Exploration of possible causes for the low temperature throughout the winter of 2011 in Yangtze River basin

Exploration of possible causes for the low temperature throughout the winter of 2011 in Yangtze River basin has been performed with the temperature data of 50 meteorological stations in the same basin, NCEP reanalysis datasets and arctic sea ice data from NSIDC. The results have shown both Siberian high and Ural blocking high which is abnormal intensified and stable continuance are main synoptic systems led to low temperature throughout the winter of 2011 in Yangtze River basin, whose intensity and lasting time all have reached to the maximum value since 1961. Further analysis indicate that the stronger intensity and more lasting time of Siberian high and Ural blocking high in winter appear easily in La Nina years, and the circulation features at 500 hPa are similar to mean circulation pattern in winter of La Nina years. Arctic sea ice area being abnormally smaller makes Siberian high strengthened and Ural blocking high lasted. La Nina and arctic sea ice area being abnormally smaller may act as the major external forcing factors causing air temperature to be continuously lower over Yangtze River basin.