In March 2012, the center of polar vortex splits into two parts. At 500hPa level, meridional circulation prevailed over the Eurasia region, with the East Asian trough being deeper than normal to lead cold air from high latitudes to influence China. Meanwhile, the intensity of Siberian high shifted from stronger than normal to weaker than normal in the month, with three cold surges intruding China. Thus, the surface air temperatures were generally below normal over central-eastern China. At low troposphere level, the westerly moisture transportation from the India-Burma region to southern China were enhanced as a result of stronger India-Burma trough, while concurrently an anomalous anti-cyclonic circulation was over the Northwest Pacific, which enhanced the southerly moisture transportation to southern China. Above anomalous circulation pattern resulted in positive precipitation anomalies over the western south China, Jiangnan, Jianghuai and Huanghuai regions.
Update monitoring in March, 2012 showed that an anomalous wave train distributed over the mid-high latitudes of Eurasia at 500hPa level, with a deeper than normal East Asia trough and India-Burma trough(Fig. 1). AO persisted positive phase (figure omitted), with the polar vortex splitting into two parts (Fig. 1), which induced the southward intrusion of cold air from polar region to East Asia. The Siberian high was stronger than normal during the first middle of month, and turned to weaker condition hereafter (Fig. 2), with occurrence of three cold surges intruding China (Fig. 3). Thus, the surface air temperatures were generally below than normal over central-eastern China(Fig. 4).
Furthermore, at 850hPa level, corresponding to stronger than normal India-Burma trough, the westerlies from India-Burma region was stronger than normal，and an anomalous anti-cyclonic circulation was also visible over the Northwest Pacific(Fig. 5). Because of the above anomalous circulation pattern, the southwesterly moisture transportations were anomalously strong, and resulted in occurrence of positive anomalies of precipitation over the western South China, Jiangnan, Jianghuai and Huanghuai region (Fig. 6).
Fig. 1 Distribution of 500hPa geopotential height anomalies in Northern Hemisphere
in Mar. 2012 (27 days)
Fig. 2 Daily variations of Siberian high index
Fig. 3 Accumulated decreasing amplitude of minimum surface temperature for the three cold surges intruding China ( 1-2(top), 17-18(middle) and 23-24(bottom)) in March, 2012
Fig. 4 Distribution of monthly mean surface air temperature anomalies in March, 2012 (28 days)
Fig. 5 Distribution of 850hPa wind anomalies March, 2012 (26 days)
Fig. 6 Distribution of precipitation anomalies inMarch, 2012 (28 days)