12 May 2003

Onset of this year's South China Sea Summer Monsoon

Tropical cyclone 1B has formed over the Bay of Bengal.  As it continues to move northwestward, and then northward, it is likely to make landfall over Bangladesh, according to the ECMWF forecast (Fig. 1).  Previous studies have suggested a few days after the decay of such a cyclone, onset of the South China Sea (SCS) summer monsoon is likely to occur (see e.g. Liu et al. 2002 and the references within).  The main idea is that the southwesterlies associated with this so-called onset cyclone would weaken the subtropical high present over the SCS.  With a triggering mechanism that could come from a vortex to the north or disturbances from the east, the onset occurs.  However, the ECMWF 500-hPa forecast chart for 18 May at 12 UTC still indicates a ridge over the SCS (Fig. 2) although a low is predicted to develop east of the Philippines.  Therefore, it may still be a few days after 18 May before the summer monsoon over the SCS will likely begin.  When that occurs, it will signal a complete switch over the circulation from the winter to the summer pattern, at least over Asia.

Future discussion will focus on the evolution of Tropical cyclone 1B and the subtropical high over the SCS.

Reference

Fig. 1.