Changing severe weather in the Arctic

Arctic weather has many faces. While the ice sheet surrounding the North Pole is frequently calm and cloudy, the warm regions with open ocean experience severe weather such as explosive mid-latitude storms, polar lows, arctic fronts and roll clouds.

The cold air does it
A common feature of these weather types is that they usually form when cold air masses wander out from over the ice sheets onto the warm ocean. In the North Atlantic such conditions arise frequently along the Gulf Stream and its branches. The North-East Atlantic (the Greenland, Iceland, Norwegian and Barents Seas) is particularly prone to such marine cold-air outbreaks (MCAOs), as they are called in the paper.

A fundamental problem with typical MCAO weather is that it is not properly resolved in cost-efficient, coarse-resolution climate models. MCAOs, on the other hand, are large-scale phenomena, and are thus easily distinguished in the same models. This led the authors to use MCAOs as ‘proxies’ (representatives) of severe weather, allowing them to assess the projected changes over the coming century.

The good news and the bad
Using the climate model results that were prepared for the fourth assessment report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), changes for the whole Arctic region were derived. In the areas that are characterized by warm, open water in the current climate a projected decrease in the strength of MCAOs was found. This is because the ocean warms less rapidly than the atmosphere, leading to less pronounced vertical temperature differences.

The decrease over the Labrador Sea is especially pronounced due to the weakness of the ocean warming in the models there. This is linked to the projected strengthening of the North Atlantic Oscillation (NAO), which pulls cold air into this region, cooling the water.

In the warming climate of the century to come, the sea ice in the Northern Hemisphere is predicted to retreat rapidly, drawing the regions of most intense MCAOs with it towards the north. This may prove to be good news for the population near the coasts of Norway, Iceland, the British Isles and northern Europe in general.
The bad news, and the second robust finding of the paper, is that regions that are now covered with sea ice during winter will be exposed to new kinds of severe weather. Large increases in the strength and frequency of MCAOs were found along the entire southern rim of the Arctic Ocean ice sheet, including the Barents, Bering and Beaufort Seas. While these areas are sparsely populated, an increasing commercial marine activity is predicted.

Reference
Kolstad, E. W. and T. J. Bracegirdle (2007): Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the Northern Hemisphere. Climate Dynamics, doi: 10.1007/s00382-007-0331-0.