J. Cilmate, submitted, May 1999

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

R. Wu

B. Wang
Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Climatological summer monsoon onset over the South China Sea (SCS) and the Western North Pacific (WNP) (defined as the region of 120-160E, 10-20N) displays three distinct stages. Around mid-May, monsoon rain commences in the SCS and Philippines. In early- to mid-June, the monsoon rain extends to 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 interannual variation of the onset in the WNP is highly correlated with the normalized sea surface temperature (SST) anomaly difference between the WNP and the equatorial central Pacific in the previous winter and spring.

The general circulation model results indicate that during El Nio 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 Nia case, the local warm SST anomalies in the WNP are more important than the cold SST anomalies in the equatorial eastern-central Pacific in generation of lower-level cyclonic wind anomalies and enhancement of convection instability.

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