Strong Cold Weather Event over Eurasia during the Winter of 2011/2012 and a Downward Arctic Oscillation Signal from the Stratosphere

The circulation evolution and possible causes of a severe cold event over the Eurasian continent during the winter of 2011/2012 were investigated with the NCEP-NCAR reanalysis data. Strong cold anomalies covering most of Europe, Mongolia, and northeastern China began in late January 2012 and lasted for about 3 weeks. Analysis results indicate that this cold event coincided with the phase of the Arctic Oscillation (AO) changing from positive to negative, which implied a possible impact of the AO. Before this AO phase change, a minor warming occurred in the stratosphere caused by an anomalous upward propagation of planetary waves. The polar night jet then decelerated and the AO changed its sign in the stratosphere. Within 2-3 weeks, the stratospheric AO signal gradually propagated downward and accordingly the tropospheric AO evolved into the negative phase. Thus, a strong Ural blocking high developed, and cold air invaded Europe and East Asia from the polar region, which induced the decreasing temperature there. Our results suggest that improvements can be made in predicting severe winter cold events over Eurasia by taking into account the stratospheric circulation anomaly.