The Asian Monsoon System

The word "monsoon" derives from the Arabic mawsim, meaning "season," a term Arab traders applied to the reversing winds of the Indian Ocean that Gulf sailors had long used to time voyages east in one season and west in the other. The physical explanation for this seasonal wind reversal was proposed as early as 1686 by the English astronomer Edmond Halley, who argued in a paper to the Royal Society that differential heating between the Asian landmass and the surrounding ocean drives the system: during the Northern Hemisphere summer, the interior of Asia, and especially the elevated Tibetan Plateau, heats far more rapidly than the adjacent Indian Ocean, creating a large area of low pressure over land that draws moist, southwesterly winds inland off the warmer ocean surface — the summer, or southwest, monsoon. In winter the pattern reverses: the Asian landmass cools faster than the ocean, high pressure builds over the continent, and dry, northeasterly winds flow outward from land to sea, producing the largely dry winter monsoon across most of South Asia. Modern research has refined Halley's land-sea contrast theory considerably, showing that the elevated heating of the Tibetan Plateau itself, acting as a high-altitude heat source, and the associated shift of the subtropical jet stream play a more decisive dynamical role than simple lowland heating contrast alone.