The Indian Monsoon

A monsoon seasonal change is characterized by a variety of physical mechanisms which produce strong seasonal winds, a wet summer and a dry winter. The main characteristics are differences in land/sea temperatures, location within the ITCZ region, and intense convective storms.

1. In March and April the Indian sub-continent begins to heat up, so by May some of the highest surface temperature of the year occur.

- a large difference between land surface temperature and sea surface temperature, resulting in a reversal of winds from seaward (towards the sea) to land-ward (towards the land).

- large low pressure cell exists over southwest Asia, intensified by the location of the Himalayas and Hindu Kush mountains which trap warm air within the Indian ocean basin. This low pressure cell along with the Earth's Coriolis force cause intense winds to blow from the southwest.

- northward shift in the ITCZ, causing a magnification of the winds. As the winds cross the Indian ocean they pick up moisture which is released (usually beginning in early June) as they pass over the sub-continent.

- topography of the sub-continent traps moisture within the region, producing tremendous convective storms.

2. In the winter a reverse process occurs. The land surface cools faster than the sea surface as a result of water's capacity to retain heat. This reverses the winds (coincides with the southward ITCZ) so that they are seaward, containing very little moisture over the Indian sub-continent. As a result the winters are very dry in India.

http://www.crseo.ucsb.edu

The Figure shows a 3-D perspective of flow during the Indian Monsoon in June 1988. The fields are from the GEOS-1 data assimilation system. The view is from the South Indian Ocean looking north. The Indian subcontinent is in the center and Maylasia is to the right and Africa to the left. High topography is in brown and lower elevations are green. The arrows show near-surface winds. The strong winds blowing from Africa, south of Arabia and onto the western shore of Indian represent the Somali jet. This wind current brings moist sea air into the subcontinent. The yellow and blue ribbons are two air parcel trajectories. They show that air of maritime origin from the eastern Indian Ocean and western Pacific, as well as air descending over Arabia converge in the Somali jet. The white shows energy released as the air ascends and gives up its moisture to precipitation. The trajectories show a small event as they cross the Ghats Mountains on the west Indian coast. Later along the trajectory, over eastern India and Bangladesh, the trajectories are nearly vertical and move together to the top of the troposphere. The air then moves westward in the upper troposphere.

Nasa

http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/atmospheric_dynamics/ad_images_dao_animations.html

Monsoonal variations in the Indian Ocean give rise to seasonal changes in phytoplankton concentrations. Here, following a period of pre-monsoon calm (May-June, composite, left), strong summer southwesterly monsoon winds generate upwelling of nutrient-rich waters, leading to the development of bloom conditions (September-October composite, right).

Hydrology of the Indian Ocean

http://www.stormsurf.com/page2/links/indiacurrnt.html

The monsoon starts over Southeast Asia early in March and April. Heavy monsoon rainfall is also found over South China before a monsoon sets up over the Indian monsoon region. In the middle of May, cross-equatorial winds become pronounced in association with heavy convective rainfall in the southern tip of the Indian subcontinent. The entire Indian monsoon region experiences the most rapid transition of circulations in May.

Strong low-level winds, the so-called Somali Jet, are continuously intensified throughout June, while progressing northward steadily. The heavy rainfall near the west coast of the Indian subcontinent is associated with high mountain terrain and abundant moisture supply transported by the Somali Jet. The northern part of the Bay of Bengal is known to have about the heaviest rainfall in the world during the summer monsoon period. The low-level cyclonic circulation east of the mountains may provide a favorable condition for the development of tropical depressions, where the westward propagating disturbances from the western Pacific are often intensified. The source of moisture for the heavy rains is not clear yet. However, the moisture evaporated from the Arabian Sea and the South Indian Ocean appears to be an important source.

In June, the monsoon rainbands continuously march northward over the East Asian region as well. While the total precipitation amount is not comparable to the tropical rainfall, the concentrated rainfall during the monsoon period, usually no longer than two weeks, is crucial for the agriculture over that region. The East Asian monsoon has different names as it progress northward; Mai-Yu in China, Baiu in Japan, and Changma in Korea. The failure of the monsoon often results in devastating drought over these countries.