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SPRING/ EARLY SUMMER 2018 WEATHER PREDICTION Apologies that this is a bit late, I have been busy during the worst blizzards to hit northern England in the last 39 years: However, I now have my seasonal prediction for Spring, covering late March, April and May 2018. Given this is so late I will add a prediction for June! At the time of writing the Circumpolar Vortex and Stratospheric Westerlies around the Arctic are much weaker than normal and are predicted to remain so for the next three weeks. Almost all (20 of 21 modellers) predict the Stratospheric Westerlies to remain weaker than usual for the next three weeks (https://www.weatheriscool.com). The predictions on some days for the Stratospheric Westerlies at 60N, both at the 10 mb and 30 mb level, show a negative speed, in other words mean easterly winds. This has implications for the weather for up to a month beyond the end of the forecast period for weather-patterns in the lower atmosphere. In other words, we are looking at a situation whereby there is likely to be frequent high-pressure over northern Scandinavia and over Greenland through the remainder of March and for much of April. The pattern of sea-surface temperatures is also interesting. After a cold February and early march sea-surface temperatures around the UK are about 1C below normal for the time of year. There are colder than usual waters in the southern Norwegian Sea and in the North Atlantic around 50 to 60N and to the west and NW of the UK. Meanwhile warmer-than-usual waters (with anomalies over 3˚C above normal) are found off the east coast of the USA. Significantly warmer than normal waters (anomaly up to 2˚C), are also found in the Bering Strait and Barents Sea. The Mediterranean Sea overall was about 1˚C warmer than normal for mid-March. Arctic sea-ice has been at record lows for early March in the Bering Strait around Alaska and in the Barents Sea and around Spitzbergen, but sea-ice extent has been close to the mid-March norm off eastern Canada, around Greenland and just north of most of northern Russia. There was more sea-ice than usual for mid-March in the Baltic Sea between Sweden, Finland and the Baltic States. The Quasi Biennial Oscillation is strongly easterly at the 30 mb level over the Equator and it has also turned easterly at the 50 mb level. The Sun is now very quiet, with few solar flares and sun-spots predicted over the next thirty days. Both these factors signify weaker Westerlies in higher latitudes than normal. In the eastern equatorial Pacific sea-surface temperatures are locally more than 1˚C below the long-term norm so we have La Niña conditions and sea-surface temperatures over all equatorial ocean surfaces are (on average) fractionally below the seasonal normal. The impact of the Madden Julian Oscillation, a convective large-scale weather-pattern that encircles the globe is diminished by La Niña. The Madden Julian Oscillation leads to large-scale convective waves that penetrate the Stratosphere and which can often disrupt the Stratospheric Circumpolar Vortex in high latitudes, La Niña often (but not always) prevents this happening. However, by May the Stratospheric Westerlies at high latitudes tend to break down and reverse due to the 24 hour solar- heating effect on the Polar Stratosphere so the workings of the Madden Julian Oscillation and the effects of La Nina become much less important. For earlier in the Spring, the starting point is very weak Westerlies both aloft in the Stratosphere and in the troposphere at higher northern latitudes. In late Spring the Circumpolar Vortex weakens anyway, though factors such as a quiet Sun and easterly QBO high over the Equator would, if anything, point to even weaker Westerlies aloft but the impact is not as great as during winter and early Spring. The patterns of sea-surface temperature, sea-ice cover and less thick ice than normal over the Central Arctic Ocean (https://www.nsidc.com/arctic-sea-ice) would suggest weaker baroclinicity in the troposphere around the Arctic but with the Vortex shifted a hundred miles or so in the direction of Europe. This will be important later in the season when other global influences, such as La Niña or the easterly QBO have less of an importance. Most of the equatorial waters being slightly cooler than normal, would hint at a weaker global circulation (though the impact on the winter hemisphere would be greater), with a predisposition towards blocking in high latitudes. We can now use the above information to make a forecast: Starting with the second half of March the above analysis indicates further periods with strong blocking patterns over Scandinavia leading to further very cold easterly or north-easterly winds with the air originating over north-west Russia. These will bring snow-showers to the North East, Yorkshire, East Anglia and the South East of England whilst frontal influences bring some heavier rain or snow to the South-West. Eastern Scotland will also get snow-showers, however amounts will be nothing like we saw at the start of March. Western Scotland, NorthWest England and the English Midlands along with much of Wales will be drier and brighter during the cold easterly/north-east spells but still cold. Daytime maxima will range from near 7˚C in the south of England to 2 to 3˚C in North East England and Scotland though upland areas in Scotland will remain below freezing-point by day during these cold spells. Frost will be widespread at night during these cold north-easterly spells as skies clear inland, minima will be locally -5˚C or colder from the Midlands northward. A brief milder spell with south-westerly winds will occur around the 25th March. This will bring rain (and mountain snow) to Scotland and NorthWest England along with parts of upland Yorkshire and Northumberland and North Wales. Coastal and upland gales are possible in all these regions. Northern Ireland can expect similar weather. Daytime maxima will be near 10C in the lowlands in these more northerly areas. For the Midlands, the North East lowlands, lowland Yorkshire along with eastern and southern England and South Wales brighter warmer weather is likely with less rain, temperatures of 14 to 15˚C can be expected quite widely and nights will be frost-free for a time. The very end of March will see a return to icy north-easterly winds with snowfalls in the North East, Yorkshire and eastern Scotland, drier brighter conditions elsewhere and the return of air-frosts at night. March looks set to be the coldest for five years with mean daily temperatures around 3.5 to 4˚C over much of England, averaging over 2˚C colder than average. The departures from normal look set to be about 2˚C below in eastern Scotland; but nearer 1˚C below in western Scotland. Rainfall looks set to be above normal in East and North East England, South West England and in Eastern Scotland but a little below normal in the West Midlands, North West England and western Scotland. As we head into April we can expect the alternation between icy north-easterlies and milder showery westerlies to continue. About half of early and mid-April will be dominated by westerlies with depressions taking a track just to the north of Scotland and moving east into the North Sea. A good deal of cold wet showery weather will affect the northern half of the British Isles, with showers in Scotland and northern England likely to be accompanied by sleet or even hail at low levels but with snow in the mountains above about 600 metres. Strong westerly winds will affect coastal areas of the NorthWest, North Wales and Scotland with gales possible in some places. Daytime temperatures will be near 10˚C in the northern and Scottish lowlands, possibly a bit more where the Sun comes out. There will still be the possibility of frost at night as the generally cool showery Maritime Polar airstream will lend itself to frequent clear skies inland with winds falling light. The Midlands, South Wales and the South and East of England won’t escape showers during the showery cool Westerlies of early-mid April, but they will be lighter than further north and there will be more in the way of sunshine. Daytime temperatures of near 14˚C can be expected to occur quite widely, so it will feel like Spring at such times. Clear skies at night will lead to temperatures falling close to freezing point and sharp ground-frost will occur. During the first three weeks of April, quite possibly as an extension of the cold north-easterlies expected to set in at the end of March, there will be a spell of five or more days when strong high-pressure over northern Scandinavia and/or near Iceland will lead to much colder drier east or north-easterly winds affecting the country. This will bring what is widely known as a “Blackthorn Winter” as it coincides with the time when blackthorn trees normally blossom, at least in the lowlands of the Midlands and South of England! The North Sea will be near its coldest by this point and with the air bring slightly less frigid coming across from northern Russia convective snowfalls near the East Coasts of Scotland, North East England, Yorkshire and East Anglia will be less (and less intense). Alas, there will still be snow-showers in most of these locations but except on higher ground in the North and Scotland will be unlikely to lead to significant snow-cover. Coastal Kent and around London is likely to see any showers fall as rain or sleet as the north-easterly winds will just not quite be cold enough to bring snow. On the western side of Scotland, to the west of the Pennines in North West England and across the Midlands, Wales and southern England the cold north-easterly winds are likely to lead to dry, bright conditions although the north-easterly wind will still feel cold. Again, the South West of England is likely to be affected by frontal systems moving into the Bay of Biscay whilst cold north-easterlies affect the rest of Britain which will lead to some rain and sleet locally, though snow will still fall on Dartmoor and Exmoor. It will be cold nationwide with maximum temperatures below 8˚C even in the South during the April spell with north-easterly winds. Daytime maxima in the lowlands of North East England and in Scotland will be near 5˚C and will remain below 0˚C in the Scottish mountains, where snow is liable to accumulate where it falls. Night skies will be clear during the spell of icy north-easterlies, except along the East Coast and in the far south-west so nighttime temperatures on the coldest nights will drop well below freezing point, particularly as the wind will fall light inland. Again minima below -5˚C will occur locally from the Midlands northwards, so gardeners beware! The cold snap will not last beyond a week and will be superseded by a return to milder showery west or south-west winds. During the last ten days of April there is confidence in high-pressure developing over the UK for a time, aided by the still cold seas around the country and the weakening of the Circumpolar Vortex as this retreats northwards. This high-pressure is likely to be centred over and to the west of the country extending as a ridge from the subtropical-high over the Azores. The vast bulk of England and Wales will enjoy fine sunny conditions for a few days; with temperatures reaching a very warm 20˚C or above inland, although coastal areas will be considerably cooler. Clear skies at night with light winds will still allow temperatures to plummet and frost is likely inland, even in the South. Scotland and the far North East of England is liable to miss out on this fine spell to some extent, chilly north-westerly winds will bring more cloud and a touch of rain to coastal areas. Temperatures in lowland Scotland will remain below 14˚C during the fine spell further south. Clear skies inland at night will still see temperatures fall below freezing-point locally. The end of April or the beginning of May will see a return to showery west or north-westerly winds across the whole country as a four-wave Circumpolar Vortex gets properly established. These winds will be lighter as the depressions responsible for them are likely to be weak and slow-moving. The air will be cooler than normal for the time of year, thanks in part to below normal sea-surface temperatures upwind so daytime maxima will be no more than 15˚C, even in the Midlands and South whilst maxima of 12˚C will be normal for Scotland and the North where hail and sleet is still likely to accompany showers. Again, the Polar Maritime airstream responsible will mean clear skies inland on most nights; this means widespread ground-frost and localised air-frost from the Midlands northwards. Average temperatures during April 2018 will be near-normal in the South of England but colder than normal elsewhere, with the departure from the seasonal normal over 1˚C below the April normal over a wide area. Mean daily temperatures will range from 9˚C along the South Coast to 7C in the lowlands of the North West and a chilly 5˚C in the north of Scotland. Rainfall totals will range from a little below normal for April in the Midlands and the South of England and in South Wales, to around the seasonal norm in South West and North West England and North Wales but a little wetter than normal for much of Scotland, North East England and Yorkshire. (CONTINUED BELOW)
This coming winter (2017/18) there are global (macroscale) influences that suggest that the UK will have some colder spells of weather. This does not mean a cold winter by any means, just that there is a stronger signal for cold spells (related to blocking over northern Europe), likely to occur during January-February 2018. There are six significant global-scale weather-patterns that will have an influence on the weather in the coming months: 1) The Quasi Biennial Oscillation (QBO) has turned Easterly: This is a wind-pattern high up over the Equator, affecting the Equatorial Stratosphere that gravitates from westerly winds to easterlies, then back to westerlies over a (approximately) thirty-month period. After an unusually long period of westerlies winds have become strongly easterly at the 30 mb level (about 38 mph as averaged through October). At the 50 mb level winds are still Westerly but are now very light (just 3 mph as averaged through October) and these winds are likely to become easterly this month (November 2017). These strong easterlies high up in the Equatorial Stratosphere are a source of negative Westerly Atmospheric Angular Momentum –or AAM for short (that is, winds blowing from east to west, rather than west to east) and its impact- once the easterlies of the QBO descend and impact the general global circulation- will be to resist (and weaken) the upper westerlies that move polewards aloft over the tropics and subtropics- and then go on to weaken the Westerlies aloft and near the surface in higher latitudes. Weaker Westerlies coming into north-western Europe mean less storms and a higher likelihood of high-pressure being able to form in high latitudes sending icy north or easterly winds towards Britain. However, one must not over-estimate the likely impact of the QBO on weather in Britain, the air at the 30 mb level is just 0.03 times the density of air at sea-level, so the 38 mph easterly would only slow down surface westerlies by just over 1 mph. Also, the easterly QBO air will mix (eventually) with the vast bulk of the atmosphere globally- further reducing its impact. But the impact is greater when the slowed-down upper westerlies reach higher latitudes (because of the reduced distance to the axis of the Earth’s rotation); hence there is likely to be a discernable impact on Britain’s weather. It takes two to three months for the easterly QBO winds at 30 mb to descend, enter the global circulation and then reach higher latitudes in its impact: Thus the signal from the strong easterly winds high up over the Equator will affect Britain as weaker intermittent westerly winds (with icy winds spilling out from northern Europe at times) during late January/ February. Other influences on the winter weather-patterns affecting the UK may well have a greater impact, as mentioned the QBO plays out in the rarefied atmosphere (at 10 to 50 mb level) high over the Equator and the proportion of the total global circulation directly involved with the QBO is small. 2) El Niño Southern Oscillation (ENSO) has entered a weak La Niña Phase: La Niña watch (12th October 2017) predicts a weak La Niña during winter 2017/18, with below normal sea-surface temperatures for the east and central Equatorial Pacific Ocean with sea surface temperatures a little above normal in the far western tropical Pacific Ocean. The impact of cooler than normal Equatorial waters would weaken the low-pressure zone of hot rising air near the Equator, thus weakening the north-easterly trade winds blowing into it from the Northern Hemisphere: Consequently the Hadley Cell weakens and westerlies aloft further from the tropics (including the subtropical jetstream and- further north- the Arctic Circumpolar Vortex are weakened in turn). Again, this suggests weaker westerlies reaching northwest Europe, increasing the chance of cold frosty spells associated with blocking highs over Scandinavia. By way of contrast, the strong El Niño of 2015/16 contributed towards winter 2015/16 being exceptionally wet, mild and stormy in the UK. However, one must not over-estimate the likely impact. A weak La Niña means conditions are not so far removed from normal. Furthermore, La Niña implies stronger easterly Trade Winds over the central and eastern Pacific Ocean (to push cool waters westwards into the central Pacific Ocean) along with stronger easterlies aloft coming across the northern (Equatorial) Andes of Ecuador and Colombia: These would result in more Westerly AAM being added to the global atmospheric circulation- resulting in a tendency towards milder stormier winter weather along western continental margins in higher latitudes; the pattern of cooler than normal Equatorial waters off South America and steamy waters in the far west (near Indonesia) would also tend to encourage stronger easterly winds across the Equatorial Pacific Ocean. However, it is mostly the south-east Trade winds blowing off northern South America that impacts the ENSO cycle; if these are stronger and causing La Niña the excess Westerly AAM is liable to affect the Southern Hemisphere atmospheric circulation rather than the Northern Hemisphere, leaving the cooler waters to weaken the Hadley Circulation and the north-east Trades that deliver Westerly AAM to the Northern Hemisphere atmospheric circulation. That being so, it means La Niña, even a weak one, would lead ultimately to a small weakening of winter westerlies reaching north-west Europe. The available literature on La Niñas suggests a tendency towards cooler drier conditions for north-west Europe in the winter months (see here “La Nina may chill Britain in run-up to Christmas” (http://www.weather.com) and here El Nino and La Nina- The Weather Outlook (https//www.theweatheroutlook.com/two)): This backs up the prognosis that I have just made. That said, the La Niña this coming winter is expected to remain weak, impacts on the UK will be small. 3) The Madden Julian Oscillation (MJO) is weak: The Madden Julian Oscillation (MJO) is a large-scale tropical pressure and wind-pattern that affects the tropical oceans; it propagates eastwards at the rate of 10 to 15 mph per day and circulates right around the world in a period of two months on average. It is, of course, modified by the ENSO Phase, the relatively cool tropical Pacific waters associated with La Niña (as is the case this year) tends to weaken the MJO Cycle- and this is indeed predicted to be the case over the next couple of months. The MJO is hemispheric in its phases- when one hemisphere of the deep tropics has increased convection and rising air the other hemisphere of the deep tropics has weak convection and even subsidence that would suppress rainfall. For the next month the MJO is predicted to be weak, but may strengthen a little as the large active (convective) zone moves (or reforms should it be totally swamped by La Niña) over the tropical Atlantic in late January/February (the position of the convective area dictates the Phase of the MJO). Phases 7 and 8 are when the convective zone is over the tropical Atlantic Ocean and if the MJO is fairly active in these phases it increases the likelihood of blocking patterns over Northern Europe. Atmospheric waves associated with an active MJO in any phases can penetrate upwards into the Stratosphere- and this can lead ultimately to possible stratospheric subsidence and associated Sudden Stratospheric Warmings over the Arctic: That in turn encourages a sharp weakening (and expansion) of the Circumpolar Vortex and much colder winter conditions affecting north-west Europe (source: http://www.cpc.ncep.noaa.gov>mjoupdate). The phase and intensity of the Madden Julian Oscillation may (slightly- in view of how weak it will be) increase the likelihood of some very cold weather from the east or north-east affecting the UK in late January and February- but it is likely to have almost no overall impact on the first half of the season. On the whole, the MJO Phase may have more of an impact than either the weak La Niña or the easterly QBO from mid-January onwards, but certainly not before then. 4) The current Sunspot Cycle is drawing towards an end as the Sun goes Quiet: The Sun is entering a quiet phase with few sunspots and solar flares: In the process the energy output from the Sun is also declining slightly compared to recent years. It is known that high levels of Sunspot activity combined with solar flares and coronal mass ejections (CMEs) result in high speed plasma crashing into the Sun-facing side as this hurtles westwards at 66,660 miles per hour as the Earth orbits the Sun (65,600 mph taking into account the Earth’s rotation from west to east). Thus these solar winds crash into the atmosphere from rather westwards of vertical and impart Westerly AAM to the global circulation- leading to a spike of deep depressions associated with stronger Westerlies in higher latitudes, especially in winter (“Clear link between solar activity and winter weather revealed”, October 2011, https://phys.org>Earth>Earth Sciences). The Sun is, of course, entering a quiet phase and, notwithstanding the large Solar Flare that affected Earth in September, expect little solar activity overall this coming winter: Less Solar Flares and CME’s mean less of the forces that would increase Westerly AAM and (with it) strong Westerlies in higher latitudes. This is yet another factor that favours a slightly increased chance of colder drier weather from the east this winter. As the Sun enters it’s quiet phase nearing the end of the current Sunspot Cycle, total solar output has been declining slightly: The Solar Constant was 1362 Watts per square metre on average during 2014, now it averages under 1361 Watts per square metre with dips to 1360 Watts per square metre (Source: Solar Irradiance and Sunspot Numbers, http://www.climate4you.com>Sun). This is a drop in total solar irradiance of 0.1% or more, one that would cause a global temperature drop of 0.1˚C (other things being equal), but nearer 0.2˚C (and greater in high latitudes) once positive feedbacks are taken into account. The additional greenhouse effect arising from the increased CO2 levels over the last three years (about 10 ppm) does not even half counter such a sharp drop in solar output. Slightly reduced solar output in itself would lead to a weakening of the Hadley Circulation in the tropics and subtropics, with slightly weaker north-easterly Trade Winds at the surface (and weaker westerlies aloft). This weakening of the Hadley Circulation would result from the 0.1% drop in solar irradiance, whether the drop in solar output led to an overall global cooling in the face of rising CO2 levels- or not. The weakening of the Hadley Circulation may be slight, but it would also lead to a weakening of the higher-latitude westerlies. Again, that means there is a small increase in the likelihood of severe cold reaching the UK from Russia this winter. The impact of the Quiet Sun with a 0.1% reduced Solar Constant has perhaps the most potent impact on the coming winter of the global climatic drivers so far discussed; yet for reasons to be discussed below this does not by any means guarantee a season like 2009/10 or 2010/11. 5) Arctic Sea-Ice remains below normal but not by much, whilst the North Atlantic Ocean and North Pacific Ocean are 1 to 2˚C warmer than the seasonal norm in early November Arctic Sea-Ice extent was overall below the normal for early November by about 1.4 million square kilometres and the southernmost extent of the pack-ice is some 250 km north of its seasonal position to the north of the Bering Strait and in the Barents Sea/Svalbard area. However, sea-ice extent is close to the seasonal normal extent in the Davis Strait and the Canadian Arctic Archipelago. (Source: Arctic Sea-Ice News and Analysis http://nsidc.org/arcticseaicenews/). Sea surface temperatures in early November were 1 to 2˚C warmer than the seasonal normal across the North Atlantic (http://weather.unisys.com>surface>sfc_daily/), about 1˚C warmer than usual across the North Pacific. However sea-surface temperatures are up to 4˚C warmer than normal in the Bering Strait between Alaska and eastern Siberia and up to 5˚C warmer than normal off the East Coast of the USA. Sea surface temperatures are actually up to 1˚C cooler than normal across much of the Equatorial Indian Ocean, parts of the Equatorial Atlantic as well as 1 to 2˚C cooler than normal for the time of year over the eastern Equatorial Pacific Ocean. By way of contrast, the subtropical North Atlantic and subtropical North Pacific sea-surface temperatures are above the long-term seasonal norm, again by about 1˚C for early November, sea surface temperatures are also about 1˚C above normal in the Norwegian Sea. The pattern of warmer than usual sea-surface temperatures in both the North Atlantic and North Pacific, with the Arctic-ice limit further north than usual in the European Arctic is supportive of the Circumpolar Vortex- the river of very strong westerly winds in the upper atmosphere over higher latitudes- being contracted and blowing closer to the Arctic. There is still likely to be ice-cover over the Arctic as this expands in extent going into winter to ensure the Arctic interior gets extremely cold by the time we reach December (i.e. below -25˚C at the surface and colder still aloft); this with warmer than usual oceans in mid-latitudes will intensify the atmospheric temperature and pressure gradients that drive sub-arctic depressions and the strong westerlies that blow on their southern flanks. Furthermore, unusually warm waters off the USA Eastern Seaboard combined with sea-ice extent and that of severe cold near normal extent around north-east Canada will help foster an intense atmospheric temperature and pressure gradient that could help send some really powerful depressions headed across the far North Atlantic, to bring strong mild south-westerlies to the UK. On the contrary there are cooler than normal sea-surface temperatures over Equatorial waters but slightly warmer than usual sea-surface temperatures across the northern sub-tropics: This would substantially weaken the crucial temperature gradients that drive the Hadley Circulation; since the normal 35˚N to Equator lower-atmospheric temperature gradient is about 15˚C averaged over the Northern Hemisphere in winter the sea-surface temperature anomalies (such as they are) are likely to lead to a weakening of the Hadley Circulation by up to 5% This means weaker north-easterly Trade Winds, and with the Circumpolar Vortex likely to be north of its seasonally normal position (due to reduced Arctic ice-extent and warmer-than-usual mid-latitude oceans) the subtropical high-pressure belt is likely to be 35 to 40˚N with the slightly weaker north-easterly Trade Winds covering a somewhat greater area of lower/tropical latitudes than usual, westerlies in higher latitudes would not need to be any stronger to provide a sink for the westerly atmospheric angular momentum generated by more extensive (but weaker) north-easterly Trade Winds. Despite all this, the fact that the Westerlies are likely to be blowing in higher latitudes (due to warmer mid-latitude oceans and reduced Arctic ice extent) means these Westerlies will be blowing closer to the axis of the Earth’s rotation; thus they will need to blow stronger to provide a sink for all the westerly Atmospheric Angular Momentum (AAM) caused by the north-easterly Trade Winds and high-latitude (Polar) Easterlies. However, snowcover and severe cold over the interior of Asia could help displace the strong Westerlies (at least aloft) to the extent that the Himalayas and Pamirs become a major sink for Westerly AAM; then all bets are off with all factors likely to weaken the mid-latitude Westerlies combining to cause a Sudden Stratospheric Warming over the Arctic: This we will now discuss. (continued below)
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: (Continued below)
La Nina Episodes Vs UK Winters 1949 - 2011 - Updated 22-OCT-2011 with futher references [color=#00ffff][b]Weak (Oceanic Nino Index SST Anomoly -0.5 to -0.9)[/b][/color] [b]1956-57[/b] Little snow overall. White Christmans for the NW of UK, though Mild & Wet with Atlantic LP dominating in the most part (w/swerlies) [b]1962-63[/b] Very Snowy,Widespread heavy snow throughout Nov - March aka "The big freeze of 63" [b]1967-68[/b] Little snow overall - Largely dry and a tad mild due to HP frequently building over us from the South & S West [b]1974-75[/b] Little snow, Remembered for the 6 year snow drought, Largely dry and mild due to HP influence large swathes of the UK. [b]1995-96[/b] Average, Snow (some very heavy) tended to fall in certain areas of the uk at certain points of the winter so not widespread and frequent enough to class as very snowy [b]2000-01[/b] Average, as with the above, some heavy snow fell late Dec through into Feb but mainly a nothern England & Scotland affair including some decent drifting [b][color=#0000ff]Mod (Oceanic Nino Index SST Anomoly -1 to -1.4)[/color][/b] [b]1954-55[/b] Very snowy, Widespread, blizzards,Dec- drifting/snow through to May [b]1950-51[/b] Very snowy, Widespread heavy falls Dec, 102 days lying. [b]1964-65[/b] Little snow , First 2/3rds of this winter dominated by mild/wet swerlies before becoming dier and colder with blocking/ northerlies. [b]1970-71[/b] Little snow ,Remembered as the start of a 6 year snow drought. Dry in December. Jan & Feb Atlantic dominant- mild & wet overall. [b]1971-72[/b] Little snow , Remembered for the 6 year snow drought, Again Dry in December but mild, Jan & Feb dominated by atlantic S/Swerly [b]1998-99[/b] Little if any snow, All 3 months in the most part dominated by Westerlies & Swerlies along side LP. Mild & Wet Overall [b]2007-08[/b] Little snow, Another mild, largely snowless winter, but followed by some noteworthy wintry spells in early to mid spring. There were localised snow events from short-lived Arctic incursions in November, one in the Midlands on the 18th and another minor snow event in north-eastern areas on the 23rd. After a snowless December (the second in a row), a brief easterly blast brought snow showers and local thunder to eastern Scotland, NE England and Northern Ireland on the 3rd January, but mild air returned on the 4th. The rest of January was mostly mild with south-westerly winds, but further localised snow events occurred during the first half, mainly on high ground in the north. February was remarkable for its sunshine over much of England and Wales, but the only snow event of note occurred on the 1st/2nd, with high ground in northern England, plus parts of Scotland and Norfolk, briefly affected. Wet & Mild overall due to LP ifluence and W/SWerlies [b]2010/11[/b] Average, No-body needs reminding of this one, however, it started off a snowfest for most of the country but sadly became a mostly dry affair Jan and February. That said, was a pretty damn cold winter at that. Snowy Start with Jan/ Feb generally cold and dry. [color=#800080][b]Strong (Oceanic Nino Index SST Anomoly -1.5 +)[/b][/color] [b]1949-50[/b] - Little snow, Generally dominated by LP to the west or NW and W/SWerly winds so plenty of mild wet weather. [b]1955-56[/b] Very snowy - Netweather claim this was same as 54-55 (Very snowy with blizzards and throughout winter) [b]1973-74[/b] Little snow, remembered for the 6 year snow drought, Again LP along with Westerly/ Swerly flow prevailed. Mild & Wet overall. [b]1975-76[/b] Little snow, remembered for the 6 year snow drought. A good deal of the UK and indeed the winter dominted by HP. Dry & Mild. [b]1984-85[/b] Very snowy, Widespread heavy snow for most of the uk Jan & Feb [b]1988-89[/b] Little snow- Warmest uk winter on record after a stormy autumn. Mild & Wet overall. [b]1999-2000[/b] Little if any snow, some say virtually nothing. With HP and LP being close most of the country would have had Wet & Mild muck [b]Notes:[/b] Info regarding these winters has been taken from Netweather british winter history page. Little, Average or Snowy classification takes into account Widespread snow fall and lying days over the 3 months of winter. EG. Heavy snow in December does not count as very snowy if Jan and Feb are dry unless of course the snow has remained into either of these months or snow still falls in these months little or not. if snow is isolated to few areas such as scotland this would also be classed as Little snow. Out of the last 21 La Nina episodes : 13 Co-incided with poor snowless winters some of which were mild 2 Were average winters where you would either see the north, south, east or west of the uk getting snow but not exactly widespread or continous throughout the winter months, with some milder interludes 5 Were real snow fest winters throughout the 3 months and in somecases longer. These were in the 1950's 60's and mid 80's Only 1 from reports I've read and the model achives I've viewed was actually Cold & Dry for the most part and that was 2011 after such very snow start. Had widespread snow continued through Jan-feb 2011 or the snow remained on the ground for longer than it did, then I would have classed that as a very snowy la nina winter also. Cheers Snowmad79 PS. Here's the above in a simple chart form: [img]http://img01.imagecanon.com/_upload/img/63/ninavswinteruk.jpg[/img]