During the summer monsoon,

• Heat is reflected off the Tibetan plateau, heating the air above it and causing it to rise. Low-level warm air also flows southward from the Arabian peninsula, the African Sahel desert, Rajasthan, and Pakistan.
• The westerly jet stream over northern India shifts north of the Himilayas. The westerly jet stream in south India changes direction and becomes an easterly.
• The Somali jet stream crosses into the Arabian Sea, changing the direction of ocean currents. Colder water welling up from lower ocean layers lowers the temperature.
• A low-pressure trough where the southern and northern trade winds meet, a border known as the intertropical convergence zone (ITCZ), moves north.
• Tropical storms, depressions, cyclones, squall lines, and daily cycles become create uncertainties.
• A low-pressure “monsoon trough” appears parallel to the Himalayan mountains in the south. The trough’s fluctuations influence when monsoon rains fall and stop.

Forces of Nature: ThinkQuest 2000 (Team #C003603)

http://library.thinkquest.org/C003603/english/monsoons/summermonsoon.shtml