Search the Community

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.