some 'help' here from the El Nino and eQBO combination this winter as the sPV continues to remain disorganised and far weaker than it can be at this point in the winter with far less influence on the troposphere too. When it comes to the AAM it is crucial to be able to accept and acknowledge when there are other overriding influences and counterbalances. Tamara et al often talk about not taking the MJO at 'face value', it is similar with the AAM, but when you combine the usefulness of the GSDM and all it incorporates then, as I have perfectly examined here, nobody can ever say the likes of the GSDM and the AAM is "flawed or useless" when it comes to pre-empting NWP, at times, and also gauging how the broader patterns may evolve and shift. As I mentioned at the start of the post, I wanted to put this one together because there is no better example of the usefulness of the GSDM than what has occurred over the last month or so. The evolution and 'story' of the atmosphere have been well played out in these plots. For those who want some winter weather after the last few weeks of very wet conditions then 'this is your time'. We approach mid-winter with such solid footing for cold synoptics that it should be a pleasure to watch the more unusual patterns being modelled in NWP and, interestingly, this continues to link in well with the majority if not all of the seasonal models with regards to how Jan and Feb and perhaps even Mar should progress. Winter is about to start... With regards, Matt. Original post: https://community.netweather.tv/topic/99706-model-output-discussion-into-2024/?do=findComment&comment=4994387

Ok I will start a new thread for dicussions along this line and perhaps I will draw on some ideas expressed in the stratospheric thread and artic sea ice thread. I guess you would be the best person to explain all this GP but for those who don't know this thread is about global angular momentum and how it oscialltes up and down (Global Wind Oscillation) along the lines discussed by Ed Berry. Angular momentum is of course a measure of the turning force in the winds, so could perhaps be considered a measure of the strength of low pressure systems, but also relates to how much the jetstream undulates and how much blocking we have. The budget of angular momentum goes up and down as energy is lost as weather systems crash into mountains and increases as cold air meets warm. Each phase of increasing and decreasing momentum suggests different types of weather for the UK. Looking at the current GWO plot we see a liklihood of going into phases 3 and 4 based on how it usually cycles round. This implies increasing angular momentum as the various torques including mountain torque diminish (i.e those things which take energy out are not active). This is certainly true of mountain torque. For frictional torque and gravity wave torque then the jury is out. Overall it looks like global angular momentum is on the increase. The tendecy during december has been upwards. Short term I think we are looking at phases 3-4 and more of an Atlantic influence. What I am guessing at though is that low pressure systems crossing the US will increase mountain torque, equally the jet stream across india is not a weak flabby one which might increase asian mountain torque. The strong jet in the western pacific along with OLR charts suggest strong trade winds with a stationary high to the north east of Australia and convectional activity to the north west of Australia. So back to phase 1-2 fairly quickly I think afterwards. All maps are available in the link below. PSD Map room for AAM I am sure GP will tell us what he expects from the MJO and convectional activity in the pacific and how and if he expects rossby wave development as a result. It is those Rossby waves which in part will affect the stratospheric vortex and the low angular momentum could be linked to a more blocked pattern and sea ice build up to our north which I talked about in associated threads. What we should always remember though that this a complex interaction of parts of which the stratosphere plays a large part during the winter. Please note that this post and subsequent comments have been copied from the forum, so the dates/times of the comments are not correct.

This is my revised version of the summer synoptics guide. The standard summer synoptic setup Traditionally, in summer, we have a strong Azores High out to the south-west, low pressure systems moving from west to east to the north of Britain, and westerly winds dominating, bringing cool cloudy weather and rain at times. Southern areas see the warmest and sunniest weather as they are closest to the influence of ridges from the Azores High. Low pressure dominated scenarios Cool, cloudy, unsettled summer weather is often associated with a conveyor belt of strong westerly winds, a flattened Azores High well to the south-west, and low pressure systems and fronts bringing bands of rain west to east at regular intervals. The Julys of 1992, 1993, 1998, 2002 and 2004 all had this pattern. When lows track further south than usual (over northern Britain, say, rather than to the north of Scotland) it can be especially wet- July 2007 was a good example. However, if we get a significant gap in between fronts, the result tends to be a mix of sunshine and showers- the most common pattern being a sunny start, a build up of cloud towards the afternoon and then sharp showers. When we have slow moving low pressure close by and no frontal activity, such "sunshine and showers" weather can persist for days on end- this situation generally arises when the jetstream is weak. However, if the low pressure is also associated with slow moving fronts, then instead of being bright and showery it tends to be cloudy and drizzly. High pressure setups For spells of warm dry sunny weather, many look out for ridges from the Azores High extending over towards the British Isles, quietening the weather down. Sometimes this can indeed herald the start of a fine spell, if high pressure can establish over the British Isles for upwards of a few days, but more often, the ridge brings just a day or two of fine weather before the next Atlantic system comes in and the high retreats to the south-west. Often the ridge just covers southern areas, giving warm dry sunny weather in the south, and dull damp weather in the north. If a ridge from the Azores High connects with high pressure over and/or to the east of Britain, however, we may get a prolonged spell of warm dry sunny weather, for instance the famous summer of 1976 was dominated by this setup. Eastern blocking Blocking over and/or to the east of Britain can sometimes bring hot sunny spells on its own, without the need for ridging from the Azores High- such a pattern typically has low pressure to the west, and the Azores High displaced to the west of its usual position. Persistence of this pattern resulted in the hot summer of 1995 and the exceptional July of 2006. It can also give rise to significant thunderstorms when Atlantic systems push against the block, bringing a "Spanish plume" event with southerlies bringing storms up from the near Continent. However, if the jetstream strengthens, such a pattern is usually temporary, as the Atlantic systems push through, the block retreats eastwards and we get a thundery breakdown followed by westerlies. Northern blocking Sometimes high pressure prevails to the north of Britain (this type of setup is far more common during June than July or August, as the westerlies are traditionally weaker). This brings a pattern of easterly winds, it is often warm, dry and sunny in the north-west, cool, dull and misty near the east coast, while central and southern areas tend to be warm and humid with thundery rain periodically moving up from the south. The mid-Atlantic high Finally, it is also possible for the Azores High to be displaced northwards into the mid-Atlantic, giving northerlies over the British Isles. This setup tends to be cool and cloudy, especially in eastern areas, as frontal systems move southwards around the periphery of the high, though western areas are often sunny. However, if we pick up an unmodified draw of air from the Arctic with a significant gap between fronts, the result tends to be sunshine and showers- similar to what I described under the low pressure setups- this setup tends to be cool but can also provide very dramatic weather with hail and thunder, particularly for eastern England.

The Cloud Works Clouds are simply condensed water droplets known as water vapour. These clouds can achieve many different things; they can give warnings, they can produce preciptation or merely tell how stable/unstable the atmosphere is. Clouds can come in many forms from the fluffy fair-weather Cumulus to the thick billowing frontal cloud of the Nimbostratus. Fair-weather Cumulus are caused when the sun causes heating of the ground and this heat known as convection causes clouds to grow on the boundary between cool air and warm air and sometimes these fluffy fair-weather Cumulus can grow into Cumulonimbus causing heavy midday showers or evening thunderstorms. Frontal cloud is the same process where warmer air bumps into cooler air or vica versa, causing moisture to form where the two air masses meet. This is also controlled by evaporating water from the sea/ocean before hand but only when a temperature gradient meet does condensation become very active and these two split areas of heat are called fronts. The Cloud Types Cirrus Cirrus are wispy clouds. These clouds are made of ice crystals and sit at the top of the layer of the atmosphere called the Troposphere where all the weather takes place. Cirrus is usually a sign that a frontal system containing strong winds is on the way or they can be found in the anvil of a Cumulonimbus. These clouds are known also as 'Mares' Tales' and are found around 4-10 miles up from sea level. Cirrostratus Cirrostratus are sheets of stratus like formation high in the sky. These are not particularly a sign of bad weather to follow. The sheet is made mostly of ice and is often seen in winter following on from periods of snow where the sun is visible through the cloud. Sometimes these can produce isolated snow flurries in winter. Again these clouds are 4-10 miles up from sea level. Cirrocumulus Cirrocumulus are uniformed sheets of cumulo-form cloud usually made of ice. These clouds indicate higher level instability particularly in summer and can be a sign of a coming thunderstorm. In winter these clouds often precede snow showers and can again give their own flurries if it is cold enough. These clouds are usually made of ice. The clouds are also known as 'Mackeral Sky' due to the fish-scale apperance they take on. These clouds are usually found around 4-5 miles up from sea level. Altocumulus Altocumulus are patches of cloud usually found in a uniformed formation and these clouds are usually made of ice or water droplets. These are very good sign of mid-high level instability and can sometimes tell of a thunderstorm or heavy shower on its way. In the winter these clouds can tell of storms and snow following but these clouds do not give any precipitation. These cloud are usually found 2-6 miles above sea level. Altostratus A sign that a front is on the way, these clouds can tell whether there will be periods of rain or snow and are often the cloud that precedes Nimbostratus on a warm front. These clouds are thick and they take on a sheet formation of water droplets or ice crystals. These clouds can even produce hours of snow themselves under the right conditions. These cloud again are found about 2-6 miles above sea level Cumulonimbus These clouds can tower as high as mount everest and are famous for their thunderstorms. A Cumulonimbus consists of a Towering Cumulus and an anvil of cirrus on the top. Although the bases of these clouds are less than 2.5 miles above sea level they can extend about 10 miles into the higher reaches of the Troposhere. The clouds are made of ice crystals and water droplets and give heavy rainfall and hail and sometimes snow associated with a cold front. Cumulus These clouds are usually fluffy fair-weather clouds built up by convection but enough sunlight can transform them into Towering Cumulus which can give torrential afternoon downpours. These clouds are made of water droplets though a Towering Cumulus can have ice crystals as the main feature at the top. These are mainly a shower cloud and can give showers of rain or snow. These clouds again form less than 2 miles above sea level. Nimbostratus These are deep layers of stratus usually associated with warm or occluded fronts. These clouds give precipitation of rain or snow and sometimes even hail and have been known to produce lightning. These clouds are slow moving and have a shallow temperature gradient associated with the warm front and usually indicate stable air is on the way. These clouds are made of water droplets and are found around about 2 miles above dea level. Stratus These clouds are low level, uniformed sheets of thick grey cloud. These clouds often appear with weak fronts and are usually a sign of stable air. Such clouds produce drizzle or light rain and when they hit ground level they are known as fog. These clouds are particularly associated with High Pressure areas of the Atlantic in the winter and summer where the land is cool and the sea is warm. These clouds are made from water droplets and form below 2 miles above sea level. Other Clouds They are the cloud types that are the mechanics of our atmosphere but there are other clouds including Mammatus which are found under Cumulonimbus anvils during a death of a severe thunderstorm. Also Pileus clouds which are a good indication of a strong shear these are wispy clouds that over ride Towering Cumulus clouds. Of course the noctilucent clouds that produce eerie shape and colours that are crafted by split colours from the sun or other sources of light.

The main Air Masses that affect the United Kingdom Air Masses are defined as a large body of air (covering many thousands of square kilometers) which at any given level has almost uniform temperatures, lapse rates(see topic on this), and humidity. Their Source Regions are large areas of the earth where air often stagnates for long periods. Examples of these are the Polar Regions, and the sub tropics. Air over any of these regions may stay for long periods and thus picks up the characteristics of the land or sea beneath it. When, because of a pressure gradient (see another topic on this) an air mass moves from its source, its properties will be modified by the land or sea over which it travels. By following the isobars on a pressure chart we can identify where this air mass has come from. Air Masses are classified by reference to the area they have come from and their subsequent track. In the UK they are known as either POLAR or TROPICAL, depending on where the airmass originated from, and are then sub divided into MARITIME or CONTINENTAL depending on whether the air has passed over land or sea. There are four major types that affect the UK Polar Maritime (Pm) Tropical Maritime (Tm) Tropical Continental (Tc) Polar Continental (Pc) Below is a diagram showing these four major air masses as they approach the UK. One has not been mentioned so far, that is Returning Polar Maritime (shown as a paler colour and approaching the UK from the sw). This is Polar Maritime air which has dome a long sweep over to the south west and is now returning with Tropical Maritime characteristics at the surface. Without going into a lot of detail the 4 air mass types can be summarized this way. Polar Maritime (Pm) In winter this will give convective type cloud with showers, often on windward coasts and hills. Depending on how cold the air is then snow, soft hail and sleet are likely. The showers can extend well inland by day but as the land cools at night the showers become mostly confined to the coastal belt. If troughs form in this air mass then showers can continue into the night and become much more widespread, especially if a Polar low forms (read up on separate section for Polar lows) In Summer then it is the inland areas that get most shower activity due to the land heating more than the sea. Coastal strips may be shower free but inland showers or thunderstorms can develop. Another version of this is called Arctic Maritime (Am)). As the map shows this comes direct from the Arctic and is thus even colder and therefore more showery than the Pm, This may well allow marked troughs to develop as it comes south over warmer seas, possibly even a Polar Low(see article on these) Tropical Maritime (Tm) It starts off from its source warm, moist and just a shallow layer. As it moves towards us it picks up more moisture, its cooled from below by the sea becoming colder as it moves north. On arrival here, usually into the southwest it has an overcast layer of Stratus and Stratocumulus (see cloud information section) and gives drizzle with extensive hill fog and sometimes coastal fog also. In winter these conditions can spread well inland with ground temperatures being low, especially at night. If it is associated with frontal weather then coming over a cold, maybe frozen ground then advection fog (see fog section) can set in with the thaw. It can sometimes to the east of high ground, with a Fohn effect(see section on special winds) give better visibility and higher cloud bases than in the west. In summer , particularly during the afternoon this Fohn effect combines with the warmer land to produce good cloud breaks well inland and only shallow Cumulus or Stratocumulus. Tropical Continental (Tc) Starting out from its source as dry, hot and at times hazy. On arrival in the UK it is stable, usually with no cloud, and is either hazy or becomes hazy if this persists for several days. Often it is the hottest air mass for these islands, especially the south and south east. In winter if this airmass reaches us then it has the characteristics of a short sea track Polar Continental due to it crossing the cold land mass of France. Polar Continental (Pc) This may have started over Russia or Scandinavia. In each case its precise track will have a great bearing on what weather it gives. Taking the winter situation first. It sets off very cold and dry. If its track is across Europe and then from the Low Countries into south east England then it picks up very little moisture. The weather is usually dry, hazy and very cold with severe night frosts. If its track takes it across the North Sea (often having originated from Scandinavia) then it picks up moisture off the sea, this is also much warmer than the air moving over it. The net result is that the bottom of the atmosphere becomes what we call unstable (see section on Stability and Instability) and large convection clouds develop (Cumulonimbus or Cumulus=again see section on clouds). This causes showers, usually wintry, even possibly with thunder and can give appreciable snowfall in eastern districts, being especially pronounced on the eastern slopes of hills. In Summer with a short sea track it is often dry and very warm (due in part to the long hours of daylight in high latitudes. So only limited Cumulus and possibly hazy if the situation lasts for any length of time. With a long sea track it has become stable due its passage over a relatively cold North Sea, and it gives overcast conditions with possibly drizzle and poor visibility, maybe even fog along the coasts. This can extend well inland during the evening. It has similar characteristics on the eastern side to those of Tropical Maritime along southern and western coasts. Finally to Returning Polar Maritime (RPm) It starts life in cold areas, maybe off Greenland is the most likely. It then moves south, possibly south east for a time and then swings in towards the UK from the south west. What happens to it during its journey is quite complex. First, as it heads southwards, it is heated strongly from the increasingly warm sea, this makes it unstable. Its moisture content is increased. Then as it moves north east it is cooled from below, so becoming stable in its lowest layers, and it continues to pick up more moisture. So when it arrives in the south west (its usual entry point) it has broken or overcast low cloud with hill and coastal fog, along with drizzle. In summer as it moves inland then the heating effect returns it to its original type (POLAR MARITIME) and convective cloud develops giving showers or thunderstorms. This topic has several links, some of which are already available on the Net Weather Guides; some are yet to be put together. One topic which follows on from this is the development of Fronts and Frontal Depressions. I hope to be able to post this in the not too distant future. John Holmes

Dewpoint: The strict definition for this is:-The temperature to which the air must be cooled in order that it is saturated with respect to a water surface and for this to occur at its existing pressure and humidity content. Dewpoint may be measured indirectly from wet- bulb and dry-bulb temperatures with the aid of humidity tables. More often now it is produced by some kind of calculator or computer programme taking these basics into consideration. Dew forms on clear nights when there is little or no wind at the surface. It can occur on summer nights but is most prevalent during the long nights from autumn through to spring. Dew forms on surfaces whose temperature falls to below the dewpoint of the air in contact with it. From what has already been said it should be clear that the dewpoint is always below the surface temperature. As in many things to do with Meteorology there is a caveat. Remember that the definition is 'become saturated with respect to a water surface'. There are a very small number of occasions when, with temperatures falling, the relationship becomes to that with respect to ice. In these few instances, briefly, the dewpoint may be a fraction higher than the air temperature. Dewpoint is really a measure of how much water vapour there is at any one time. Obviously the higher the air temperature then the higher the dewpoint can be. Compare the temperature in winter say of 3C with that on a hot summer day, maybe 25C. Obviously much more water vapour is possible with higher temperatures. This is partially responsible for the intensity of thunderstorms in summer compared to those of winter, and to the Tropics having much more intense downpours than in Temperate latitudes. The frost point is that temperature at which the air is saturated with respect to an ice surface. Dew point is closely associated with humidity. Thus in warm frontal zones the humidity is high. The arrival of a cold air mass will usually bring a sharp drop in the dewpoint. Dewpoint is an important tool for forecasters to use when forecasting many weather phenomena, be it thunderstorms, human comfort levels, or the likelihood of snow or frost. by John Holmes

A quickie on the abbreviations... WZ- Wetterzentrale ( German website for viewing charts) NAO- North atlantic oscillation PNA- Pacific North american SOI./ENSO- El-Nino Southern oscillation WAA- Warm air advection CAA- Cold air advection 528 DAM- is the line drawn on the maps that equated to the temperature ( MAX) that snow can be often observed at GFS- Global forecasting sytem METO- Met office model UKMO- United kingdom Met office ECM( Or ECMWF) European centre of medium range weather forecasts.. ASL- Above sea level- PPN- Precipitation Ensembles- 10 GFS model runs- Control run is the one seen on the models SST'S sea surface temperatures PM- Polar maritime air MT- Maritime Tropical air PC- Polar Continental air LRF's - Long range forecasts MRF's- meduim range forecasts Trough- Upper level equivalent to a surface Low pressure Ridge- Upper level equivalent to a surface High pressure Blocking- The jet stream being moved AROUND CLOCKWISE a large area of high pressure. Regards Steve

For those that have read Steve's interesting post and his reference to shortwaves here's a description courtesy of weatherprediction. com. PRESSURE TROUGHS AND SHORTWAVES METEOROLOGIST JEFF HABY When analyzing a surface chart you will notice the isobars bend in the vicinity of the warm front and the cold front. The isobars do not make perfect circles around low-pressure centers because of the pressure troughs created by the fronts. Pressure can decrease in the atmosphere by: (1) causing the air to rise (2) decreasing the density of the air (3) decreasing the mass of the air (i.e. upper level divergence). Causing the air to rise counteracts some the downward force created by gravity. This lowers pressure just as if someone started pushing up on you when standing on a scale; your weight would decrease. Fronts force the air to rise. This causes the surface pressure to decrease in the vicinity of the front. Cold fronts have a more defined pressure trough than warm fronts because the average cold front has a steeper slope and stronger temperature gradient than the average warm front. A warm front raises the air gradually while a cold front lifts the air more quickly in the vertical. The faster the air rises, the more pressure will lower. A mid-latitude cyclone and a front will both cause the air to rise and pressure to lower. The stronger the front, the more well defined the pressure trough will be. Now to shortwaves. A shortwave is an upper level front or a cool pocket aloft. Just as a surface front causes the air to rise, upper level fronts can do the same. First, let's start with a general description of a shortwave: (1) It is smaller than a longwave trough (shortwave ranges from 1 degree to about 30 degrees in longitude (the average one is about the size of two U.S. states put together (Iowa and Missouri put together is a good example) (2) Isotherms cross the height contours (if it is a baroclinic shortwave). This creates an upper level temperature gradient and therefore an upper level front (3) They are best examined on the 700 and 500 millibar charts (4) They generate positive vorticity (mainly due to the counterclockwise curvature within the shortwave). This creates positive curvature and positive shear vorticity. If PVA occurs with the shortwave then the shortwave will deepen and strengthen due to lift created by upper level divergence. (5) They can create an environment conducive to surface based convection or elevated convection due to the cooling aloft. It is important to see how much moisture is associated with the shortwave. A shortwave moving over a warm and moist lower troposphere has a better chance of producing precipitation and strengthening than one moving over a dry lower troposphere. If the low level dew point depressions are low, the instability and lift associated with the shortwave can enhance cloudiness and precipitation. In summary, a pressure trough is associated with a low-level front while a shortwave is associated with an upper level front or a cool pocket aloft. Both are associated with rising air and can add instability to the atmosphere.

Here are the current Papers & Articles under the research topic Madden-Julian Oscillation (MJO). Click on the title of a paper you are interested in to go straight to the full paper. Papers and articles covering the basics (ideal for learning) are shown in Green. What is the MJO, and why do we care? No abstract, so a snippet: Imagine ENSO as a person riding a stationary exercise bike in the middle of a stage all day long. His unchanging location is associated with the persistent changes in tropical rainfall and winds that we have previously described as being linked to ENSO. Now imagine another bike rider entering the stage on the left and pedaling slowly across the stage, passing the stationary bike (ENSO), and exiting the stage at the right. This bike rider we will call the MJO and he/she may cross the stage from left to right several times during the show. So, unlike ENSO, which is stationary, the MJO is an eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average. Learning All About the MJO No abstract, but this fascinating article mainly focusses on how the MJO impact spreads globally and includes some great illustrations. Fifty Years of Research on the Madden-Julian Oscillation: Recent Progress, Challenges, and Perspectives Published July 2020. Abstract: Since its discovery in the early 1970s, the crucial role of the Madden-Julian Oscillation (MJO) in the global hydrological cycle and its tremendous influence on high-impact climate and weather extremes have been well recognized. The MJO also serves as a primary source of predictability for global Earth system variability on subseasonal time scales. The MJO remains poorly represented in our state-of-the-art climate and weather forecasting models, however. Moreover, despite the advances made in recent decades, theories for the MJO still disagree at a fundamental level. The problems of understanding and modeling the MJO have attracted significant interest from the research community. As a part of the AGU's Centennial collection, this article provides a review of recent progress, particularly over the last decade, in observational, modeling, and theoretical study of the MJO. A brief outlook for near-future MJO research directions is also provided. Improving the prediction of the Madden-Julian Oscillation of the ECMWF model by post-processing Published: March 2022 Abstract: The Madden-Julian Oscillation (MJO) is a major source of predictability on the sub-seasonal (10- to 90-days) time scale. An improved forecast of the MJO, may have important socioeconomic impacts due to the influence of MJO on both, tropical and extratropical weather extremes. Although in the last decades state-of-the-art climate models have proved their capability for forecasting the MJO exceeding the 5 weeks prediction skill, there is still room for improving the prediction. In this study we use Multiple Linear Regression (MLR) and a Machine Learning (ML) algorithm as post-processing methods to improve the forecast of the model that currently holds the best MJO forecasting performance, the European Centre for Medium-Range Weather Forecast (ECMWF) model. We find that both MLR and ML improve the MJO prediction and that ML outperforms MLR. The largest improvement is in the prediction of the MJO geographical location and intensity. Deep learning for bias correction of MJO prediction Published: May 2021 Abstract: Producing accurate weather prediction beyond two weeks is an urgent challenge due to its ever-increasing socioeconomic value. The Madden-Julian Oscillation (MJO), a planetary-scale tropical convective system, serves as a primary source of global subseasonal (i.e., targeting three to four weeks) predictability. During the past decades, operational forecasting systems have improved substantially, while the MJO prediction skill has not yet reached its potential predictability, partly due to the systematic errors caused by imperfect numerical models. Here, to improve the MJO prediction skill, we blend the state-of-the-art dynamical forecasts and observations with a Deep Learning bias correction method. With Deep Learning bias correction, multi-model forecast errors in MJO amplitude and phase averaged over four weeks are significantly reduced by about 90% and 77%, respectively. Most models show the greatest improvement for MJO events starting from the Indian Ocean and crossing the Maritime Continent. ENSO Modulation of MJO Teleconnections to the North Atlantic and Europe Published Nov 2019 Abstract: The teleconnection from the Madden-Julian Oscillation (MJO) provides a source of subseasonal variability and predictability to the North Atlantic-European (NAE) region. The El Niño-Southern Oscillation (ENSO) modulates the seasonal mean state, through which the MJO and its teleconnection pattern propagates; however, its impact on this teleconnection to the NAE region has not been investigated. Here we find a robust dependence of the teleconnections from the MJO to NAE weather regimes on the phase of ENSO. We show that the MJO to NAO+ regime tropospheric teleconnection is strongly enhanced during El Niño years, via enhanced Rossby wave activity, and suppressed during La Niña. Conversely, the MJO to NAO− regime stratospheric teleconnection is enhanced during La Niña years and suppressed during El Niño. This dependence on the background state has strong implications for subseasonal predictability, including interannual variations in subseasonal predictive skill. Verification of medium-range MJO forecasts with TIGGE Published June 2011 Abstract: The Madden-Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Accurate simulations of the MJO are important for studies of weather and climate in the tropics and extratropics. This study assesses the forecast performance of operational medium-range ensemble forecasts, available at THe Observing system Research and Predictability EXperiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) data portal, regarding the MJO for the past 3 years. The results indicate that ECMWF (European Centre for Medium-range Weather Forecasts) and UKMO (United Kingdom Meteorological Office) generally yield the best performances in predicting the MJO; however, they do not always show similar skills. ECMWF performs well in simulating the maintenance and onset of the MJO in phases 1-4, whereas UKMO and NCEP (National Centers for Environmental Prediction) perform well in simulating the maintenance and onset of the MJO in phases 5-8. Thus, the best-performing numerical weather prediction (NWP) centre varies with the phase of the MJO. With advance knowledge of the forecast characteristics of each NWP centre, we can ensure more reliable forecasts of the MJO in operational uses, based on the MJO phase. This represents an advantage of the multi-centre grand ensemble approach. The Role of MJO Propagation, Lifetime, and Intensity on Modulating the Temporal Evolution of the MJO Extratropical Response Published May 2019 Abstract: The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability. Many studies have found that the MJO has significant impacts on extratropical weather. Since the MJO can act as a tropical heat source that excites Rossby waves, midlatitude weather is modulated by the MJO due to the Rossby waves that propagate into the midlatitude and modulate the midlatitude circulation. Heat sources of individual MJO events are different since each event has different eastward propagation speed, lifetime, intensity, and structure. The background flow is also different for each event. These result in different Rossby waves and different extratropical response for each MJO event. In this study, the role of MJO propagation speed, lifetime, and intensity on modulating the structure and temporal evolution of the MJO extratropical response is systematically explored by using an idealized general circulation model. By adding the MJO-associated heating into the general circulation model as an external forcing, the extratropical response in the Reanalysis is captured reasonably by the model. However, large ensemble model simulations show that the response in the Reanalysis is not robust. Experiments with MJO events of different propagation speed, lifetime, and intensity show that to excite a strong extratropical response, the MJO has to propagate through specific phases (Phases 1–3 and 5–7). The intensity, timing, and duration of the extratropical response strongly depend on when the MJO is initiated and when the MJO decays. The extratropical impacts of slow- and fast-propagating MJO also have significant differences, especially on intensity and duration. Changes in Madden‐Julian Oscillation Precipitation and Wind Variance Under Global Warming Published Jul 2018. Abstract: The Madden‐Julian oscillation (MJO) is the leading mode of tropical intraseasonal variability, having profound impacts on many weather and climate phenomena across the tropics and extratropics. Previous studies using a limited number of models have suggested complex changes in MJO activity in a warmer climate. While most studies have argued that MJO precipitation amplitude will increase in a future warmer climate, others note that this is not necessarily the case for MJO wind variability. This distinction is important since MJO wind fluctuations are responsible for producing remote impacts on extreme weather through teleconnections. In this study, we examine projected changes of MJO precipitation and wind variance at the end of the 21st century in Representative Concentration Pathway 8.5 using the multimodel Coupled Model Intercomparison Project phase 5 data set. Under global warming, most models show an increase in MJO band precipitation variance, while wind variability decreases. The discrepancy between MJO precipitation and wind variance changes under global warming is shown to be due to increases in tropical static stability in a warmer climate. The multimodel mean shows a 20% increase in both the 500‐hPa vertical tropical dry static energy gradient and the ratio of intraseasonal precipitation to 500 hPa omega fluctuations, consistent with scaling by weak temperature gradient theory. These results imply that tropical static stability increases may weaken the MJO's ability to influence extreme events in future warmer climate by weakening wind teleconnections, even though MJO precipitation amplitude may increase. A Cautionary Note on the Long‐term Trend in Activity of the Madden‐Julian Oscillation During the Past Decades Published Dec 2019. Abstract: Recent studies suggest that frequency of active phases of the Madden‐Julian Oscillation (MJO) over the Maritime Continent and western Pacific, that is, the MJO Phases 4–6 defined by the real‐time multivariate MJO (RMM) index, has increased in recent winters. A robust positive trend in MJO Phase 4–6 days during 1979–2015 winters is confirmed in this study. Our analyses, however, suggest that this trend could be exaggerated due to the blended low‐frequency variability signals in the RMM. When the winter RMM is reconstructed using anomalous fields after removing their winter mean instead of the previous 120‐day mean as for the original RMM, the robust trend in MJO Phase 4–6 days can no longer be detected. Therefore, cautions need to be exercised when applying the RMM for studies on the low‐frequency variability and climate trend in MJO activity and using the derived MJO variability to interpret associated changes in climate systems. A Madden‐Julian Oscillation event remotely accelerates ocean upwelling to abruptly terminate the 1997/1998 super El Niño Abstract: The termination of the super intense 1997/1998 El Niño was extraordinarily abrupt. The May 1998 Madden-Julian Oscillation (MJO), a massive complex of stormy tropical clouds, is among possible contributors to the abrupt termination. Despite having been sensationally proposed 18 years ago, the role of the MJO remained controversial and speculative because of the difficulty of sufficiently simulating the El Niño and MJO simultaneously. An ensemble simulation series using a newly developed, fully ocean-coupled version of a global cloud system resolving numerical model replicated the specific atmosphere and ocean conditions of May 1998 in unprecedented detail, extending the prediction skill of the MJO to 46 days. Simulation ensemble members with stronger MJO activities over the Maritime Continent experienced quicker sea surface temperature drop in the eastern Pacific, confirming that the easterly winds associatedwith the remote MJO accelerated ocean upwelling to abruptly terminate the El Niño. An All-Season Real-Time Multivariate MJO Index: Development of an Index for Monitoring and Prediction Abstract: A seasonally independent index for monitoring the Madden–Julian oscillation (MJO) is described. It is based on a pair of empirical orthogonal functions (EOFs) of the combined fields of near-equatorially averaged 850-hPa zonal wind, 200-hPa zonal wind, and satellite-observed outgoing longwave radiation (OLR) data. Projection of the daily observed data onto the multiple-variable EOFs, with the annual cycle and components of interannual variability removed, yields principal component (PC) time series that vary mostly on the intraseasonal time scale of the MJO only. This projection thus serves as an effective filter for the MJO without the need for conventional time filtering, making the PC time series an effective index for real-time use. The pair of PC time series that form the index are called the Real-time Multivariate MJO series 1 (RMM1) and 2 (RMM2). The properties of the RMM series and the spatial patterns of atmospheric variability they capture are explored. Despite the fact that RMM1 and RMM2 describe evolution of the MJO along the equator that is independent of season, the coherent off-equatorial behavior exhibits strong seasonality. In particular, the northward, propagating behavior in the Indian monsoon and the southward extreme of convection into the Australian monsoon are captured by monitoring the seasonally independent eastward propagation in the equatorial belt. The previously described interannual modulation of the global variance of the MJO is also well captured. Applications of the RMM series are investigated. One application is through their relationship with the onset dates of the monsoons in Australia and India; while the onsets can occur at any time during the convectively enhanced half of the MJO cycle, they rarely occur during the suppressed half. Another application is the modulation of the probability of extreme weekly rainfall; in the “Top End” region around Darwin, Australia, the swings in probability represent more than a tripling in the likelihood of an upper-quintile weekly rainfall event from the dry to wet MJO phase. Circulation Response to Fast and Slow MJO Episodes 2017 paper. Abstract: Fast and slow Madden–Julian oscillation (MJO) episodes have been identified from 850- and 200-hPa zonal wind and outgoing longwave radiation (OLR) for 32 winters (16 October–17 March) 1980/81–2011/12. For 26 fast cases the OLR took no more than 10 days to propagate from phase 3 (convection over the Indian Ocean) to phase 6 (convection over the western Pacific). For 8 slow cases the propagation took at least 20 days. Fast episode composite anomalies of 500-hPa height (Z500) show a developing Rossby wave in the mid-Pacific with downstream propagation through MJO phases 2–4. Changes in the frequency of occurrence of the NAO1 weather regime are modest. This Rossby wave is forced by anomalous cooling over the Maritime Continent during phases 2 and 3 (seen in phase-independent wave activity flux). The upper-level anticyclonic response to phase-3 heating is a secondary source of wave activity. The Z500 slow episode composite response to MJO phases 1 and 2 is an enhanced Aleutian low followed by a North American continental high. Following phase 4 the development of an NAO1 like pattern is seen over the Atlantic, transitioning to a strong NAO2 pattern by phase 8. A dramatic increase in frequency of the NAO1 weather regime follows phases 4and 5, while a strong increase in NAO2 regime follows phases 6 and 7. The responses to MJO-related heating and cooling over the Indian and western Pacific Oceans in phases 1–4 provide a source for wave activity propagating to North America, augmented by storm-track anomalies. Combined effect of the QBO and ENSO on the MJO Abstract: This study investigates the combined effect of the El Niño–Southern Oscillation (ENSO) and stratospheric quasi-biennial oscillation (QBO) on the Madden Julian Oscillation (MJO). Here, the authors investigate the combined effect of the QBO and ENSO on the MJO. The results show that the western Pacific MJO originating from the Indian Ocean during La Niña/QBO easterly years is stronger than that during El Niño years. This relation, however, disappears during La Niña/QBO westerly years. The reason is that ENSO and the QBO have different effects on each MJO event. For an El Niño year, there is only about one MJO event, and the QBO effect is small. During a La Niña/QBO easterly year, there are 1.7 MJO events, while during a La Niña/QBO westerly year, there are only 0.6 MJO events. El Niño can reinforce the MJO over the western Pacific because of the positive moisture advection of the El Niño mean state by MJO easterly wind anomalies. The QBO mainly affects the MJO over the Maritime Continent region by changing the high-cloud-controlled diurnal cycle; and the Maritime Continent barrier effect is enhanced during the QBO westerly phase because of the strong diurnal cycle. During El Niño years, even the MJO over the Maritime Continent is suppressed by the QBO westerly phase; the MJO can be reinforced over the western Pacific. During La Niña/QBO westerly years, the MJO over the Maritime Continent is suppressed because of the strong Maritime Continent diurnal cycle, and it is further suppressed over the western Pacific because of the lack of a reinforcement process. Impact of the MJO on the boreal winter extratropical circulation Published August 2014. Abstract: The effect of the Madden‐Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex is evaluated using a meteorological reanalysis data set and a modern atmospheric general circulation model. The MJO influences the tropospheric North Pacific; and in particular, it modulates the heat flux that is in phase with the climatological planetary waves in both the troposphere and stratosphere. The phase of the MJO in which convection is propagating into the tropical central Pacific leads to a weakened vortex, while suppressed MJO convection in this region is associated with a stronger vortex. Subsequently, the MJO modulates the phase of the tropospheric North Atlantic Oscillation (also known as the Arctic Oscillation or the Northern Annular Mode). While the responses in the model and in the reanalysis data differ in some respects, they both indicate that the MJO can remotely impact the extratropical tropospheric circulation via the stratosphere. Influence of the QBO on MJO prediction skill in the subseasonal-to-seasonal prediction models Abstract: Recent studies have shown that the Madden-Julian Oscillation (MJO) is significantly modulated by the stratospheric Quasi Biennial Oscillation (QBO). In general, boreal winter MJO becomes more active during the easterly phase of the QBO (EQBO) than during the westerly phase (WQBO). Based on this finding, here we examine the possible impacts of the QBO on MJO prediction skill in the operational models that participated in the WCRP/WWRP subseasonal-to-seasonal (S2S) prediction project. All models show a higher MJO prediction skill during EQBO winters than during WQBO winters. For the bivariate anomaly correlation coefficient of 0.5, the MJO prediction skill during EQBO winters is enhanced by up to 10 days. This enhancement is insensitive to the initial MJO amplitude, indicating that the improved MJO prediction skill is not simply the result of a stronger MJO. Instead, a longer persistence of the MJO during EQBO winters likely induces a higher prediction skill by having a higher prediction limit. Influence of the Stratospheric Quasi-Biennial Oscillation on the Madden–Julian Oscillation during Austral Summer Abstract: Influence of the stratospheric quasi-biennial oscillation (QBO) on the Madden–Julian oscillation (MJO) and its statistical significance are examined for austral summer (DJF) in neutral ENSO events during 1979–2013. The amplitude of the OLR-based MJO index (OMI) is typically larger in the easterly phase of the QBO at 50 hPa (E-QBO phase) than in the westerly (W-QBO) phase. Daily composite analyses are performed by focusing on phase 4 of the OMI, when the active convective system is located over the eastern Indian Ocean through the Maritime Continent. The composite OLR anomaly shows a larger negative value and slower eastward propagation with a prolonged period of active convection in the E-QBO phase than in the W-QBO phase. Statistically significant differences of the MJO activities between the QBO phases also exist with dynamical consistency in the divergence of horizontal wind, the vertical wind, the moisture, the precipitation, and the 100-hPa temperature. A conditional sampling analysis is also performed by focusing on the most active convective region for each day, irrespective of the MJO amplitude and phase. Composite vertical profiles of the conditionally sampled data over the most active convective region reveal lower temperature and static stability around the tropopause in the E-QBO phase than in the W-QBO phase, which indicates more favorable conditions for developing deep convection. This feature is more prominent and extends into lower levels in the upper troposphere over the most active convective region than other tropical regions. Composite longitude–height sections show similar features of the large-scale convective system associated with the MJO, including a vertically propagating Kelvin response. Intraseasonal interaction between the Madden–Julian Oscillation and the North Atlantic Oscillation 2008 paper. Abstract: Bridging the traditional gap between the spatio-temporal scales of weather and climate is a significant challenge facing the atmospheric community. In particular, progress in both medium-range and seasonal-to-interannual climate prediction relies on our understanding of recurrent weather patterns and the identification of specific causes responsible for their favoured occurrence, persistence or transition. Within this framework, I here present evidence that the main climate intra-seasonal oscillation in the tropics—the Madden–Julian Oscillation1,2 (MJO)—controls part of the distribution and sequences of the four daily weather regimes defined over the North Atlantic–European region in winter3. North Atlantic Oscillation4 (NAO) regimes are the most affected, allowing for medium-range predictability of their phase far exceeding the limit of around one week that is usually quoted. The tropical–extratropical lagged relationship is asymmetrical. Positive NAO events mostly respond to a mid-latitude low-frequency wave train initiated by the MJO in the western–central tropical Pacific and propagating eastwards. Precursors for negative NAO events are found in the eastern tropical Pacific–western Atlantic, leading to changes along the North Atlantic storm track. Wave-breaking diagnostics tend to support the MJO preconditioning and the role of transient eddies in setting the phase of the NAO. I present a simple statistical model to quantitatively assess the potential predictability of the daily NAO index or the sign of the NAO regimes when they occur. Forecasts are successful in ∼70 per cent of the cases based on the knowledge of the previous ∼12-day MJO phase used as a predictor. This promising skill could be of importance considering the tight link4 between weather regimes and both mean conditions and the chances of extreme events occurring over Europe. These findings are useful for further stressing the need to better simulate and forecast the tropical coupled ocean–atmosphere dynamics, which is a source of medium-to-long range predictability and is the Achilles’ heel of the current seamless prediction suites. Circulation Response to Fast and Slow MJO Episodes 2008 paper. Abstract: In this study, we detected the spatial and temporal characteristics of Madden-Julian Oscillation (MJO) using zonal winds at the surface and outgoing long-wave radiation (OLR) from the NCEP-NCAR (U.S. National Center of Environmental Prediction-National Center for Atmospheric Research) reanalysis product from 1981–2003. The results show that MJO activity, represented by these two variables, has large variances around 10° off the equator and over the near-equatorial western Pacific. One central issue addressed in this study is MJO-ENSO (El Niño and Southern Oscillation) relationship. It has been found that there exists a statistically significant relationship between MJO in spring-summer and ENSO in autumn-winter. The relationship of MJO-ENSO is nonlinear in nature and has a decadal variation. A much stronger statistical relationship of MJO-ENSO was found in the 1990s as compared to that in the 1980s. These findings were further verified using ECMWF (European Center for Medium-Range Weather Forecasts) reanalysis product. The potential mechanisms responsible for MJO-ENSO relationship are also discussed. MJO‐Related Tropical Convection Anomalies Lead to More Accurate Stratospheric Vortex Variability in Sub-seasonal Forecast Models Abstract: The effect of the Madden‐Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex in the period preceding stratospheric sudden warmings is evaluated in operational subseasonal forecasting models. Reforecasts which simulate stronger MJO‐related convection in the Tropical West Pacific also simulate enhanced heat flux in the lowermost stratosphere and a more realistic vortex evolution. The time scale on which vortex predictability is enhanced lies between 2 and 4 weeks for nearly all cases. Those stratospheric sudden warmings that were preceded by a strong MJO event are more predictable at ∼20 day leads than stratospheric sudden warmings not preceded by a MJO event. Hence, knowledge of the MJO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere polar stratosphere. Modulation of equatorial Pacific westerly/easterly wind events by the Madden–Julian oscillation and convectively‑coupled Rossby waves Abstract: Synoptic wind events in the equatorial Pacific strongly influence the El Niño/Southern Oscillation (ENSO) evolution. This paper characterizes the spatio-temporal distribution of Easterly (EWEs) and Westerly Wind Events (WWEs) and quantifies their relationship with intraseasonal and interannual large-scale climate variability. We unambiguously demonstrate that the Madden–Julian Oscillation (MJO) and Convectively-coupled Rossby Waves (CRW) modulate both WWEs and EWEs occurrence probability. 86 % of WWEs occur within convective MJO and/or CRW phases and 83 % of EWEs occur within the suppressed phase of MJO and/or CRW. 41 % of WWEs and 26 % of EWEs are in particular associated with the combined occurrence of a CRW/MJO, far more than what would be expected from a random distribution (3 %). Wind events embedded within MJO phases also have a stronger impact on the ocean, due to a tendency to have a larger amplitude, zonal extent and longer duration. These findings are robust irrespective of the wind events and MJO/CRW detection methods. While WWEs and EWEs behave rather symmetrically with respect to MJO/CRW activity, the impact of ENSO on wind events is asymmetrical. The WWEs occurrence probability indeed increases when the warm pool is displaced eastward during El Niño events, an increase that can partly be related to interannual modulation of the MJO/CRW activity in the western Pacific. On the other hand, the EWEs modulation by ENSO is less robust, and strongly depends on the wind event detection method. The consequences of these results for ENSO predictability are discussed. Modulation of the Global Atmospheric Circulation by Combined Activity in the Madden–Julian Oscillation and the El Niño–Southern Oscillation during Boreal Winter 2010 paper. Abstract: Composite global patterns associated with the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) are frequently applied to help make predictions of weather around the globe at lead times beyond a few days. However, ENSO modulates the background states through which the MJO and its global response patterns propagate. This paper explores the possibility that nonlinear variations confound the combined use of composites based on the MJO and ENSO separately. Results indicate that when both modes are active at the same time, the associated patterns in the global flow are poorly represented by simple linear combinations of composites based on the MJO and ENSO individually. Composites calculated by averaging data over periods when both modes are present at the same time more effectively describe the associated weather patterns. Results reveal that the high-latitude response to the MJO varies with ENSO over all longitudes, but especially across the North Pacific Rim, North America, and the North Atlantic. Further analysis demonstrates that the MJO influence on indexes of the North Atlantic Oscillation is greatest during La Niña conditions or during periods of rapid adjustment in the phase of ENSO. More Frequent Sudden Stratospheric Warming Events due to Enhanced MJO Forcing Expected in a Warmer Climate Abstract: Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Observations show SSW events to be correlated with certain phases of the Madden–Julian oscillation (MJO), but the effect of the MJO on SSW frequency is unknown, and the teleconnection mechanism, its planetary wave propagation path, and time scale are still not completely understood. The Arctic stratosphere response to increased MJO forcing expected in a warmer climate using two models is studied: the comprehensive Whole Atmosphere Community Climate Model and an idealized dry dynamical core with and without MJO-like forcing. It is shown that the frequency of SSW events increases significantly in response to stronger MJO forcing, also affecting the averaged polar cap temperature. Two teleconnection mechanisms are identified: a direct propagation of MJO-forced transient waves to the Arctic stratosphere and a nonlinear enhancement of stationary waves by the MJO-forced transient waves. The MJO-forced waves propagate poleward in the lower stratosphere and upper troposphere and then upward. The cleaner results of the idealized model allow identifying the propagating signal and suggest a horizontal propagation time scale of10–20 days, followed by additional time for upward propagation within the Arctic stratosphere, although there are significant uncertainties involved. Given that the MJO is predicted to be stronger in a warmer climate, these results suggest that SSW events may become more frequent, with possible implications on tropospheric high-latitude weather. However, the effect of an actual warming scenario on SSW frequency involves additional effects besides a strengthening of the MJO, requiring further investigation. Northern Hemisphere mid‐winter vortex‐displacement and vortex‐split stratospheric sudden warmings: Influence of the Madden‐Julian Oscillation and Quasi‐Biennial Oscillation Abstract: We investigate the connection between the equatorial Madden-Julian Oscillation (MJO) and different types of the Northern Hemisphere mid-winter major stratospheric sudden warmings (SSWs), i.e., vortex-displacement and vortex-split SSWs. The MJO-SSW relationship for vortex-split SSWs is stronger than that for vortex-displacement SSWs, as a result of the stronger and more coherent eastward propagating MJOs before vortex-split SSWs than those before vortex-displacement SSWs. Composite analysis indicates that both the intensity and propagation features of MJO may influence the MJO-related circulation pattern at high latitudes and the type of SSWs. A pronounced Quasi-Biennial Oscillation (QBO) dependence is found for vortex-displacement and vortex-split SSWs, with vortex-displacement (-split) SSWs occurring preferentially in easterly (westerly) QBO phases. The lagged composites suggest that the MJO-related anomalies in the Arctic are very likely initiated when the MJO-related convection is active over the equatorial Indian Ocean (around the MJO phase 3). Further analysis suggests that the QBO may modulate the MJO-related wave disturbances via its influence on the upper tropospheric subtropical jet. As a result, the MJO-related circulation pattern in the Arctic tends to be wave number-one/wave number-two ~25–30 days following phase 3 (i.e., approximately phases 7–8, when the MJO-related convection is active over the western Pacific) during easterly/westerly QBO phases, which resembles the circulation pattern associated with vortex-displacement/vortex-split SSWs. Observed Changes in the Lifetime and Amplitude of the MJO Associated with Interannual ENSO Sea Surface Temperature Anomalies Abstract: The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May andOctober–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño,consistent with theoretical arguments concerning equatorial wave speeds.The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is foundin 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter,and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture,this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or ashorter dataset (as in some previous studies), no relationship is found. Observed connection between stratospheric sudden warmings and the Madden-Julian Oscillation Abstract: The effect of the Madden-Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex and major, mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis dataset. The MJO influences the region in the tropospheric North Pacific sector that is most strongly associated with a SSW.Consistent with this, SSWs in the reanalysis record have tended to follow certain MJO phases. The magnitude of the influence of the MJO on the vortex is comparable to that associated with the Quasi-Biennial Oscillation and El Niño. The MJO could be used to improve intra-seasonal projections of the Northern Hemisphere high latitude circulation, and in particular of the tropospheric Northern Annular Mode, at lags exceeding one month. Ocean Rossby waves as a triggering mechanism for primary Madden-Julian events Abstract: The Madden–Julian Oscillation (MJO) is sporadic, with episodes of cyclical activity interspersed with inactive periods. However, it remains unclear what may trigger a Madden–Julian (MJ) event which is not immediately preceded by any MJO activity:a ‘primary’ MJ event. A combination of case-studies and composite analysis is usedto examine the extent to which the triggering of primary MJ events might occur in response to ocean dynamics. The case-studies show that such events can be triggered by the arrival of a downwelling oceanic equatorial Rossby wave, which is shown to be associated with a deepening of the mixed layer and positive sea-surface temperature(SST) anomalies of the order of 0.5–1◦C. These SST anomalies are not attributable to forcing by surface fluxes which are weak for the case-studies analysed. Furthermore,composite analysis suggests that such forcing is consistently important for triggering primary events. The relationship is much weaker for successive events, due to themany other triggering mechanisms which operate during periods of cyclical MJO activity. This oceanic feedback mechanism is a viable explanation for the sporadic and broadband nature of the MJO. Additionally, it provides hope for forecasting MJ events during periods of inactivity, when MJO forecasts generally exhibit low skill. On the emerging relationship between the stratospheric Quasi-Biennial oscillation and the Madden-Julian oscillation Abstract: A strong relationship between the quasi-biennial oscillation (QBO) of equatorial stratospheric winds and the amplitude of the Madden-Julian oscillation (MJO) during the boreal winter has recently been uncovered using observational data from the mid-1970s to the present. When the QBO is in its easterly phase in the lower stratosphere, it favors stronger MJO activity during boreal winter, while the MJO tends to be weaker during the westerly phase of the QBO. Here we show using reconstructed indices of the MJO and QBO back to 1905 that the relationship between enhanced boreal winter MJO activity and the easterly phase of the QBO has only emerged since the early 1980s. The emergence of this relationship coincides with the recent cooling trend in the equatorial lower stratosphere and the warming trend in the equatorial upper troposphere, which appears to have sensitized MJO convective activity to QBO-induced changes in static stability near the tropopause. Climate change is thus suggested to have played a role in promoting coupling between the MJO and the QBO. Predictability and Prediction of the North Atlantic Oscillation (large section on MJO impact) 2010 article. No abstract, but the introduction: The North Atlantic Oscillation (NAO) is one of the most important modes of variability in the Northern Hemisphere extratropical atmosphere. The NAO has a wide range of time scales from days to decades. It has larger amplitude in boreal winter than in summer. On interannual time scale, the NAO accounts for about 30% of the hemispheric surface air temperature variance over the past 60 winters. The NAO has long been recognized as a major circulation pattern influencing the weather and climate from eastern North America to Europe. It is generally accepted that the primary mechanism for the NAO is the internal dynamics of the extratropical circulation. This implies a lack of predictability for the NAO variability beyond the time scale of a baroclinic wave. Processes external to the extratropical atmosphere have also been found to contribute to the NAO variability on intraseasonal and seasonal scales. For example, tropical diabatic heating anomalies associated with the Madden-Julian Oscillation (MJO) can influence the extratropical circulation and the NAO. Sea surface temperature (SST) and snow cover anomalies may have an impact on the NAO on seasonal time scale. In this article, focus of discussion is made on the forecast skill of the NAO on intraseasonal and seasonal time scales. QBO Influence on MJO Amplitude over the Maritime Continent:Physical Mechanisms and Seasonality Abstract: The quasi-biennial oscillation (QBO) is stratified by stratospheric zonal wind direction and height intofour phase pairs [easterly midstratospheric winds (QBOEM), easterly lower-stratospheric winds, westerly midstratospheric winds (QBOWM), and westerly lower-stratospheric winds] using an empirical orthogonal function analysis of daily stratospheric (100–10 hPa) zonal wind data during 1980–2017. Madden–Julian oscillation (MJO) events in which the MJO convective envelope moved eastward across the Maritime Continent (MC) during 1980–2017 are identified using the Real-time Multivariate MJO (RMM) index and theoutgoing longwave radiation (OLR) MJO index (OMI). Comparison of RMM amplitudes by the QBO phase pair over the MC (RMM phases 4 and 5) reveals that boreal winter MJO events have the strongest amplitudes during QBOEM and the weakest amplitudes during QBOWM, which is consistent with QBO-driven differences in upper-tropospheric lower-stratospheric (UTLS) static stability. Additionally, boreal winter RMM events over the MC strengthen during QBOEM and weaken during QBOWM. In the OMI, those amplitude changes generally shift eastward to the eastern MC and western Pacific Ocean, which may result from differences in RMM and OMI index methodologies. During boreal summer, as the northeastward-propagating boreal summer intraseasonal oscillation (BSISO) becomes the dominant mode of intraseasonal variability, these relationships are reversed. Zonal differences in UTLS stability anomalies are consistent with amplitude changes of eastward-propagating MJO events across the MC during boreal winter, and meridional stability differences are consistent with amplitude changes of northeastward-propagating BSISO events during boreal summer. Results remain consistent when stratifying by neutral ENSO phase. Stratospheric Control of the Madden–Julian Oscillation Abstract: Interannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Niño–Southern Oscillation (ENSO). For example, during El Niño winters, seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Niño winters and contract toward the western Pacific during La Niña winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-related MJO change explains up to 40% of interannual variation of the boreal winter MJO amplitude. This result suggests that variability of the MJO and the related tropical–extratropical teleconnections can be better understood and predicted by taking not only the tropospheric circulation but also the stratospheric mean state into account. The seasonality of the QBO–MJO link and the possible mechanism are also discussed. Subseasonal Forecasts of Convectively Coupled Equatorial Waves and the MJO: Activity and Predictive Skill Abstract: In this study, the contribution of low-frequency (.100 days), Madden–Julian oscillation (MJO), and con-vectively coupled equatorial wave (CCEW) variability to the skill in predicting convection and winds in the tropics at weeks 1–3 is examined. We use subseasonal forecasts from the Navy Earth System Model (NESM);NCEP Climate Forecast System, version 2 (CFSv2); and ECMWF initialized in boreal summer 1999–2015.A technique for performing wave number–frequency filtering on subseasonal forecasts is introduced and applied to these datasets. This approach is better able to isolate regional variations in MJO forecast skill than traditional global MJO indices. Biases in the mean state and in the activity of the MJO and CCEWs are smallest in the ECMWF model. The NESM overestimates cloud cover as well as MJO, equatorial Rossby, and mixed Rossby–gravity/tropical depression activity over the west Pacific. The CFSv2 underestimates convectively coupled Kelvin wave activity. The predictive skill of the models at weeks 1–3 is examined by decomposing the forecasts into wavenumber–frequency signals to determine the modes of variability that contribute to forecast skill. All three models have a similar ability to simulate low-frequency variability but large differences in MJO skill are observed. The skill of the NESM and ECMWF model in simulating MJO-related OLR signals at week 2 is greatest over two regions of high MJO activity, the equatorial Indian Ocean and Maritime Continent, and the east Pacific. The MJO in the CFSv2 is too slow and too weak, which results in lower MJO skill in these regions. The global response to tropical heating in the Madden–Julian oscillation during the northern winter 2004 paper. Abstract: A life cycle of the Madden–Julian oscillation (MJO) was constructed, based on 21 years of outgoing long-wave radiation data. Regression maps of NCEP–NCAR reanalysis data for the northern winter show statistically significant upper-tropospheric equatorial wave patterns linked to the tropical convection anomalies, and extratropical wave patterns over the North Pacific, North America, the Atlantic, the Southern Ocean and South America. To assess the cause of the circulation anomalies, a global primitive-equation model was initialized with the observed three-dimensional (3D) winter climatological mean flow and forced with a time-dependent heat source derived from the observed MJO anomalies. A model MJO cycle was constructed from the global response to the heating, and both the tropical and extratropical circulation anomalies generally matched the observations well. The equatorial wave patterns are established in a few days, while it takes approximately two weeks for the extratropical patterns to appear. The model response is robust and insensitive to realistic changes in damping and basic state. The model tropical anomalies are consistent with a forced equatorial Rossby–Kelvin wave response to the tropical MJO heating, although it is shifted westward by approximately 20◦longitude relative to observations. This may be due to a lack of damping processes (cumulus friction) in the regions of convective heating. Once this shift is accounted for, the extratropical response is consistent with theories of Rossby wave forcing and dispersion on the climatological flow, and the pattern correlation between the observed and modelled extratropical flow is up to 0.85. The observed tropical and extratropical wave patterns account for a significant fraction of the intraseasonal circulation variance, and this reproducibility as a response to tropical MJO convection has implications for global medium-range weather prediction. The MJO‐SSW Teleconnection: Interaction Between MJO‐Forced Waves and the Midlatitude Jet Abstract: The Madden Julian Oscillation (MJO) was shown to affect both present-day Sudden Stratospheric Warming (SSW) events in the Arctic, and their future frequency under global warming scenarios, with implications to the Arctic Oscillation and mid-latitude extreme weather. This work uses a dry dynamic core model to understand the dependence of SSW frequency on the amplitude and longitudinal range of the MJO, motivated by the prediction that the MJO will strengthen and broaden its longitudinal range in a warmer climate. We focus on the response of the mid-latitude jets and the corresponding generated stationary waves, that are shown to dominate the response of SSW events to MJO forcing. Momentum budget analysis of a large ensemble of spinup simulations suggests the climatological jet response is driven by the MJO-forced meridional eddy momentum transport. The results suggest that the trends in both MJO amplitude and longitudinal range are important for the prediction of the mid-latitude jet response and for the prediction of SSWs in a future climate. The Relationship between Northern Hemisphere Winter Blocking and Tropical Modes of Variability 2016 paper. Abstract: In the present study, the influence of some major tropical modes of variability on Northern Hemisphere regional blocking frequency variability during boreal winter is investigated. Reanalysis data and an ensemble experiment with the ECMWF model using relaxation toward the ERA-Interim data inside the tropics areused. The tropical modes under investigation are El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), and the upper-tropospheric equatorial zonal-mean zonal wind [U1^50]E. An early (late) MJO phase refers to the part of the MJO cycle when enhanced (suppressed) precipitation occurs over the western Indian Ocean and suppressed (enhanced) precipitation occurs over the Maritime Continent and the western tropical Pacific. Over the North Pacific sector, it is found that enhanced (suppressed) high-latitude blocking occurs in association with El Niño (La Niña) events, late (early) MJO phases, and westerly (easterly)[U1^50]E. Over central to southern Europe and the east Atlantic, it is found that late MJO phases, as well as a suppressed MJO, are leading to enhanced blocking frequency. Furthermore, early (late) MJO phases arefollowed by blocking anomalies over the western North Atlantic region, similar to those associated with a positive (negative) North Atlantic Oscillation. Over northern Europe, the easterly (westerly) phase of[U1^50]Eis associated with enhanced (suppressed) blocking. These results are largely confirmed by both the reanalysis and the model experiment. The Role of Zonal Asymmetry in the Enhancement and Suppression of Sudden Stratospheric Warming Variability by the Madden–Julian Oscillation Abstract: Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Previous work showed the Arctic stratosphere to be influenced by the Madden–Julian oscillation(MJO) and that the SSW frequency increases with an increase of the MJO amplitude, expected in a warmer climate. It is shown here that the zonal asymmetry in both the background state and forcing plays a dominant role, leading to either enhancement or suppression of SSW events by MJO-like forcing. When applying a circumglobal MJO-like forcing in a dry dynamic core model, the MJO-forced waves can change the general circulation in three ways that affect the total vertical Eliassen–Palm flux in the Arctic stratosphere. First,weakening the zonal asymmetry of the tropospheric midlatitude jet, and therefore preventing the MJO-forced waves from propagating past the jet. Second, weakening the jet amplitude, reducing the waves generated in the midlatitudes, especially stationary waves, and therefore the upward-propagating planetary waves. Third,reducing the Arctic lower-stratospheric refractory index, which prevents waves from upward propagation.These effects stabilize the Arctic vortex and lower the SSW frequency. The longitudinal range to which the MJO-like forcing is limited plays an important role as well, and the strongest SSW frequency increase is seen when the MJO is located where it is observed in current climate. The SSW suppression effects are active when the MJO-like forcing is placed at different longitudinal locations. This study suggests that future trends in both the MJO amplitude and its longitudinal extent are important for predicting the Arctic stratosphere response. The tropical Madden-Julian oscillation and the global wind oscillation Abstract: The global wind oscillation (GWO) is a subseasonal phenomenon encompassing the Madden-Julian Oscillation (MJO) and mid-latitude processes like meridional momentum transports and mountain torques. A phase space is defined for the GWO following the approach of Wheeler and Hendon (2004) for the MJO. In contrast to the oscillatory behavior of the MJO, two red noise processes define the GWO. The red noise spectra have variance at periods that bracket the 30-60 day band generally used to define the MJO. The MJO and GWO correlation accounts for 25% of their variance and cross-spectra show well-defined phase relations. However, considerable independent variance still exists in the GWO. During MJO and GWO episodes, key events in the circulation and tropical convection derived from composites can be used for monitoring and for evaluating prediction model forecasts, especially for weeks 1-3. A case study during April-May 2007 focuses on the GWO and two ~30 day duration orbits with extreme anomalies in GWO phase space. The MJO phase space projections for the same time were partially driven by mountain torques and meridional transports. The case reveals the tropical-extratropical character of subseasonal events and its role in creating slowly evolving planetary-scale circulation and tropical convection anomalies. Time-Lagged Response of the Antarctic and High-Latitude Atmosphere to Tropical MJO Convection Abstract: Intraseasonal tropical variability has important implications for themid- and high-latitude atmosphere, and inrecent studies has been shown to modulate a number of weather processes inthe Northern Hemisphere, such as snow depth, sea ice concentration, precipitation, atmospheric rivers, and air temperature. In such studies, the extratropical atmosphere has tended to respond to the tropical convection of the leading mode of intraseasonal variability, the Madden–Julian oscillation (MJO), with a time lag of approximately 7 days. However, the time lag between the MJO and the Antarctic atmosphere has been found to vary between less than 7 and greater than 20 days. This study builds on previous work by further examining the time-lagged response of SouthernHemisphere tropospheric circulation to tropical MJO forcing, with specific focus on the latitude belt associated with the Antarctic Oscillation, during the months of June (austral winter) and December (austral summer) using NCEP–DOE Reanalysis 2 data for the years 1979–2016. Principal findings indicate that the timelag with the strongest height anomalies depends on both the location of the MJO convection (e.g., the MJO phase) and the season, and thatthe lagged height anomalies in the Antarctic atmosphere are fairly consistent across different vertical levels and latitudinal bands. In addition, certain MJO phases in December displayed lagged height anomalies indicative of blocking-type atmospheric patterns, with an approximate wavenumber of 4, whereas in June most phases were as-sociated with more progressive height anomaly centers resembling a wavenumber-3-type pattern. Tropical–extratropical interactions (section from the 2011 publication Intraseasonal Variability in the Atmosphere–Ocean ClimateSystem) No abstract, but this from theclosing 'Discussion' chapter: The MJO is associated with significant signals that extend around the globe andthroughout the seasonal cycle. Organized convection in its active convective phaseresults in a planetary wave response that modulates teleconnection patterns aroundthe globe. Convection organized in the tropics associated with the MJO is alsostrongly modulated by extratropical waves, including those that previouslyoriginated within the MJO itself. Such associations suggest the possibility thatcoupling between the tropics and the extratropics might be fundamental to theevolution and structure of the MJO. The associated global signals modulateextreme weather events. Since numerical weather prediction models tend to poorlysimulate the MJO, MJO modulation of extreme events would reduce our ability topredict such events at long range by deterministic means until simulation of the MJOimproves. MJO Prediction Skill of the Subseasonal-to-Seasonal Prediction Models Published May 2018. Abstract: The Madden–Julian oscillation (MJO), the dominant mode of tropical intraseasonal variability, provides a major source of tropical and extratropical predictability on a subseasonal time scale. This study conducts a quantitative evaluation of the MJO prediction skill in state-of-the-art operational models, participating in the subseasonal-to-seasonal (S2S) prediction project. The relationship of MJO prediction skill with model biases in the mean moisture fields and in the longwave cloud–radiation feedbacks is also investigated. The S2S models exhibit MJO prediction skill out to a range of 12 to 36 days. The MJO prediction skills in the S2S models are affected by both the MJO amplitude and phase errors, with the latter becoming more important at longer forecast lead times. Consistent with previous studies, MJO events with stronger initial MJO amplitude are typically better predicted. It is found that the sensitivity to the initial MJO phase varies notably from model to model. In most models, a notable dry bias develops within a few days of forecast lead time in the deep tropics, especially across the Maritime Continent. The dry bias weakens the horizontal moisture gradient over the Indian Ocean and western Pacific, likely dampening the organization and propagation of the MJO. Most S2S models also underestimate the longwave cloud–radiation feedbacks in the tropics, which may affect the maintenance of the MJO convective envelope. The models with smaller bias in the mean horizontal moisture gradient and the longwave cloud–radiation feedbacks show higher MJO prediction skills, suggesting that improving those biases would enhance MJO prediction skill of the operational models.

Here are the current Papers & Articles under the research topic Brewer-Dobson Circulation. Click on the title of a paper you are interested in to go straight to the full paper. Papers and articles covering the basics (ideal for learning) are shown in Green. (About the) Brewer-Dobson Circulation The Brewer‐Dobson circulation What drives the Brewer-Dobson Circulation? The Brewer–Dobson Circulation: Dynamics of the Tropical Upwelling Brewer–Dobson Circulation: Recent-Past and Near-Future Trends Simulated by Chemistry-Climate Models