Arctic sea ice extent reached the second lowest minimum value on record in September 2011.
Note that this article was originally published in September 2011. This has now been updated for 2012.
Before this, the lowest minimum value was observed in September 2007. The low ice extents increase the possibility of shipping routes opening up through the Arctic; ships were routed along the Siberian coast (the Northern Route) from mid-August this year, while parts of the route through the Canadian Archipelago (the Northwest Passage) were also open.
A large volume of sea ice melted during June this year driven by a dipole pressure pattern with high pressure over Greenland and the central Arctic and low pressure over Siberia. This produced warm southerly winds that promote melting in the Pacific sector, and can transport the ice across the Arctic Basin and out through the Fram Strait. The pressure pattern was similar to the pattern which persisted through most of the summer in 2007 contributing to the record low minimum.
Attempts to predict the summer Arctic sea ice are now underway. The SEARCH sea ice Outlook contained 21 predictions of the September 2011 ice extent, ranging from 3.96 to 5.4 x 106 km2 with a median value of 4.6 x 106 km2. This compares with the actual September mean value of 4.5 x 106 km2. The Met Office contributed one of only four model-based predictions to the Outlook using the seasonal prediction system GloSea4 (Arribas et al. 2011) which assimilates sea ice concentration from satellites as well as ocean temperature and salinity to initialise the predictions.
Timeseries of September monthly mean sea ice extent from HadISST As can be seen in the figure to the right, sea ice has been on a gradually declining trend since satellite records began in 1979. This has led to an increase in the fraction of first year (thinner) ice in the Arctic and a decrease in the fraction of multiyear (thicker) ice. Greater first year ice will precondition the ice so that it is more vulnerable to melting and is likely to result in greater interannual variability in the ice extent.
We are now starting to see evidence that the rate of decline of the September ice extent has increased (Comiso et al, 2008, Stroeve et al 2011). For example, the mean rate of decline over the most recent 16 year period was approximately 1.5 x 106 km2 per decade, which is significantly different to the equivalent value for the preceding 16 years (0.4 x 106 km2 per decade). Decadal periods of accelerating ice loss are not unusual in climate models and we are investigating the causes of accelerating and slowing ice loss.
Are the recent decreases in Arctic ice cover likely to have an impact on European climate? As the sea ice decreases, the immediate atmospheric response is for a local warming of the lower Arctic atmosphere by the relatively warm Arctic Ocean (e.g. Kumar et al., 2010). However, there is also evidence that depleted sea ice alters atmospheric circulation patterns outside the Arctic, throughout the following months and into winter. This appears to result in high pressure over the Arctic and low pressure over the mid-latitudes which in turn is balanced by more easterly winds. While other factors are also involved in determining winter climate, this raises the risk of anomalously cold conditions over Eurasia (Wu and Zhang, 2010; Petoukhov and Semenov, 2010). However, the relative importance of sea ice conditions and other factors in producing cold conditions are being investigated.
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