THE IMPACT OF AN ICE-FREE ARCTIC OCEAN ON WEATHER-PATTERNS IN WESTERN EUROPE

[iapennell]

Dear Readers

The impact of increasing global temperatures over the last forty years on Arctic sea-ice cover is clear: There has been a consistent decline in sea-ice extent compared to the long-term normal (see here for an illustration: http://nsidc.org/soac/sea-ice.html#seaice). The possibility of the Arctic Ocean becoming ice-free in summer, then remaining so into the winter is one that is of increasing concern for remote communities on the Arctic coastline and for wildlife. Less appreciated, perhaps is how an Arctic Ocean free of pack-ice year round would impact upon the prevailing (and possible extreme) weather- conditions further afield.

It is certain that the loss of Arctic sea-ice would occur in the context of global temperatures just 2.5 to 3C above those of today. As the Arctic warms and the edge of the pack-ice recedes north of Spitzbergen the ice-free water absorbs heat from the Sun rather than reflecting it back to space and this helps warm the local ocean surfaces further. If Greenland remains very cold and ice-covered the baroclinic zones of sharp atmospheric temperature and pressure- gradients would extend north-eastwards and northwards from the tip of southern Greenland, this would encourage more North Atlantic depressions to push deep into the Arctic. That process would, in addition bring warm southerly winds from the North Atlantic right up into the Arctic- further exacerbating regional warming. It is clear to see that if this feedback between more ice-free waters absorbing the Sun's heat in the summer, then attracting depressions into the area in winter bringing more warmth- really took off then the entire Arctic could become free of ice one summer, then remain free of ice as autumn and winter storms moved in over the ice-free areas with warm air from lower latitudes. Once the Arctic is ice-free and the surface waters stay above the freezing-point of ocean water year-round (-1.8C) the ice-free ocean would release a large amount of warmth into the low atmosphere. This would lower surface pressures but would also have a smaller warming influence on the high atmosphere over the Arctic. The main influence of the lowest 3,000 metres of the atmosphere being some 10C warmer in Autumn and 15 to 20C warmer in Winter would be a drop in average surface-pressures over the central Arctic by 15 millibars in Autumn and 20 to 25 millibars in Winter- that fact alone combined with the stronger atmospheric temperature gradient between much warmer conditions in the low troposphere and still very-cold air in the upper-atmosphere would be strongly conducive to storm activity in Autumn and Winter. The article referred to here lends support to this thesis (https://journals.ametsoc.org/view/journals/clim/31/19/jcli-d-18-0109.1.xml).

In Summer, an ice-free Arctic would be little warmer than the part-thawed pack-ice there today, unless sea-surface temperatures could get well above freezing-point: It is thus likely that the Central Arctic would not experience more summer storms in a warmer world, but hotter conditions on the continents surrounding the Arctic in a warmer would still increase the crucial baroclinic atmospheric temperature contrasts to increase cyclogenesis at the margins of the Arctic- with worrying implications for coastal erosion along the Arctic coast-lines of Canada, Alaska and Russia. This still implies, for the purposes of discussing the wider implications for mid-latitude weather-conditions, a general northwards shift in storm- tracks.

We now look at what the implications of increased temperatures and storminess in the ice-free Arctic Ocean mean for regions well to the south of the Arctic Circle. There is a school of thought that believes an ice-free Arctic ushers in wetter conditions with stronger winds in temperate latitudes, with severe winter cold spells becoming a thing of the past (not least because the World would be 3C warmer and middle-latitudes are not far from the unfrozen Arctic Ocean region that will be 20C or more warmer in winter). Certainly this means that any northerly winds coming from the Arctic interior will not bring severe cold to Britain during the winter months, but this does not rule out cold air coming from the east even if sea-surface temperatures around Britain are 4C warmer at the start of the winter. And there is no guarantee that that will be so because if the North Atlantic Drift weakens in response to weaker Westerlies sea surface temperatures around the UK may not be much warmer than today.

The main influence of an ice-free Arctic Ocean, at least in Autumn and Winter will, from the above, be to steer the deep depressions (that normally push north-east from Iceland to north of Norway in Autumn and Winter nowadays) much more northwards along the East Greenland Coast and into the Arctic interior. Furthermore a much warmer Arctic in Winter would even ensure that the upper-atmosphere warmed a little (as cyclonic cloudy convection transfers heat into the upper-air)- increasing the 500 mb heights by 200 metres or more. That process alone would help decrease the strength of the Circumpolar Vortex by 10% or more, not least over the North Atlantic west of the UK. On the other side of the ledger, if the North Atlantic is 4C warmer in early winter that almost doubles the latent heat available to fuel deep depressions and could help tighten and strengthen the Circumpolar Vortex twice as much as the reduced Circumpolar Vortex might weaken it. That is particularly possible if Greenland is still ice-covered and Greenland/ north-east Canada still get very cold in winter, the sharp baroclinic temperature gradients in the atmosphere between Greenland/ NE Canada and a warmer North Atlantic would be a very potent source of cyclogenesis- but the question then becomes where do the depressions go, i.e. eastwards or northwards up the East Greenland Coast into the ice-free Arctic Ocean. If the depressions that normally headed just north of Norway in the past travel up the East Greenland Coast towards the North Pole in a warmer world that has very profound implications for the climate of western Europe (including the UK).

Continued below.         

 

[iapennell]

Continued.

A 3C warmer Earth with an ice-free Arctic Ocean year-round has profound implications for the Global Westerly Atmospheric Angular Momentum (GLAAM) fluxes. In the absence of an outside force a rotating mass maintains the same total angular momentum, this is the Law of Conservation of Angular Momentum and for the Earth and its atmosphere this does closely approximate to being true (outside forces such as lunar tides and meteorites are miniscule compared to the exchanges in Westerly Atmospheric Angular Momentum (AAM) between the surface of the Earth and the atmosphere. This means that the total axial Angular Momentum of the Earth, its oceans and atmosphere must remain constant though it does not preclude exchanges between them- or for the atmosphere to rotate faster whilst the Earth and oceans slow down a minute amount.

In practice, and under most climatic conditions on Earth that one might care to consider, what this means is that the atmosphere gains Westerly AAM (which makes the atmosphere rotate west to east more compared to the underlying surface) at the same rate that it loses Westerly AAM elsewhere. Under current climatic conditions, the Earth's atmosphere gains Westerly AAM as a result of the frictional impact of tropical and sub-tropical Trade Winds (that tend to blow from the east but also towards the Equator) and tropical easterlies coming up against mountains (mountain torque). The atmosphere loses Westerly AAM where Westerlies blow over the Earth's surface in higher latitudes, although where strong Westerlies impact mountains in mid-latitudes there is also a mountain torque (due to pressure being lower on the lee side of mountains with respect to the wind), which also removes Westerly AAM from the atmosphere. In high latitudes there is usually high surface-pressure over frozen lands and seas and the Polar Easterlies add Westerly AAM to the atmosphere, again through frictional interaction with underlying surfaces (again, this is balanced by the Westerlies in mid-latitudes).

However, how Global Warming impacts on the GLAAM fluxes and the sources and sinks of Westerly AAM also has very profound implications for the North Atlantic Westerlies that bring winter warmth and moisture to western Europe: So far as the Northern Hemisphere Circulation is concerned an ice-free Arctic attracting increased storminess means that mean wind- directions will become strongly Westerly over extensive areas of the Arctic, particularly in winter. And unlike mid-latitude depressions a deep depression centred near the North Pole would have its centre north of just about everywhere else in the Arctic Ocean so the depression would bring strong westerly winds everywhere (there would be no "north of the depression" where there are easterlies). Thus, deep Arctic depressions would become  significant sinks for Westerly AAM, though the impact of the Westerlies would be muted by the fact that they blow close to the axis of the Earth's rotation. All the same, if the Arctic north of 60N had Westerlies in winter with a similar speed to the mean speed of winter Polar Easterlies nowadays that reduces the pressure on mid-latitude regions and ocean- surfaces to act as a sink for Westerly AAM.

Increased deep tropical depressions in the tropics (never at the Equator) are also expected with a World 3C warmer than today as hotter tropical seas (with little warming in the high atmosphere) means more convection and the potential for massive releases of latent heat-energy. As far as Westerly GLAAM is concerned, a deep tropical depression is a net sink for Westerly AAM as the very strong Westerlies equatorward of the "eye" blow slightly further from the axis of Earth's rotation than the very strong Easterlies on the poleward side of the "eye": Thus more deep tropical depressions slightly weaken the tropical source of Westerly AAM, other things being equal. However, other influences in the tropics such as stronger Easterlies in the upper atmosphere near the Equator (as a result of the zone of hot, steamy rising air, the ITCZ moving well north and south of the Equator into the Hemisphere experiencing summer) would add Westerly AAM due to more Easterlies blowing against equatorial mountains. Global warming is also likely to push the zones of strong upper-atmosphere Westerlies further north and south, so that these miss major mountain areas in lower latitudes. Against that, the ITCZ moving well north or south of the Equator also encourages the normally-easterly Trade Winds to "re-curve" on crossing the Equator- leading to more extensive North West Trade Winds penetrating northern Australia or interior South America from December to March and more extensive South-Westerly Monsoon Winds over South Asia and West Africa during the Northern Summer.

Hotter, steamier conditions with the ITCZ would (on the one hand) increase the strength of the easterly trade winds and the source of Westerly AAM in the tropics, but greater warming in higher latitudes- particularly over sub-tropical oceans would also help weaken the temperature and pressure gradients between the sub-tropical high-pressure belts and the ITCZ- leading to weaker easterly Trade Winds. Overall, a slight weakening of the North East and South East Trade Winds will help reduce the extent to which easterlies in the tropics and sub-tropics (both at the surface and aloft) add Westerly AAM to the global circulation. All things considered, less Westerly AAM generated by easterlies blowing over land and seas in low latitudes leaves less to be transported to higher latitudes.    

All these factors together- slightly weaker easterly Trade Winds, more intense tropical depressions and more vigorous monsoons in the summer Hemisphere outweighs the effects of more Easterlies and less Westerlies affecting tropical mountains in a 3C warmer world. The result is less Westerly AAM transfeered to higher latitudes and this is consistent weaker Westerlies in middle latitudes. An ice-free Arctic Ocean attracts the depressions that would otherwise move east across the far North Atlantic. This article here also lends support towards weaker Westerlies reaching western Europe (https://journals.ametsoc.org/view/journals/clim/29/2/jcli-d-15-0315.1.xml).

A global climate 3C warmer than today with a totally ice-free Arctic Ocean, with weaker Westerlies from the North Atlantic (with depressions often moving north rather than east after the southern tip of Greenland) means Britain and north-west Europe will come more under the influence of blocking high-pressure systems, particularly during the winter months. Hotter conditions (and a steeper atmospheric temperature gradient) mean more sharp thunder-storms are possible in summer (bringing localised flooding), but also a lot of very hot sunny weather even across northern Britain during spells of high-pressure. Winter is likely to feature rather more frequent high-pressure systems building over northern Europe- extending westwards to bring very cold air from Russia to the UK- even with 3C global warming a week of easterlies from the far side of the Urals would still bring bitterly cold conditions with hard frost and snowfalls in the winter months. Other winters would be very mild with the high-pressure further east allowing warm south-westerlies to reach Britain (with the warmer North Atlantic encouraging heavy rainfall across the north at times). Continental Europe is likely to become quite arid, with very hot summers and dry, often bitterly cold winters: Severe droughts would become a major problem right across western Europe.

Judging from the changes in positions of depressions, the reduced Arctic to sub-tropic atmospheric temperature gradients and changes in the strength and positions of the GLAAM fluxes (up and down) the Circumpolar Vortex is likely to weaken by 10% or more: This alone will make the Circumpolar Vortex more wavy and spread the sink for Westerly AAM towards lower mid-latitudes (where westerly winds do not have to blow as strongly over the surface to act as a sink for AAM)- as well as permitting depressions to penetrate the high Arctic. In a world that is 3C warmer this will not be good news for those with health problems that make them susceptible to extreme heat or cold- or for farmers and horticulturalists that depend on rain for their crops to grow.      

                    

[Gray-Wolf]

Thanks for that iapennel!

I suspect 'aspects' of what you sketch out are already becoming increasingly established across our hemisphere and the 'weather patterns' they generate ever more common?

[Gray-Wolf]

double post!

Edited April 27, 2021 by Gray-Wolf