Arctic Oscillation and the Northern Hemisphere Cold Surge at 2009/2010 Winter

Abstract

The cause of the cold surge occurred at 2009/2010 winter over northern hemisphere was investigated. During the boreal winter (December-January-February) of 2009/2010, the surface temperature was extremely lower than normal years by more than 5 degree over North-east Asia, Europe, and North America. The cold air outbreak was due to the stronger northerly winds in troposphere at mid latitudes associated with the substantially weaker Arctic Oscillation (AO) polarity. The weaker AO is associated with the weaker zonal-mean zonal winds and higher geopotential height and associated marked warming anomalies at high northern latitudes, whose signals propagated from the mid-latitude stratosphere at autumn down to troposphere at winter. The marked stratospheric warm anomaly in autumn is produced by the stronger upward propagation of the stationary planetary waves which led to the weaker znoal-mean zonal winds. Analyses indicate that the stronger wave activity is originated from Eurasia/Siberia associated with the anomalously larger snow cover during October and November. The autumn snow cover over these regions tends to increase with time and its increasing rate is becoming larger towards present with the global warming. This result indicates that the anomalously increased autumn snow cover over Siberia at 2009 autumn in part led to the weakening of the AO polarity and consequent cold air outbreak for the 2009/2010 winter.