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Regional Surface Temperature & Wind Fields (quicktime movie, 9.25 mb)
Regional Precipitation & Wind Fields (quicktime movie, 9.25 mb)
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Monsoon is a term originally coined by Arab mariners in reference to the seasonally shifting winds in the Indian Ocean and surrounding regions, including the Arabian Sea. The Indian ocean monsoon winds blow from the southwest during summer (wet) and from the northeast in winter (dry). The Asian monsoon, which affects the Indian subcontinent and southeast Asia, is the best-known example, although monsoonal climates are also found in tropical regions of Australia, Africa and Central America.
The Monsoon occurs as a part of larger phenomenon characterized by the InterTropical Convergence Zone (ITCZ). The ITCZ separates the wind circulations of the northern and southern hemispheres. This zone moves north and south with the annual changes of the sun's declination, and is defined where the northeast and southeast trade winds flow together. This region is characterized by strong upward motion and heavy rainfall, as a result of intense heating by the direct rays of the Sun. The heating also results in a three-dimensional atmospheric circulation referred to as a Hadley Cell. The Hadley cell consists of rising air at the equator and descending air at 30 degrees north and south. This results in strong equatorial surface winds which blow from the east due to the Earth's Coriolis force. The Indian Monsoon is unique in that the winds blow from the southwest.
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.
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. This dramatic heat-up causes 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). During the monsoon period a 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. This coincides with the 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. The orography of the sub-continent traps moisture within the region, producing tremendous convective storms.
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.
It is the combined effect of these mechanisms which produces the monsoon's characteristic reversals of high winds and precipitation. In the case of the Indian Ocean Monsoon the land/sea heat differential and intense convection, as a result of orography, produce more intense effects than any other place in the world. Of particular interest is the "wet summer" phase from June to September with prevailing winds from the southwest and heavy rainfall.
The strong Indian summer monsoon years are generally associated with positive tropospheric temperature anomalies over Eurasia and negative temperature anomalies over the Indian Ocean and the Eastern Pacific but positive sea surface anomalies in the Western Pacific. Several connections between Eurasian snow cover, the Indian Monsoon and the El Niño /Southern Oscillation (ENSO) have been established. Based on 80 years of data, major droughts have been associated with warmer than normal sea surface temperatures (SST) in the equatorial Eastern Pacific for time periods spanning a monsoon season. Floods, on the other hand, have been associated with cooler SST events in the tropical Eastern Pacific. Also, anomalously high winter Eurasian snow has been linked to weak rainfall in the following summer Indian monsoon.
The monsoon is one of the most dramatic climate phenomena on the planet. The large areas involved in monsoons and the grand scale of the weather within monsoons suggest that they play a significant role in modulating global climate.
With a population of well over one billion people, on the Indian sub-continent a failure in the monsoon can have extreme negative regional and global human and economic effects, as a result of crop failure. A too intense monsoon can also have extreme negative effects as a result of sever flooding, crop destruction and the displacement of thousands or even millions of people.
Some key parameters for monsoons include:
Bodies of water have a much higher capacity for storing heat than do land surfaces. The same amount of solar radiation will heat up the ground more than it will a body of water. Heat absorbed by a body of water is also distributed, through mixing, over a greater depth than is the heat absorbed by land surfaces. Air expands and rises in regions of higher surface temperature, producing lower surface pressure. The air in regions of low pressure is replaced by cooler air from regions of high pressure.
If the region of low pressure is over a continent and the region of high pressure is over an ocean, the air over the continent is replaced by moist air (high water vapor) from the ocean. When the moist air passes over the hot land it heats up and rises. As it rises it cools reaching the dew point where precipitation is possible. If the land is elevated the air currents are forced upward even more and the cooling process is intensified. This results in increased precipitation.
Allan, R.J., J.A. Lindesay, & C.J.C. Reason, 1995 : Multidecadal Variability in the Climate System over the Indian Ocean Region During the Austral Summer, J. of Climate, 8(7), 1853-1873.
Li, C., and Yanai, M., 1996 : The Onset and Interannual Variability of the Asian Summer Monsoon in Relation to Land-Sea Thermal Contrast, J. of Climate, 9(2), 358-375.
Chowdhury, A.; Mhasawade, S.V. : Variations in meteorological floods during summer monsoon over India, Mausam, 42(2), 167-70.
Garnett, E. R.; Khandekar, M. L. 1992 : The impact of large-scale atmospheric circulations and anomalies on Indian monsoon droughts and floods and on world grain yields - a statistical analysis, Agricultural & Forest Meteorology, 61(1-2), 113-128.
Khandekar, M.L., 1991 : Eurasian Snow Cover, Indian Monsoon and El Nino/Southern Oscillation - A Synthesis", Atmosphere-Ocean, 29(4), 636-747.
Whetton, P.; Rutherfurd, I. : Historical ENSO teleconnections in the eastern hemisphere, Climatic Change, 28(3), 221-53.
Vernekar, A.D., Zhou, J., & Shukla, J., 1995 : The Effect of Eurasian Snow Cover on the Indian Monsoon", J. of Climate, 8(2), 248-266.
Zhu, Y., & Houghton, D.D., 1996 : "The Impact of Indian Ocean SST on the Large-Scale Asian Summer Monsoon and the Hydrological Cycle", International J. of Climatology, 17, 617-632.
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Last update:Wed Oct 1 12:30:42 EDT 1997