DEFINITION OF MONSOONS

A wind system that changes direction with the

seasons is known as a monsoon, after the Arabic term

mausim, meaning seasons. The Arabian Sea is characterized

by monsoons, with the wind blowing from the northeast for

six months, then from the southeast for the other half of the

year. Seasonal reversal of winds is probably best known from

India and southern Asia, where monsoons bring seasonal

rains and floods.

The Asian and Indian monsoon originates from differential

heating of the air over the continent and ocean with the

seasons. In the winter monsoon, the air over the continents

becomes much cooler than the air over the ocean, and a large,

shallow high-pressure system develops over Siberia. This produces

a clockwise movement of air that rotates over the South

China Sea and Indian Ocean, producing northeasterly winds

and fair weather with clear skies over eastern and southern

Asia. In contrast, in the summer monsoon the air pattern

reverses itself as the air over the continents becomes warmer

than the air over the oceans. This produces a shallow lowpressure

system over the Indian subcontinent, within which

the air rises. Air from the Indian Ocean and Arabian Sea

rotates counterclockwise into the low-pressure area, bringing

moisture-laden winds into the subcontinent. As the air rises

due to convergence and orographic effects, it cools below its

saturation point, resulting in heavy rains and thunderstorms

that characterize the summer monsoons of India from June

through September. Some regions of India, especially the

Cherrapunji area in the Khasi Hills of northeastern India,

receive more than 40 inches (1,000 cm) of rain during a summer

monsoon. A similar pattern develops over Southeast Asia.

Other less intense monsoons are known from Australia, South

America, Africa, and parts of the desert southwest, Pacific

coast, and Mississippi Valley of the United States.

The strength of the Indian monsoon is related to the El

Niño–Southern Oscillation. During the El Niño events, surface

water near the equator in the central and eastern Pacific

is warmer than normal, forming excessive rising air, thunderstorms,

and rains in this region. This pattern causes air to

sink over eastern Asia and India, leading to a summer monsoon

with much lower than normal rainfall totals.

See also EL NIÑO.

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