monsoon

monsoon, a seasonal wind in the Indian Ocean, generated by the large land mass to the north of the ocean, which heats up in summer and cools down in winter faster than the sea. The cycle of monsoon winds dominates the weather of the Indian subcontinent and South-East Asia, and strongly influences the ocean. When the land is hot, the air over the continent rises, and strong onshore winds develop carrying moisture-laden air that deposits torrential monsoon rains. The foothills of the Himalayas and the Ghats may receive as much as 10 metres (32.8 ft) of rain. When the continents cool down the winds reverse and blow offshore, and the climate becomes arid. Over the north-west Indian Ocean the wet south-west monsoon blows from May until September steadily at force 7–8 on the Beaufort Scale. As these winds blow parallel to the Arabian coastline, the effects of Coriolis force means they push surface water offshore and generate a strong seasonal upwelling.

Off Somalia a huge current gyre, the Great Whirl, develops creating surface currents as fast as 7 knots. Locally sea surface temperatures may fall to less than 15 °C (59 °F), and temperature fronts develop across which the sea temperature may change by 7 °C. The cool upwelled water is rich in nutrients and stimulates intense blooms of algae. These algae become too abundant for the grazing plankton to eat them all and as they die, they sink down, and their decomposition uses up all the oxygen in the water. In October, after a brief windless period, the winds start to blow from the north-east. The climate reverts to being hot and arid, the upwelling ceases, and the sea becomes unproductive.

On the equator, the reversing winds reverse the flows of the equatorial currents. These reversing winds were first described by Arab navigators in ad 800 who were regularly sailing dhows from the Persian Gulf and Oman to their trading posts along the east coast of Africa at Zanzibar, Mombasa, and Dar es Salaam during the north-east monsoon, and returning on the south-west monsoon with slaves and spices.

http://yang.gmu.edu/~yang/nasacd/www/indian_monsoon.html

M. V. Angel