Features and Possible Causes for East Asian Winter Monsoon in 2012/2013
The East Asian winter monsoon (EAWM) was in the phase stronger than normal in the inter decadal variation, and the EAWM index was 0.83 in winter 2012/2013, which was the 6th consecutive year with strong intensity. During the winter from December 2012 to February 2013, the daily Arctic Oscillation index was negative, leading to a colder than normal situation over China. While the Siberian High (SH) exhibited strong intra seasonal variations, the temperature over China had two stage variations last winter, warmer in the early winter and colder in the late winter. Further research indicated that the reduced Arctic sea ice extent in the last autumn was responsible for the positive sea level pressure (SLP) in the Northern Eurasia in winter, resulting in the strengthening of Siberian High which was favorable for cold front to move southward into China. The intra seasonal variation of the SH and EAWM was mainly affected by the downward propagation of positive geopotential height anomalies in stratosphere.The positive stratospheric anomalies over the high latitude areas in Northern Hemisphere in mid January had an evident influence on the mid and low levels of troposphere, causing the periodical weakening of SH and EAWM.