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First of all, apologies for not getting the complete Spring 2017 forecast out in time. I was on holiday in Churchill (Canada) with my brother in late February, then when I got back I had exams to study for. I was also beset with sickness for much of March so little time to get onto the Netweather Forum let alone make seasonal forecasts! I will rectify my shortcomings by producing a forecast for the next two months, but I will not be tempting providence by producing a Summer 2017 prediction before the middle of May. In the summer half-year the Circumpolar Vortex is weak and regional variations in sea-surface temperatures provide the better hope of making a prediction for the season. Even that can get blown off course by a deep depression pushing into Scandinavia or a "Spanish Plume" that drastically alters local temperature and pressure patterns which can flip the upper-level Rossby waves into a completely different configuration that could prove to invalidate earlier predictions of the season. With these provisos in mind I will provide details of the weather that we can expect in April and May this year: A number of important controls still look like they will shape the weather we get in the UK over the next couple of months. Firstly, sea-surface temperatures are some 2 to 3C warmer than usual around the United Kingdom but the North Atlantic south-west of Iceland is some 2C colder than usual and waters off Newfoundland are up to 3C colder than usual. Much of the north Pacific is also chillier than normal for the start of April but further north the Arctic waters are warmer than usual and the same is true of sea-surface temperatures in the Barents Sea (north of Norway) where sea-surface temperatures are 3 to 4C warmer than usual. Arctic sea-ice, as you may be aware, has started to recede from it's lowest maximum extent on record- particularly in the European Arctic sector and in the Bering Sea. Sea-ice extent, however, is close to normal early April extent off eastern Canada and in the Davis Strait between northern Canada and Greenland. This means that the Circumpolar Vortex would, other things being equal, travel a little closer to the Arctic than normal whilst an upper trough will form (more often than not) a little downwind (i.e. to the east) of the anomalously cold surface waters in the mid North Atlantic with the upper flow re-curving northwards as it reaches north-west Europe. This suggests high pressure over Europe but with deeper depressions than normal moving north-eastwards between Scotland and Iceland. This would suggest a warmer than normal April and May but with frequent spells of wet cooler conditions for Scotland, Northern Ireland and North West England. However, there remain global macro-scale controls that could mean the emphasis is going to be more on wet and breezy rather than warm: The Quasi Biennial Oscillation (QBO) is a wind-pattern high up in the Equatorial Stratosphere over 15 miles above sea-level and it has been blowing at record-breaking speeds from the west (at 34 mph on average at the 30 mb level and over 20 mph at the 50 mb level) and this has ramifications for the weather-patterns in higher latitudes. The excess west-to-east momentum of this vast river of air eventually descends and it enters the global tropospheric circulation within a couple of months- within a further few weeks this excess westerly atmospheric angular momentum (AAM) finds its way into mid-latitudes (of both the Southern and Northern Hemispheres). It is the excess westerly AAM transferred into the Northern Hemisphere that we are interested in and unless of course it encounters the Himalayas and Rockies first this will eventually feed into stronger (and more extensive) westerly winds at higher latitudes. The Himalayas and the Rocky Mountains will intercept some of this excess westerly atmospheric angular momentum but by no means a substantial portion of it (let alone all of it) because during the spring lower latitude continents and oceans warm up along with the Eurasian and North American landmasses: Hence strong upper westerlies associated with the subtropical and polar-front jet-streams are pushed to slightly higher elevations in the subtropics and to higher latitudes over mid-latitudes- less of the westerly AAM is stopped by mountain ranges before reaching higher northern latitudes as a result. This can only mean one thing- stronger westerly winds associated with deeper depressions bringing more rain!! Upper-air forecasts hint at a weakening (even a reversal) of westerlies at 60N over the Stratosphere going through to mid April and there are hints that this could result in a cold snap with northerly winds mid-month as high-pressure builds over Greenland. However, it must be remembered that in summer the stratospheric winds over the Arctic and sub-arctic are usually easterly- but clearly Britain still gets wet and unsettled weather then! A strongly westerly QBO does not portend lots of dry fine spring weather in higher northern latitudes and though the hope must be that with sea-surface temperatures around Britain warmer than usual the excessive storms are guided north of the country: However, the large patch of icy water in the mid-north Atlantic will encourage the jet-stream to swing south over and east of there. That means there are likely to be depressions heading towards Britain- and given the effects of the record westerly QBO these depressions will be stronger than normal for the season. Furthermore the south-westerlies associated with these depressions will blow on their southern flank over warmer-than-usual waters just north-west of Britain and they will pick up moisture. I would be inclined, on the strength of these factors alone, to assert that north-west Scotland will have a rather changeable May at best and there will be plenty of rain. Another important factor are developments in the equatorial Pacific Ocean: Sea-surface temperatures for the time of year are 1 to 2C above normal off the coast of Ecuador and these positive temperature anomalies push west towards the central Pacific: This suggests ENSO-neutral bordering on El-Nino conditions which, if anything, will tend to strengthen the upper westerlies associated with the sub-tropical jet-stream over the northern Pacific. This, through a dynamical impact on the structure of the upper westerlies over the North Pacific and North America can lead to increased amplitude between the waves and troughs associated with the Circumpolar Vortex. This could result in fine warm weather over Britain if depressions are carried well to the north of the country but, given the temperature-patterns as they are over the North Atlantic is as likely to mean the jet-stream pushes depressions in from the west on a more southerly track- before they turn north up the western side of Scotland. However temperature and pressure patterns across the Equatorial Pacific are only just bordering on El- Nino so it's overall impact on the United Kingdom weather through April and May will remain minimal. The Sun is also an important influence: Currently the Sun is declining in the extent of it's electromagnetic activity, solar flares and sunspots as we get towards the quiet final stages of the current Sunspot Cycle: An active Sun produces solar flares that (through interaction with the Earth's magnetic field) increase the intensity of winter depressions and attendant westerlies in sub-arctic latitudes. A quiet Sun, by implication, should mean weaker depressions and weaker westerlies in higher latitudes. Also of note is that the Solar Constant has dropped from 1366 Wm-2 a few years ago to nearer 1361 Wm-2 today (a fall in solar output of close to 0.4%), it is of course known that the Solar output drops from Sunspot Maximum to Sunspot Minimum but there are indications that the Sun is going unusually quiet even by the standards of past Solar minima. However, this is not the thread to discuss whether changes in the Sun will mean an end to (and reversal of) global warming; though a quiet Sun ought to point towards a drier, cooler spring in Britain the strength of the Westerly QBO Phase and the sea-surface temperature patterns around and well west of Britain are liable (together) to well overcome the effect of a quiet Sun: The season as a whole does not look to be settled for the north and west of the country. (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)
2nd May 2016 Following my mixed successes (although in some regions of Scotland my spring forecasts have proved a little wide of the mark) I am making a provisional outlook for Summer 2016 for the United Kingdom. It is based on sound meteorological principles and the influence of sea-surface temperatures, sea-ice anomalies and the Northern Hemisphere snow-cover extent observed in March this year. The El-Nino in the equatorial Pacific seems to have ended recently with sea-surface temperatures actually falling below normal off the coasts of Peru and Ecuador. However temperaures in the tropical North Atlantic and tropical North Pacific are (on average) 1C warmer than normal for the stat of May whilst the Indian Ocean just north of the Equator is up to 2C warmer than normal. This is likely to support a strong Intertropical Convergence Zone (ITCZ) going into summer which will sustain NE Trade Winds stronger than normal. Also supportive of stronger-than-usual NE Trade Winds is the fact that most of the North Pacific between 30 and 45N is significantly colder than normal for the start of May with the cold anomaly reaching 2C in places. It is a similar story in the NE Atlantic north of 40N with cooler than normal waters. Cooler than normal waters in the subtropical/low mid-latitude oceans means that the subtropical high-pressure belt will be stronger than normal and so NE Trade Winds on the southern flank will be correspondingly stronger. Other factors, however, such as the fact that snow-cover during March 2016 was some 3 million square kilometres less than normal over North America and Eurasia means that these vast continents will warm up faster with less snow-cover to reflect the sun's heat going into summer. This in turn means less cool air filtering into the subtropics of these continents, southern Asia and the southern USA/Mexico will heat up faster with the result being that the subtropical high will weaken sooner in the season and weaken further. This will weaken the NE Trade Winds further south. On the whole, the overall impact is going to be of stronger NE Trade Winds covering a slightly larger geographical area than normal for the season leading to a bigger-than-normal demand for higher-latitude Westerlies to satisfy Conservation of Angular Momentum laws. The hotter-than-usual tropical waters just north of the Equator will trigger more hurricanes and typhoons in the North Atlantic and Pacific respectively. These tend to move westwards along the ITCZ once the ITCZ is well north of the Equator with the very strong easterlies associated with them feeding into the Northern Hemisphere circulation whilst the very strong westerlies to the south of such storms feed into the Southern Hemisphere circulation. For angular momentum considerations vis-a-vis the Northern Hemisphere circulation this means stronger Westerlies in higher latitudes. However, the hurricanes are not likely to get going until the ITCZ is well north of the Equator over the oceans- that is not likely before July. This paints a picture of stronger Westerlies reaching Britain bringing cool unsettled conditions off the North Atlantic during the summer. However, other factors will influence the position of ridges and troughs associated with the Circumpolar Vortex of very strong Westerlies in the upper air. The position of the upper Westerlies will be influenced (overall) by the extent of sea-ice which (during April) has retreated north of 70N in the Eurasian sector of the Arctic whilst in the North American Arctic Hudson Bay is still frozen and there is still ice in the Davis Strait. The Baring Strait has just thawed out, however and waters off the southern coast of Alaska and western Canada are over 2C warmer than normal at the start of May. Sea-surface temperatures in the NE Atlantic to the west of Britain are 1 to 2C colder than normal and the North Sea is (at the time of writing) also colder than usual but the NW Atlantic south of Greenland is now warmer than usual. The Circumpolar Vortex ridges over and just downwind of areas of warm water where sea-ice is retreated well to the north but upper troughs occur over and slightly downwind of cold seas where sea-ice extent comes south. It is well-known that topographical locking of the jet-stream to the North American Rockies occurs (where an upper ridge often forms) and there is no reason why this will not happen over the coming months. On the whole, angular momentum considerations suggest that the upper (and lower) higher-latitude Westerlies will be a bit stronger than usual so I envisage that a four-wave pattern of upper ridges and troughs will occur through much of the summer, at least until around mid-July. A ridge is likely to occur over the Rockies (helped by warmer-than-usual water south of Alaska) but slow-to-warm Hudson Bay will encourage an upper trough over that region. Further east slightly warmer-than-usual waters south of Greenland will cause the upper Westerlies to re-curve a bit further north than usual so there may be more cyclonic activity over the NW Atlantic but with higher pressure further south and east. Colder waters over the NE Atlantic will cause the upper Westerlies to re-curve southwards and an upper trough would tend to occur over NW Europe. However, a caveat: sea-ice is retreated well to the north of its normal position in the European Arctic at the time of writing and sea-surface temperatures are above normal in the northern Norwegian and Barents seas. This distribution of sea-surface temperature anomalies and sea-ice extent could cause the upper Westerlies to split in the vicinity of western Europe with a northern section of the upper Westerlies carrying depressions north of Scandinavia to be re-invigorated by warmer-then-normal waters there and to follow the margins of the ice whilst a southern branch of Westerlies follows an intensified temperature gradient between the cold NE Atlantic and the rapidly-heating Med/continental Europe with depressions and bad weather affecting central Europe as a result. In between and away from these branches of the jet-stream the still cool waters around the United Kingdom could encourage high-pressure and a run of hot sunny days (not excessive heat owing to the tempering influence of relatively cool coastal waters). CONTINUED BELOW