submitted, May 1999
Interannual Variability of Summer Monsoon Onset
over the Western North Pacific and the Underlying Processes
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 120-160øE, 10-20øN) 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 Ni¤o 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 Ni¤a 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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