Climate model projections for the later part of the 21st century under scenarios of increasing greenhouse gases indicate an amplification of global warming in northern high latitudes, from feedbacks associated to the decrease in sea-ice and snow cover, the so-called polar amplification phenomenon.
While many models predict the near-disappearance of summer Arctic sea-ice by the end of the century, they largely fail to reproduce the rapid decrease in summer sea-ice observed in the last decades. Arctic sea-ice is disappearing faster than the average of climate models predict. State-of-the-art models have still many shortcomings, for example in the treatment of Arctic clouds or of air-sea-cryosphere interactions.
One crucial component of the atmospheric circulation is the travelling cyclones that largely influence precipitation and cloudiness. In middle latitudes, the two main storm tracks are over the Pacific and Atlantic oceans. But in summer, Northern Eurasia is also a major centre of cyclone activity, which develops in response to the heating contrast between the Eurasian continent and the Arctic Ocean. There is even a band of enhanced cyclone activity stretching from Siberia across the Arctic Ocean in summer, termed the Arctic Ocean Cyclone Maximum.
We have recently examined summer cyclone tracks, upper-level winds and surface temperatures in the Bergen Climate Model (BCM), with a focus on the Arctic and Northern Eurasia. We analysed both the current climate and scenario simulations lasting until the end of the 21st century. Our method relies on tracking cyclones in a Lagrangian manner.
One intriguing finding is that the Arctic Ocean at large and the Arctic coast of Russia in particular, are projected to harbour more, and more intense, summer storms, while occurrences are decreasing over the mid-latitudes of Eurasia. These Arctic cyclone changes are consistent with a strengthening of the subpolar jet and changes in surface temperature: as the continent warm, and the sea-surface stays at near constant temperature during the ice melting, the meridional thermal gradient across Northern Eurasia increases.
The increase is quite weak, only a few percent. Don’t expect hurricanes in the Arctic! Nevertheless, studies of this type are important to unravel the internal dynamics of climate models, despite their shortcomings in reproducing the observed changes in the ocean and atmosphere that we observed today over the Arctic.
Further comparative studies of climate models are needed to assess how well they reproduce the Arctic summer circulation. The summer Arctic Ocean Cyclone Maximum in particular seems difficult to capture.
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| Summer-mean cyclone track density for the current climate (1971-2000) (A) NCEP re-analyses, (B) for the Bergen Climate Model. Black isoline indicates the 25 cyclones/(season 5º). In B, green isoline is 25 cyclones/(season 5º) from NCEP, plotted again on the model results for comparison. |
Reference:
1) Yvan J. Orsolini: Norwegian Institute for Air Research, Kjeller, Norway
2) Asgeir Sorteberg: Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway