Wintry Winters

On June 6, 2010, Scinexx published a feature (in German) on the possibility that extreme winter conditions – like in 2009/2010 – may become common features. Following the presented argumentation, rapid Arctic warming und reduction in sea-ice change the “Grosswetterlagen” with more cold and snowy conditions over all northern landmasses.

Communicating results presented at the International Polar Year Oslo Science Conference (IPY-OSC) 2010, the feature relies mainly on quotes from James Overland of NOAAs Pacific Marine Environmental Laboratory. Natural variability and anthropogenic factors lead to enhanced warming in the northern polar region especially in the summer season with subsequent rather mild winters in high latitudes. These large scale high latitude changes in turn may give rise to colder winters in the mid-latitudes. The amplification in the Arctic is due to regional feedback processes. Warming and loss of sea-ice reduce the albedo in the region and enhance the heat transfer to the ocean. These regional and larger scale changes, modify the patterns of atmospheric flow. This concert of effects in turn further reduces the volume of sea-ice and its growth. Overland assumes the losses in sea-ice to be irreversible.

The mechanism of Arctic influences on the mid-latitude winter climate is not properly discussed in the feature. Without taking a look at the data, a warming in the Arctic reduces the temperature gradient, alters the structure of the zonal winds and would result in a negative North Atlantic Oscillation phase. This intuitive assumptions are indeed confirmed by the study by Wang et al. (2010, doi: 10.1002/asl.278). According to them, the NAO was extremely negative for one notably “wintry” period in winter 2009/2010. The resulting anomalous northerly winds advect the cold air from the Arctic to the northern land masses. As the tendency towards negative NAO is now persistent since the early 1990s a continuation may result in the proposed more frequent wintery winters.

An extensive comment focusing on the sea-ice development may be found at the klimalounge-blog (in German, by Dirk Notz).