Interannual Variability of Summer Monsoon Onset over the Western North Pacific and the Underlying Processes

Climatological summer monsoon onset over the South China Sea (SCS) and the western North Pacific (WNP) (defined as the region of 108–208N, 1208–1608E) displays three distinct stages. Around mid-May, monsoon rain commences in the SCS and the Philippines. In early to mid-June, the monsoon rain extends to the southwestern Philippine Sea. After mid-July, the rainy season starts in the northeastern part of the WNP. The onset anomaly, however, displays an in-phase interannual variation across the entire WNP domain. The standard deviation of the onset date increases eastward from 3 pentads in the SCS to 5 pentads in the northeastern part of the domain. The large onset variability in the WNP is mainly attributed to large year-to-year changes of the seasonal cycle. The role of the intraseasonal oscillation is secondary but important especially in the SCS region. The El Niño– Southern Oscillation (ENSO)-related thermal contrast between the WNP and the equatorial central Pacific modulates significantly the seasonal migration of the monsoon trough, the subtropical high, and the convection zone over the WNP during late spring–early summer in the ENSO decay phase. Thus, ENSO plays a dominant role in the interannual variation of the WNP summer monsoon onset. The general circulation model results suggest that during El Niño events, the warm SST anomalies in the equatorial eastern-central Pacific play a major role in generation of large-scale upper-level convergence and descent anomalies over the WNP. Meanwhile, the cold SST anomalies in the WNP induce lower-level anticyclonic wind anomalies and reduce convective instability. Both the remote and local SST forcing are important for delaying the seasonal movement of the monsoon trough and the western Pacific subtropical high and hence the onset of the monsoon rain. In the La Niña case, the local warm SST anomalies in the WNP are more important than the cold SST anomalies in the equatorial eastern-central Pacific in the generation of lower-level cyclonic wind anomalies and enhancement of convective instability.