Comparison of the Annual Cycles of Moisture Supply over Southwest and Southeast China

Abstract

Abstract The variation in regional precipitation over southeast and southwest China depends strongly on externally imported moisture rather than local evaporation. Associated with the different climate over the two regions, great discrepancies appear in the annual cycles of the moisture supply. Stationary moisture transport dominates externally imported moisture to a large extent, with transient transport being much weaker. The stationary moisture sink over southeast China is strong during spring and summer due to strong moisture input via the southern boundary and weak during fall and winter due to the offset between the output via the southern boundary and the net zonal boundary atmospheric flux. Zonal stationary moisture transport dominates the variation in moisture supply over southwest China. Negative net zonal boundary atmospheric flux countervails (collaborates) with positive meridional transport during the dry (wet) season. Stationary moisture circulations dominate regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while the reverse appears over southwest China in spring. The east-to-west location of the abnormal anticyclone determines whether strong moisture converges over southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, remotely forced by El Niño, is crucial to the strong moisture sink over southeast China from winter to spring, while it does not play a role in the abnormal moisture sink over southwest China.