Today´s paradox: European cold winters associated with Arctic warming – and positive(!) AO

Warming in the Arctic would increase the chance of cold winters in Europe and parts of North America and Asia. There have now been three consecutive studies that reach this conclusion in as many years.

That means it is about time we paid attention to the hypothesis. But to us still natural climate variability seems an equally convincing candidate to explain a recent set of near-average-temperature winters.

Arctic warming colder winters
The latest study was conducted by a group of German scientists of [hold tight for lengthy description] the Research Unit Potsdam of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, and published in the journal Tellus A, which is hosted by the International Meteorological Institute in Stockholm.

Seasonal changes in Arctic weather

From observations in recent years these scientists conclude climate change leads to a new seasonal pattern over the Arctic Ocean. In summer more sea ice melts, which leads to decreased albedo, a climate feedback that enhances the warming of both the open ocean water and the atmosphere directly above it.

In autumn and winter the upper atmosphere cools down quickly as the permanent polar night sets in. As however it takes longer than before for the sea ice to recover and extend across most of the Arctic Ocean, the lower atmosphere is being warmed by the (partly) open water beneath it – like a take away coffee without a lid emits heat to the air above it.

Arctic depressions forming during autumn and early winter

This the researchers say, leads to what meteorologists call an unstable atmosphere, with relatively warm air at sea level and colder air higher up in the troposphere. As warm air rises this leads to the formation of depressions at sea level, or low air pressure weather systems.

These in turn can affect typical air circulation patterns like the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). When depression activity over the Arctic is high, the indices of both AO and NAO are more easily inclined towards a positive state, with a stronger west circulation around the northern hemisphere and improved temperature isolation for the Arctic region – and that’s the real paradox of this story, because positive AO and NAO are actually associated with milder winters in Europe and for instance the US.

Arctic Oscillation more often negative, because it is more often positive(!?)

The authors however believe a positive AO and NAO leads to atmospheric wave train patterns which may have delayed opposite effects – leading to a negative AO and NAO later on in the season – and that is where especially associated European cold spells would originate from.

Whether based on empirical evidence or climate model findings the relation between increased positive AO (in autumn) leading to negative AO and NAO (some time later) is of course a bit counterintuitive. We must give this a better look and promise to get back on the matter.

If the findings prove correct it could very well be a temporary relation. As the warming trend continues and the Arctic Ocean loses more and more ice, the winter sea ice recovery gets further delayed with passing of time. In future one can probably expect stretches of open ocean even at the start of the Arctic winter, so increasingly unstable atmosphere – and therefore positive AO (and NAO) even during the northern hemisphere cold months. It would decrease the chance of experiencing the December cold spells Europe and the US witnessed in recent years.

Empirical evidence perhaps overstated

Another problem for the hypothesis is that the empirical evidence in support could be weaker than the past winter references may suggest. That is because there are decent competitors to explain negative mode in the AO and NAO in recent years – namely the recent solar minimum. And if we look at the current European cold February, firstly NAO isn’t negative – and secondly a La Niña influence is possible.

There are however other research groups who would agree there could be a connection between Arctic warming and cold boreal winters – especially cold European winters.

In 2010 there was a publication in the Journal of Geophysical Research by [two Vladimirs – can’t ignore such trivialities – of] the German Potsdam Institute for Climate Impact Research and Leibniz Institute of Marine Sciences at the University of Kiel.

One way or the other, NAO and AO behaviour must be explained

These researchers think they have found a direct relation between decreased Arctic winter sea ice extent and cold winters in Eurasia – or at least that is what their ECHAM climate model runs suggested. Here too an increasingly unstable atmosphere over stretches of open Arctic Ocean would be leading to first depression formation, followed by the formation of high pressure systems later on – and therefore a transition from a positive to a negative AO index. Parts of Eurasia would therefore experience 1.5 degrees Celsius colder winters.

And then in January of this year there was another publication, that came up with a perhaps better convincing route to explain how indeed Arctic depressions can lead to anticyclonal weather systems later on than atmospheric wave patterns: snow cover.

Researchers [including – can you believe it – yet another Vladimir] of Atmospheric and Environmental Research, Inc., the University of Massachusetts and the University of Alaska, Fairbanks, wrote in Environmental Research Letters how they think climate change leads to precipitation increases in different high latitude regions, especially in Eurasia. This leads to a larger snow cover in autumn and winter. As snow isolates the relatively warm soil, the air above it can cool quicker, causing it to compact – a process that increases air pressure and may lead to the formation (and fixation) of high pressure systems, and indeed a negative phase in the Arctic Oscillation.

The cold European winter month of February 2012

Currently indeed the Arctic Oscillation is deeply negative, with a high pressure system stretching across much of northern Europe, including Arctic Russia – and an especially cold start of February for countries like Poland, Germany and the Netherlands.

Whether this high pressure build-up was caused by increased snow cover is hard to tell, but seems unlikely, as the mild December and January did lead to high precipitation, but mostly in the form of rain, even in Scandinavia, the Baltic countries and parts of Russia.

There could be another factor at play in the current stabilisation of European winter weather: the 2012 La Niña. With a somewhat colder Gulf Stream Atlantic depressions are confined to the Atlantic Northwest (so NAO does not turn negative), where these may feed a ridge of high air pressure stretching from the Azores to Scandinavia and North Russia – one that proves more stable than we had anticipated two weeks back.

© Rolf Schuttenhelm |

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