Coupled Southward Propagation of Intraseasonal Variations in the South China Sea during Boreal Winter

Abstract:

Eastward and northward propagation of intraseasonal signals is a prominent feature during boreal winter over the tropical Northern Hemisphere. Here, an obvious southward propagation is revealed over the South China Sea (SCS) in sea surface temperature (SST), surface wind, and latent heat flux anomalies. The SST in the SCS displays prominent intraseasonal variations during boreal winter with a spectrum peak in the 10-30-day time period. These intraseasonal SST variations are closely associated with intraseasonal variations of the East Asian winter monsoon (EAWM). A weak EAWM is preceded by cooler SST and followed by warmer SST in the SCS and subtropical western North Pacific. The SST and surface wind anomalies display a coherent southward propagation over the SCS during boreal winter. This southward propagation is attributed to the wind-evaporation-SST effect under climatological northerly winds in winter, which differs from summer when climatological winds are westerly. SST-induced wind speed anomalies are larger to the north side of SST anomalies. This induces larger surface evaporation anomalies to the north side, leading to a southward displacement of large SST anomalies. In turn, wind and evaporation anomalies move southward. There appears to be a positive feedback between circulation and precipitation that leads to amplification of meridional wind anomalies when the SST anomalies are weak. Surface latent heat flux is a dominant factor for the SST change in the SCS and the Yellow Sea. Shortwave radiation has a complementary contribution in the SCS, but has a negative effect in the Yellow Sea. The wind-induced Ekman advection appears important for the SST warming in the Yellow Sea.