Interdecadal change in the South China Sea summer monsoon withdrawal around the mid-2000s

Abstract

In this study, a significant interdecadal change in the South China Sea (SCS) summer monsoon (SCSSM) withdrawal, which occurred around the mid-2000s, is revealed using NCEP-DOE reanalysis, CMAP rainfall, and OLR data. The withdrawal of the SCSSM occurred much later (about 2 weeks) after the mid-2000s. The westerlies and rainfall are stronger around the SCS during the period after the mid-2000s compared to those that occurred before the mid-2000s, which is consistent with the delayed SCSSM withdrawal. The robust and significant increases in rainfall and convection around the SCS and Philippine Sea are dynamically associated with the appearance of an anomalous low-level cyclone around the northern SCS, and the anomalous westerlies at approximately 10°N extend eastward from the Indo-China Peninsula to the western North Pacific (WNP). Anomalous mid-level ascending motion and upper-level divergence were also observed around the Philippine Sea, together with anomalous descending motion and upper-level convergence over the equatorial eastern Indian Ocean. Correspondingly, an anomalous zonal circulation formed between the WNP (upward motion) and eastern Indian Ocean (downward motion). Further analysis indicates that the increasing number and frequent visits to the SCS by the tropical cyclones and enhanced quasi-biweekly oscillation activities may both contribute to the delayed SCSSM withdrawal around the mid-2000s.