Recent Trends in Arctic Sea Ice and the Evolving Role of Atmospheric Circulation Forcing, 1979-2007

This study documents the evolving trends in Arctic sea ice extent and concentration during 1979-2007 and places them within the context of overlying changes in the atmospheric circulation. Results are based on 5-day running mean sea ice concentrations (SIC) from passive microwave measurements during January 1979 to October 2007. Arctic sea ice extent has retreated at all times of the year, with the largest declines (0.65 × 106 km2 per decade, equivalent to 10% per decade in relative terms) from mid July to mid October. The pace of retreat has accelerated nearly threefold from the first half of the record to the second half, and the number of days with SIC less than 50% has increased by 19 since 1979. The spatial patterns of the SIC trends in the two halves of the record are distinctive, with regionally opposing trends in the first half and uniformly negative trends in the second half. In each season, these distinctive patterns correspond to the first two leading empirical orthogonal functions of SIC anomalies during 1979-2007. Atmospheric circulation trends and accompanying changes in wind-driven atmospheric thermal advection have contributed to thermodynamic forcing of the SIC trends in all seasons during the first half of the record and to those in fall and winter during the second half. Atmospheric circulation trends are weak over the record as a whole, suggesting that the long-term retreat of Arctic sea ice since 1979 in all seasons is due to factors other than wind-driven atmospheric thermal advection.