On the weakening relationship between the South China Sea summer monsoon onset and cross-equatorial flow after the late 1990s

The weakening in the relationship between the South China Sea summer monsoon onset (SCSSMO) and the low-level cross-equatorial flow (CEF) in May is investigated using the ERA-Interim reanalysis data sets during 1979-2016. The SCSSMO-SCSCEF relationship has experienced a significant inter-decadal change, and the correlation coefficient becomes weaker after the late 1990s. The correlation has shifted from the significant negative value in the earlier decade to insignificant in the later decade. This inter-decadal change is robust under several sensitive tests and largely independent of the El Nino-Southern Oscillation signal. One possible explanation is the change in mechanisms driving the SCSSMO over the course of the two periods. Before the late 1990s, the northwards march of the intertropical convergence zone, which has a close relationship with the SCSCEF, is mainly responsible for the SCSSMO. After the late 1990s, warming of the western North Pacific favours more tropical cyclones and disturbances during May. The westwards movement of these tropical disturbances would affect the SCSSMO and help explain why the SCSSMO-SCSCEF relationship became weaker in the later decade.