Zonal Oscillation of Western Pacific Subtropical High and Subseasonal SST Variations during Yangtze Persistent Heavy Rainfall Events

Thursday, 18 December 2014
Xuejuan Ren, Xiu-Qun Yang and Xuguang Sun, Nanjing University, Nanjing, China
This study examines the relationship between zonal oscillation of western Pacific subtropical high (WPSH) and underneath sea surface temperature (SST) variation on subseasonal time-scale, associated with the persistent heavy rainfall (PHR) events over the middle-lower reaches of the Yangtze River Valley (MLYRV) in China. A total of 76 PHR events and 45 break events in the summers of 1979-2011 are firstly identified over the MLYRV, and divide into early and late summer groups. During the PHR events over the MLYRV for both groups, the WPSH stretches more westward, accompanied by the positive anomalies of 500-hPa geopotential height field over East Asia and its coastal region south of 30ºN, and the subseasonal warmer SSTs beneath the WPSH western edge. The time-lagged composites suggest that the WPSH western edge exhibits westward-then-eastward migration on subseasonal time-scale for the PHR events. The zonal changes of the WPSH and anomalous circulation and SSTA signals for break events is almost the mirror image of that for PHR ones for the early summer group. Accompanied by the WPSH westward extension, the increased incident solar radiation and decreased latent heat flux over the coastal region of East Asia contribute to the positive SSTAs beneath the western part of the WPSH. The positive SSTAs construct a convective instability that provides adverse condition for maintaining the anti-cyclonic anomalies in the mid-lower levels. The persistent SST warming is also favorable to the transition of low-level circulation from anti-cyclonic to cyclonic anomalies over the coastal region. Resultantly, the WPSH withdraws eastward after the peak of the rainfall events over the MLYRV.