Unusual extremes in the negative phase of the Arctic Oscillation during 2009
Abstract
During 2009, the Arctic Oscillation (AO) index was exceptionally negative during four months, which was unprecedented in a 60-year record extending back to 1950. The negative phase of the AO corresponded to a nearly zonally symmetric anomaly of temperature and pressure over the middle-to-high latitudes of the Northern Hemisphere. Consistent with the strong negative phase of the AO, below-average temperatures were also observed over North America and parts of Eurasia during June, July, October, and December 2009. Although the impact of the AO on 2-meter temperatures was near zero when averaged over the entire hemisphere (20°–90°N), positive anomalies were evident in the residual pattern of observed temperature. The residual, along with observed below-average stratospheric temperatures, are consistent with anthropogenic climate projections. The possible implication of the recent negative values on the long-term AO trend is also discussed.
Received 24 March 2010; accepted 20 April 2010; published 21 May 2010.
¹Climate Prediction Center, NCEP, NWS, NOAA, Camp Springs, MD, U.S.A.
²WYLE Information Systems, Camp Springs, MD, U.S.A.
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