High-Resolution Image

These figures show the April through June monthly mean precipitation (in mm/day) in south Asia and its surrounding oceans averaged from 1979 to 1998. The data used in generating these figures are from Xie and Arkin of NCEP/NOAA. The rainy belt stretching from southeast China to the east of Japan is called a middle-latitude storm track. In April the heavy rainy region in the tropics is called the intertropical convergence zone (ITCZ). The ITCZ strength diminishes somewhat over the maritime continent (which includes Indonesia and New Guinea). In May the ITCZ protrudes into the Bay of Bengal. The ITCZ protrusion arrives at high enough (>10º) latitude for the Earth’s rotation to turn the surface wind in the rainy region to a southwesterly direction and this protrusion is called a monsoon.

Monsoon onset is a sudden shift of the ITCZ away from the equator—at a speed of about 7º latitudes in a week. A monsoon is no more than an ITCZ removed sufficiently away from the equator—i.e., an ITCZ after a monsoon onset jump. Click here for additional information. These figures show that monsoon onset in the southern Bay of Bengal and Myanmar (formerly Burma) takes place in May. In June, monsoon onset takes place elsewhere in the Bay of Bengal (extending into southwest China), Arabian Sea and the South China Sea. By the end of June the whole rainbelt in the area stretching from the Arabian Sea to the South China Sea and further eastward, called the South Asian monsoon, is well entrenched. Notice that the South Asian monsoon diminishes over land--India and Indochina. It retreats towards the equator in September, and by October only some remnants of it can be found in the South China Sea.

(Contributed by Winston Chao)