Mechanisms of Northward-Propagating Intraseasonal Oscillation over the South China Sea during the Pre-Monsoon Period

Abstract

AbstractIn the present study, the spatiotemporal structures of the northward-propagating intraseasonal oscillation (ISO) over the South China Sea (SCS) in the premonsoon period are analyzed by using the TropFlux air?sea flux and the JRA-55 reanalysis datasets. It is found that the SCS ISO is significant in the premonsoon season with a strong component of the northward propagation and that the mean state is different from that of summertime. Moreover, there are similar structures to those of a boreal summer ISO event except for the perturbation vorticity with no obvious phase leading. An internal atmospheric dynamics mechanism is proposed to understand the cause of the northward propagation of the ISO during the premonsoon period based on the spatial and temporal structures of the ISOs. The key process associated with this mechanism is the barotropic vorticity advection by the mean barotropic southerly winds, and the main barotropic vorticity around the convection center can be induced by the vertical advection of the mean vorticity. Low-level moisture convergence caused by anomalous flow is a supplementary mechanism to drive the ISOs northward during the premonsoon period, particularly over the northern SCS. In this mechanism, the SST-induced wind anomalies play a more important role than the convection-induced wind anomalies. The summer monsoon circulation has not built up during the premonsoon period, and thus the vertical wind shear effect and the barotropic vorticity effect associated with the meridional advection of baroclinic vorticity are not essential to cause the northward propagation of the ISOs over the SCS.