As promised I have piece together macroscale developments of sea-surface temperature and regional wind/pressure anomalies to provide a preliminary forecast for the coming winter.During October the global winds, pressure and temperature-patterns across the Northern Hemisphere gravitate towards their winter states, which they will tend to retain until late March.
First thing though we need to list what we know so far:
1) Sea surface temperatures are, in general well above normal across the North Atlantic with anomalies close to 4C for early October in the European Arctic section with anomalies of +6C off the eastern coast of the USA and in the Baltic. The section is part of the mid-North Atlantic about 45 to 55N and 20 to 40W where sea surface temperatures are up to 2C colder than usual. Such warmer than usual waters around the UK would directly warm any winds blowing over them more and would tend to support milder weather and more evaporation from the warmer seas would support increased rainfall. The cool patch in the North Atlantic is sufficiently far west for it to cause the southern part of the strong upper Westerlies to re-curve south over it and just to the east whilst the upper air would be encouraged to "re-curve" northwards having crossed the warmer waters around Britain: This would place an upper trough near to the UK and enhance wet, windy weather.
2) The North Pacific north of 20N is substantially warmer than normal with sea surface temperature anomalies generally 3 to 4C warmer than normal for early October. However the Equatorial central and eastern Pacific is colder than usual with anomalies up to 2C below normal. The development of La Nina with cool equatorial waters would promote weaker north-easterly Trade Winds over the Pacific between the Equator and a weaker subtropical high-pressure belt centred over warmer than usual waters of the North Pacific around 30 to 35N: Weaker NE Trade Winds impart less westerly atmospheric angular momentum (AAM) to the Northern Hemisphere's atmospheric circulation through frictional interaction with the sea-surface- particularly as less wind means a calmer sea-surface with very low coefficient of friction. There is correspondingly need for less of a sink for accumulated westerly momentum in higher latitudes which implies weaker westerlies reaching Britain with a correspondingly higher chance of cold-air outbreaks from Russia or the Arctic.
3) Arctic sea-ice extent has recovered remarkably during September and it's extent is close to the seasonal norm east of Greenland but the sea-ice extent remains some 500 km north of its normal October extent north of Alaska and the extreme east of Siberia. Open waters in the Arctic Ocean surrounding the sea-ice remains substantially (i.e. widely up to 4C warmer than normal for October however): This is likely to encourage the Circumpolar Vortex to be contracted as well as displaced towards the UK by up to 200 km, however the warmth of Arctic seas would encourage the strong baroclinic gradients to be shifted towards the Arctic. This lends support to deeper depressions encircling the Arctic close to 70N, particularly in the North Atlantic sector and the warmth of the oceans just to the south of them means rather more moisture latent-heat potential to fuel these storms. The northwards displacement of the Westerlies is likely to encourage them to be strong in any case because they have to blow harder closer to the axis of the Earth's rotation to offset the tropical, subtropical and polar easterlies as required by Conservation of Angular Momentum laws.
4) Also supportive of a mild wet and windy winter is the fact that the Quasi Biennial Oscillation (QBO) at 30mb high above the Equator remains in Westerly phase. During August these stratospheric Equatorial Winds averaged just over 10 metres per second (23 mph) from the west. These stratospheric winds feed down into the general circulation and reach the mid-latitude jet-streams and Westerlies over three or four months. This suggests (strongly) that the coming winter will be mild wet and stormy.
5) The Sun is now entering the quiet phase towards the end of Schwabe cycle 24: Indications are that the Sun is indeed going quieter than it has been for a few years. An active Sun produces Solar Flares which interact with the atmospheric circulation to increase the strength of the Circumpolar Vortex. Instead few (if any) magnetic storms from the Sun will be interacting with the Earth's atmosphere and instead (if anything) that just leaves tidal friction due to the Sun and Moon which affects the atmosphere as well as the oceans. The tidal effects on the atmosphere are very weak but these act to reduce the Earth's rotation by very mall amounts (these are significant over time, which is why Leap Seconds are added at the end of each year). The net effect of all this (weak phase of Solar Cycle, atmospheric tidal friction) would be to weaken the Westerlies a little.
6) At least until mid November, the fact that sea-surface temperatures in the tropical Atlantic and Pacific just north of the Equator is likely to enhance tropical storm activity. More hurricanes and typhoons with strong easterlies on their northern flanks that enter the Northern Hemisphere circulation add Westerly AAM to the global atmospheric circulation. This increases the need for stronger Westerlies in higher latitudes to counter-balance them: This strongly hints to late autumn/early winter being wet, mild and stormy. However, from late January onwards the Intertropical Convergence Zone (ITCZ) will be south of the Equator and the fact that sea-surface temperatures in tropical waters just south of the Equator are also warmer than normal now suggests more tropical storms will occur there; Southern Hemisphere tropical depressions (sliding westwards along the ITCZ) have strong westerlies on their northern flank and it is these that will affect the Angular Momentum Budget of the Northern Hemisphere circulation by removing Westerly AAM through frictional impact with the underlying surface: This points to weaker Westerlies coming across the North Atlantic in January/February which would, other things being equal, increase the chances of much colder, drier spells reaching Britain from the east.
We can now put all this together to get some sort of prediction for Winter 2016/17:
I am offering my provisional outlook for Spring 2016 for the United Kingdom. In some ways the spring and summer are harder to forecast from a month or more out than winter because the Circumpolar Vortex weakens in spring; this makes it prone to sudden flips and higher latitude blocking-patterns which can cause remarkable changes in prevailing weather patterns in the space of a few days. That said, there are enough of the pan-regional temperature and circulation anomalies to gain some insight into the processes likely to affect the weather in coming months.
Firstly El Nino: This is still in a positive phase with very warm water stretching to coastal Peru and Ecuador. This means the ITCZ remains strong and that will maintain strength in the NE Trade Winds in the tropical North Pacific. This implies a need for stronger Westerlies at higher latitudes to satisfy conservation of angular momentum laws.
Secondly there is now a sizeable pool of really cold water in the mid NW Atlantic (this water gets really cold by February anyway but it is colder than normal by up to 3C) but the NE Atlantic is warmer than the seasonal norm (but not by a lot). Arctic Ice extent is less than normal in the Eurasian Arctic but extending a bit further south than normal around Newfoundland and of course Greenland and NE Canada are very cold indeed at present.
Third, the QBO high over the Equator remains in Westerly Mode and we are still in active phase of the current Sunspot Cycle (Schwabe Cycle number 24, I do believe). We can now put the pieces together to arrive at a prediction:
Given the recent references in the model discussion thread regarding the effect on the weather in the UK of the vortex sitting over Greenland, which I imagine is related to El Nino via. its effects on the vortex, I find myself wondering whether El Nino can explain the seemingly unmovable Euro high. Is there any research that supports my speculation?
A record-breaking day in the Eastern and Central Pacific: 3 consecutive major hurricanes showing up at onceBy Vorticity0123
Well, what a record-breaking day it has been in the Eastern and Central Pacific it has been today! As of speaking, three (3!) major hurricanes are roaming the Pacific waters at the same time, which is unpreceded in these areas. It is likely that this activity has been aided by the ongoing El Nino event, which has caused anomalously warm waters in the Central and Eastern Pacific. In this post I will provide some details about the cyclones individually, as well as a short look into the causes of this record-breaking activity.
A sight to behold
Below is an impressive satellite image showing all three systems in daylight:
Satellite image of (from left to right) Kilo, Ignacio and Jimena. Courtesy: NOAA.
All three systems show up clearly as well-organized hurricanes with an eye visible surrounded by intense convection.
Kilo: a very stubborn cyclone
The leftmost one, and probably also the one with the most interesting history, is hurricane Kilo. Just 24 hours ago, the system was still a 60-knot tropical storm, and now it has almost doubled its intensity up to 110 kt. Exactly what one could call rapid intensification. This make the system a category 3 hurricane on the Saffir Simpson Hurricane Scale.
However, the most remarkable thing is that this system has been more noteworthy for its lack of intensification so far. During the past several days, Kilo was continuously forecast to become a hurricane, which it refuse to do, up to now. It stubbornly stayed as a tropical depression in seemingly favourable environment.
Furthermore, its track has also defied forecasts quite a few times (also partly because it stayed so weak), as also alluded to by Somerset Squall in the thread about this cyclone. Originally, Kilo was forecast to strike Hawaii as a hurricane a week ago as well. Fortunately, this was not the case. Here's a link to the appropriate thread.
Ignacio to possibly threaten Hawaii
The center one is hurricane Ignacio. This cyclone developed in the Eastern Pacific and crossed 140 degrees longitude into the Central Pacific. Initially refusing to intensify quickly as a category 1 hurricane, Ignacio also put up a burst of rapid intensification. As of writing, the cyclone is now a category 4 hurricane with 120 knot winds.
What is noteworthy about Ignacio is that it may be a threat for Hawaii in a few days, as it moves closer to the islands from the southwest. Currently, the CPHC forecasts the cyclone to pass safely to the north of the islands, only causing high surf among the islands.
Another unusual thing about the forecast of Ignacio is that it is anticipated to stay a hurricane while passing north of the islands. Cyclones like Ignacio seldom retain hurricane intensity while passing to the north of the Hawaiian islands from the east.
More to be found here:
Jimena to undergo eyewall replacement cycle
Finally, the easternmost cyclone that can be seen here is Jimena. As of speaking, Jimena has already past her peak, and is now a 120 knot system, making it a category 4 hurricane. Her intial peak was reached 6 hours ago at 130 knots. Talking about rapid intensification, Jimena managed to get from 25 kt to 130 kt in merely 3 days!
The NHC has noted that Jimena has developed concentric eyewalls, which means it is likely to embark onto an eyewall replacement cycle. In such a cycle, the inner eyewall weakens and dissipates, while a new, larger outer eyewall becomes better defined. In this process, the eye becomes much larger and the system itself usually weakens a bit. After completing such a cycle, a new round of intesification can begin assuming that favourable environmental conditions prevail. This could also be the case for Jimena. So far, the system is not forecast to hit land.
Here is her own topic: https://forum.netweather.tv/topic/83870-major-hurricane-jimena/
Anomalously warm waters due to El Nino
One of the major causes of this unique event appears to be related to the El Nino that is currently active. Below is a map of the SST (sea surface temperature) anomalies of the 27th of August:
SST anomalies as of 27 August. Courtesy: NOAA.
For clarity, the black box roughly indicates the area in which the tropical cyclones are residing. Note that this area does not explicitly overlap with the most significant warm waters near the Equator associated with the El Nino event. Still, sea surface temperatures inthe encircled area are much warmer than average, contributing in the increased tropical activity.
An impressive event to say the least, three consecutive major hurricanes active in the Eastern and Central Pacific. Possibly we will even be facing one or two category five hurricanes in the very near future. Much more can be said about these systems, so do not hesitate to add any facts/statistics/any other things you might think of .
Finally, just because of the amazing sight, below is a loop of the three tropical cyclones at major hurricane intensity. Click to activate.
Satellite loop of the Eastern and Central Pacific. Click to activate. Courtesy: NOAA.
With winter soon approaching it is time for a new thread. This is the sixth winter that the strat thread will be running!
As ever, the first post will become both a reference thread and basic learning thread for those wanting to understand how the stratosphere may affect the winter tropospheric pattern. And then I will have a look at how we may expect the stratosphere to behave this year.
The stratosphere is the layer of the atmosphere situated between 10km and 50km above the earth. It is situated directly above the troposphere, the first layer of the atmosphere that is directly responsible for the weather that we receive. The boundary between the stratosphere and the troposphere is known as the tropopause. The air pressure ranges from around 100hPa at the lower levels of the stratosphere to around 1hPa at the upper levels. The middle stratosphere is often considered to be around the 10-30hPa level.
Every winter the stratosphere cools down dramatically as less solar UV radiation is absorbed by the ozone content in the stratosphere. The difference in the temperature between the North Pole and the latitudes further south creates a strong vortex â€“ the wintertime stratospheric polar vortex. The colder the stratosphere, the stronger this vortex becomes. The stratospheric vortex has a strong relationship with the tropospheric vortex below. The stronger the stratospheric vortex, the stronger the tropospheric vortex will be.
The strength and position of the tropospheric vortex influences the type of weather that we are likely to experience. A strong polar vortex is more likely to herald a positive AO with the resultant jet stream track bringing warmer wet southwesterly winds. A weaker polar vortex can contribute to a negative AO with the resultant mild wet weather tracking further south and a more blocked pattern the result. A negative AO will lead to a greater chance of colder air spreading to latitudes further south such as the UK. So cold lovers will look out for a warmer than average polar stratosphere.
The stratosphere is a far more stable environment then the troposphere below it. However, there are certain influences that can bring about changes - the stratospheric ozone content, the phase of the solar cycle, the Quasi Biennial Oscillation ( the QBO), wave breaking events from the troposphere and the autumnal Eurasion/Siberian snow cover to name but a few.
The ozone content in the polar stratosphere has been shown to be destroyed by CFC's permeating to the stratosphere from the troposphere but there can be other influences as well. Ozone is important because it absorbs UV radiation which creates warming of the stratosphere. The Ozone is formed in the tropical stratosphere and transported to the polar stratosphere by a system known as the Brewer-Dobson â€“Circulation (the BDC). The strength of this circulation varies from year to year and can in turn be dictated by other influences.
One of these influences is the QBO. This is a tropical stratospheric wind that descends in an easterly then westerly direction over a period of around 28 months. This can have a direct influence on the strength of the polar vortex in itself. The easterly (negative ) phase is though to contribute to a weakening of the stratospheric polar vortex, whilst a westerly (positive) phase is thought to increase the strength of the stratospheric vortex. However, in reality the exact timing and positioning of the QBO is not precise and the timing of the descending wave is critical throughout the winter.
The direction of the QBO when combined with the level of solar flux has been shown to influence the BDC. When the QBO is in a west phase during solar maximum there are more warming events (increased strength BDC) in the stratosphere as there is also during an easterly phase QBO during solar minimum.( http://strat-www.met...-et-al-2006.pdf) (http://onlinelibrary....50424/abstract)
The QBO is measured at 30 hPa and has entered an easterly phase for this winter. As mentioned warming events are more likely during solar minimum â€“ solar flux below 110 units. Currently, we have just experienced a weak solar maximum and the solar flux heading into winter is slightly above 110 units. This doesnâ€™t rule out warming events, but they will not be as likely unless the solar flux continues to drop prior to winter.
One warming event that can occur in the stratospheric winter is a Sudden Stratospheric Warming (SSW) or also known as a Major Midwinter Warming (MMW). This as the name suggests is a rather dramatic event. Normally the polar night jet at the boundary of the polar vortex demarcates the boundary between warmer tropical and cooler polar stratospheric air (and ozone levels) and is very difficult to penetrate. SSWs can be caused by large-scale planetary waves being deflected up into the stratosphere and towards the North Pole, often after a strong mountain torque event. These waves can introduce warmer temperatures into the polar stratosphere and can seriously disrupt the stratospheric vortex, leading to a slowing or even reversal of the vortex. This year if the solar flux drops below 110 units then the chances of a SSW increase - as can be seen by the following chart.
Any SSW will be triggered by the preceding tropospheric pattern - in fact the preceding troposheric pattern is important in disturbing the stratospheric vortex even without creating a SSW. Consider a tropospheric pattern where the flow is very zonal - rather like the positive AO phase in the diagram above. There has to be a mechanism to achieve a more negative AO or meridional pattern from this scenario and there is but it is not straightforward. It just doesn't occur without some type of driving mechanism. Yes, we need to look at the stratosphere - but if the stratosphere is already cold and a strong polar vortex established, then we need to look back into the troposphere. In some years the stratosphere will be more receptive to tropospheric interactions than others (such as the eQBO this year) but we will still need a kickstart from the troposphere to feedback into the stratosphere. This kickstart will often come from the tropics in the form of pulses of convection interacting with long wave undulations in the polar vortex which influence the positions of the sub tropical jet stream and polar jet streams respectively. The exact positioning of the large scale undulations (or Rossby waves) will be influenced by (amongst other things) the pulses of tropical convection (aka the phase of the MJO) and that is why we monitor that so closely. These waves will interact with land masses and mountain ranges which can absorb or deflect the Rossby waves disrupting the wave pattern further - and this interaction and feedback between the tropical and polar systems is the basis of how the Global Wind Oscillation influences the global patterns. The ENSO state will influence the GWO base state
If the deflection of the Rossby Wave is great enough then the wave can be deflected into the stratosphere. This occurs a number of times during a typical winter and is more pronounced in the Northern Hemisphere due to the greater land mass area. Most wave deflections into the stratosphere do change the stratospheric vortex flow pattern - this will be greater if the stratosphere is more receptive to these wave breaks (and if they are substantial enough, then a SSW can occur). The change in the stratospheric flow pattern can then start to feedback into the troposphere - changing the zonal flow pattern into something with more undulations and perhaps ultimately to a very meridional flow pattern especially if a SSW occurs - but not always. If the wave breaking occurs in one place then we see a wave 1 type displacement of the stratospheric vortex, and if the wave breaking occurs in two place then we will see a wave 2 type disturbance of the vortex which could ultimately squeeze the vortex on half and split it â€“ a split vortex SSW. The SSW is defined by a reversal of mean zonal winds from westerly to easterly at 60ÂºN and 10hPa. This definition is under review as there have been suggestions that other warmings of the stratosphere that cause severe disruption to the vortex could and should be included. http://birner.atmos.colostate.edu/papers/Butleretal_BAMS2014_submit.pdf
The effects of a SSW can be transmitted into the troposphere as the propagation of the SSW occurs and this can have a number of consequences. There is a higher incidence of northern blocking after SSWâ€™s but we are all aware that not every SSW leads to northern blocking. Any northern blocking can lead to cold air from the tropospheric Arctic flooding south and colder conditions to latitudes further south can ensue. There is often thought to be a time lag between a SSW and northern blocking from any downward propagation of negative mean zonal winds from the stratosphere. This has been quoted as up to 6 weeks though it can be a lot quicker if the polar vortex is ripped in two following a split SSW.
One noticeable aspect of the recent previous winters is how the stratosphere has been susceptible to wave breaking from the troposphere through the lower reaches of the polar stratosphere - not over the top as seen in the SSWs. This has led to periods of sustained tropospheric high latitude blocking and repeated lower disruption of the stratospheric polar vortex. This has coincided with a warmer stratosphere where the mean zonal winds have been reduced and has led to some of the most potent winter spells witnessed in recent years.
We have also seen in recent years following Cohen's work the importance of the rate of Eurasian snow gain and coverage during October at latitudes below 60ÂºN. If this is above average then there is enhanced feedback from the troposphere into the stratosphere through the Rossby wave breaking pattern described above and diagrammatically below.
And it appears that the reduction in Arctic sea ice may be contributing to this mechanism and this should be factored in to any forecast. http://web.mit.edu/jlcohen/www/papers/Cohenetal_NGeo14.pdf
So that leaves us to the try and forecast what will happen in the stratosphere this year. Out of the many variables what we do know at the moment is that the QBO is descending easterly and that we are probably entering El Nino conditions â€“ although weak presently. And as mentioned earlier, the level of solar flux is slightly above conditions that are favourable for SSWâ€™s. Despite this, conditions are favourable enough to suggest that we will see a warmer than average stratosphere this year. Evidence of this may already be suggested by an enhanced BDC in the Southern Hemisphere leading to a possible early final warming.
If we look at 500hPa analogue composites for comparable easterly QBO/ El Nino 'lite' years (holding off on solar flux analogues just yet) then we see that the suggestions are that the polar vortex will have positive anomalies in December and January, before the vortex gains strength later in the winter. I would put the likelihood of an SSW at around 80% with the peak time for this to occur around early January. http://www.columbia.edu/~lmp/paps/butler+polvani+deser-ERL-2014.pdf
Itâ€™s a little too early to suggest how exactly this will effect the troposphere until we see other data - including the updated solar flux and ENSO as well as the SAI, SCE values. But all in all, if the stratosphere behaves as we expect at this point, then tropospheric northern blocking would be favoured during the winter leading to a negative AO index and mid latitude polar episodes being experienced.
But after last year, when the stratosphere cooled dramatically, it is best that we remain cautious and wait to see how cold the stratosphere becomes over the next 6 weeks prior to winter, and how this may subsequently affect the strength of the polar vortex.
As ever the best sites to monitor the stratosphere and forecasts are listed below:
ECM/Berlin Site: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html
Instant weather maps: http://www.instantweathermaps.com/GFS-php/strat.php
Analysis can be found here: http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.html
Previous NW stratosphere monitoring threads:
Here's hoping for another exciting and intriguing season.
PS I look forward to all the contributers on this thread. It has grown from strength to strength over the years which has helped increase our knowledge of this fascinating and important subject - and there have been a core of extremely knowledgeable contributers from both national and international quarters and I thank them all and ask them to keep the discussion coming!
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