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Hello. Apologies for my absence for several months- I have been busy with my new business! Those of you who have a basic grasp of meteorology will know that the large- scale circulation in both the Northern and Southern hemisphere's consists of three cells: 1) The Hadley Cell- where air rises at the Equator over the hottest lands and ocean surfaces moves at high- levels towards the subtropics and descends in the subtropical high- pressure belts. Low- level north- east and south- east Trade Winds complete the circulation by bringing this air beack towards the Equator. 2) The Ferrel Cell- where air descends on the polewards flanks of the subtropical-highs and blows towards higher latitudes at lower levels. These winds, deflected by the coriolis effect cause surface south- westerlies in the Northern Hemisphere and surface north-westerlies in the Southern Hemisphere. This air then rises in the sub- polar low- pressure belts, rises and returns to the polewards flanks of the subtropical highs at high altitudes. 3) The Arctic/ Antarctic Cells- where air descends over frozen seas and lands near the North and South Pole's respectively, blows outward near the surface, rises on the polewards flank of the sub-polar lows and returns to the interior polar areas at high elevation. The Hadley and Arctic/ Antarctic Cells are thermally direct, being driven by the principle of warm air rising and cool air sinking. The Ferrel Cell tries to operate in reverse and the only reason for it's existence is the fact that the Earth rotates and because constraints placed on the Global Circulation by the Law of Conservation of Angular Momentum require that the extent and strength of surface Westerlies counterbalance that of easterlies in both high and low latitudes. The Ferrel Cell is not, perhaps a uniform feature of weather maps, as the weather- patterns in areas dominated by the Ferrel Cell are affected by individual deep depressions that track eastwards (usually between 50 and 75N in the Northern Hemisphere) and the ridges of high pressure that (usually) extend north from the subtropical- highs in between the passage of depressions further north: In the Northern Hemisphere persistent high- pressure over Siberia may block out the Ferrel Cell over much of northern Asia, but even here depressions skirting around the north edge of the high still demonstrate the Ferrel Cell is able to punch through cold-air anticyclones near the surface. The main feature of the Ferrel Cell, overall, is it's propensity to bring warmer air polewards at the surface over wide areas of mid- latitudes: It's reach is extensive and dominant over large areas of mid- latitudes. The Ferrel Cell brings damp mild weather: The northwards movement of warmer air below and an equatorwards movement of colder air aloft means an unstable atmosphere prone to producing cloud and rain- certainly over the UK where south-westerlies blow off the North Atlantic. The cloud cover and persistent advection of mild air does mean- at this time of year- an absence of freezing temperatures; cloud, dampness and no frost has so totally dominated this month of December even in the north of England! If there were some major geo- engineering initiative that could weaken the Ferrel Cell enough at this time of year, those who like frosty snowy winter weather ought to favour it. The Ferrel Cell, as I have mentioned, owes it's existence to a "need" for the frictional force of easterly winds in high and low latitudes to be counter-balanced by Westerlies elsewhere. Sadly, the Westerly atmospheric angular momentum (AAM) that the global winds pick up from the frictional interaction of the NE and SE Trade winds in the tropics and sub-tropics never magically drifts off into space (which would allow the Earth to slow down a bit- so we could fit more into 25 hour days) it always returns to the surface. The excess Westerly AAM is transferred polewards in the vicinity of the subtropical jet- stream- about 12 km up over 30N and 30S. So Donald Trump needs to be persuaded to build a huge north- south 12-km-high wall in the Mojave Desert to intercept the subtropical jet-stream (his US-Mexico border wall is not going to be high enough and it would be orientated the wrong way), slowing it down so that this very high wall becomes the sink for Westerly AAM in the Northern Hemisphere. Without the "need" for so much of a sink for Westerly AAM elsewhere, there would be less mild south- westerlies encroaching Europe, less depressions in the North Atlantic and North Pacific and this would eliminate coastal erosion along the US and Canadian coasts as well as those of the UK and northern Europe. Maybe Donald Trump could be persuaded to spend £1 trillion dollars to save on coastal erosion costs for the next century- maybe Britain could offer to give the USA Bermuda and all its coastal waters free of charge as an additional incentive! Think how many well -paid jobs that would provide for the USA We would have much more high -pressure in Britain to bring cold frosty autumn mornings and dry winters with hard frosts. Summers would be warm and sunny! It remains for me to wish all on here a happy and peaceful Christmas and all the best for 2019 Ian Pennell
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)