Unusual extremes in the negative phase of the Arctic Oscillation during 2009

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 degrees-90 degrees 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. Citation: L'Heureux, M., A. Butler, B. Jha, A. Kumar, and W. Wang (2010), Unusual extremes in the negative phase of the Arctic Oscillation during 2009, Geophys. Res. Lett., 37, L10704, doi:10.1029/2010GL043338.