ARCUS 13th Annual Meeting and Arctic Forum 2001

The Arctic Oscillation as the Driver of Spring Warmings

James E. Overland¹, Muyin Wang², Nicholas A. Bond^3
1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration (NOAA), 7600 Sand Point Way NE, Seattle, WA, 98115, USA, Phone 206/526-6795, Fax 206/526-6485, overland@pmel.noaa.gov
²Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Box 354235, Seattle, WA, 98195, USA, Phone 206/526-4532, Fax 206/685-3397 , muyin@atmos.washington.edu
³JISAO, University of Washington, Box 354235, Seattle, WA, 98195-4235, USA, Phone 206/526-6459, Fax 206/685-3397, bond@pmel.noaa.gov

Conditions in the Arctic in the 1990s were substantially different than previous decades. Six of nine recent years (1990-1998) had major cold temperature anomalies in March in the stratosphere. The remaining three years had weaker cold anomalies. These cold anomalies are part of the Arctic Oscillation (AO), and models suggest that the persistence of the anomalies is driven by CO₂ increases and ozone chemistry at cold temperatures in spring. Cold stratospheric anomalies are associated with warm tropospheric anomalies. Because the location of the stratospheric vortex varies from year to year, anomalous surface winds and warm temperatures occur over Alaska and Northwest Canada in years when the vortex was shifted to the western Arctic; i.e., in 1990, 1993, 1995 and 1997. Both the North Atlantic Oscillation (NAO) and the AO are influenced by increases in zonal winds from the polar vortex associated with the cold stratosphere; however, the NAO has winter variability reinforced by longitudinal sea-surface temperature gradients, while the Arctic proper has a hemispheric component driven by the stratospheric change in late winter. Although the stratospheric/tropospheric connection decouples in May, earlier snow and ice melt in late spring in the western Arctic may precondition summer and fall conditions through albedo and cloud/radiative feedbacks.

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