by Leon Clifford, reportingclimatescience.com, December 2, 2010
Global warming may be contributing to the current heavy snow and subzero temperatures across Europe, which has closed major airports, caused hundreds of highway accidents and several deaths. Cold weather in Europe is often associated with a weather system known as the North Atlantic Oscillation (NAO) but there is also a significant impact from current low levels of sea ice in the Barents-Kara Sea, according to a leading climate scientist. Low levels of sea ice in the Barents-Kara Sea are currently close to the record lows seen in the harsh winter of 2005 and 2006.
Recent research published last month found a link between low levels of sea ice in the Barents-Kara Sea, north of Norway and Russia, and an increased probability of harsh winters across Europe such as the cold winter between 2005 and 2006. These sea ice declines associated with warmer oceans could triple the probability of cold winter extremes in Europe and northern Asia, according to Vladimir Petoukhov, lead researcher for the study, and a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.
Sea ice is currently very low
“Just yesterday I checked the sea ice in the Barents-Kara and it was very low. Nearly as low as it was between 2005 and 2006,” Petoukhov told Reporting Climate Science .Com on 2 December 2010.Typically cold weather in northern Europe is associated with the NAO which consists of a high pressure system over the Azores and a low pressure system over Iceland. The so-called negative phase of the NAO was particularly strong during the second half of November, according to Petoukhov. The NAO is the dominant mode of atmospheric variability in the north Atlantic throughout the year and particularly prominent in the winter. The pressure difference drives the winds and the weather fronts.
Petoukhov explained that it is difficult to separate out the different contributions from the NAO and from the Barents-Kara sea ice decline but that there would be a contribution from both effects.
“We had a strong negative phase of the NAO in the second half of November. The pressure gradient between the Azores high and the Icelandic low was very small. This is the favourite situation for cold winters even without the Barents-Kara effect," said Petoukhov. “I would say that both the NAO and the Barents-Kara sea ice effect are playing a role at the moment. Both effects can work simultaneously as they will support each other. By the end of December we will have a much clearer idea as to the contribution of the Barents-Kara effect. We can not say which is the stronger at the moment but let's wait until the end of December and then we might have an answer.”
Warmer oceans melt sea ice and lead to the effect
Warming oceans, thought by many to be associated with climate change, are contributing to reductions in sea ice in the Arctic area. Computer models suggest that a reduction in sea ice in the eastern Arctic leads to a loss of ocean heat and a consequent warming of the lower atmosphere which can trigger atmospheric circulation anomalies that can in turn lead to an overall cooling of northern continents, according to Petoukhov's research which was published in the Journal of Geophysical Research in November. This can result in a continental-scale winter cooling reaching, on average, −1.5 °C colder than it would otherwise have been.A drastic reduction of sea ice was observed in the Barents-Kara Sea north of Norway and Russia during the cold European winter of 2005 and 2006 and the exposed sea surface lost a lot of warmth to the normally cold and windy arctic atmosphere. Warming of the air over the Barents-Kara Sea seems to be associated with bringing cold winter winds across Europe.
The researchers simulated this situation using a general circulation climate model and incrementally reduced the sea ice cover of the eastern Arctic in the model from 100% cover down to 1% cover in order to analyse the relative sensitivity of wintertime atmospheric circulation. The simulations demonstrated that lower-troposphere heating over the Barents-Kara Sea in the eastern Arctic caused by the sea ice reduction may result in a strong anticyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents and a consequent cooling. An abrupt transition between different regimes of the atmospheric circulation in the sub-polar and polar regions may also be very likely.
This correlation between the sea ice reduction and the continental cooling is strong, according to the research. Other explanations linking cold winters and global warming include reduced solar activity and changes in the Gulf Stream are less strongly correlated. However, the NAO could interact with sea-ice decrease, the study concludes and one could amplify the other. This is what may be happening at the moment, suggested Petoukhov.
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