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    Monsoon

    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. These winds blow from the southwest during one half of the year and from the northeast during the other. There are, therefore, seasonal changes which are particularly noticed as northeast winds prevailing in the winter in the Indian subcontinent and southwest in the summer. Other monsoons occur in Australia and Africa. As monsoons have come to be better understood, the definition now denotes climatic systems anywhere in which the moisture increases dramatically in the warm season. The Asian monsoon, which affects the Indian subcontinent and southeast Asia, is the best-known example, although monsoonal climate is also found in northern Australia, West Africa, and elsewhere. The North to South presence of the Andes prevents similar phenomena in both S. and N. America. 

    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. Phenomena related to regional monsoons when precipitation is heavy are droughts where the opposite occurs. 

    A monsoon seasonal change is characterized by a variety of physical mechanisms which produce strong seasonal winds, a wet summer and a dry winter. All monsoon share three basic physical mechanisms: differential heating between the land and oceans; Coriolis forces due to the rotation of the Earth; and the role of water which stores and releases energy as it changes from liquid to vapor and back (latent heat). It is the combined effect of these three mechanisms which produces the monsoon's characteristic reversals of high winds and precipitation. In the case of the Indian Ocean Monsoon the first and third mechanisms 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 Asian 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 Nino/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. 

    Some key parameters for monsoons include: surface temperature (both land and ocean), precipitation, and wind fields.
 

Animations:

PCP-Wind-Monsoon

TG-Wind-Monsoon
 

References

Allan, R.J., J.A. Lindesay, & Reason, C.J.C., "Multidecadal Variability in the Climate System over the Indian Ocean Region During the Austral Summer", Journal of Climate, V. 8, n. 7, 95, p. 1853-1873 (????). 

Li, C., and Yanai, M., "The Onset and Interannual Variability of the Asian Summer Monsoon in Relation to Land-Sea Thermal Contrast", Journal of Climate, v. 9, n. 2, p.358-375, (Feb. 1996).

Khandekar, M.L., "Eurasian Snow Cover, Indian Monsoon and El Nino/Southern Oscillation - A Synthesis", Atmosphere-Ocean, v. 29, n. 4, 91, p. 636-747, (1991). 

Vernekar, A.D., Zhou, J., & Shukla, J., "The Effect of Eurasian Snow Cover on the Indian Monsoon", Journal of Climate, v. 8, n. 2, p.248-266 (Feb. 95). 

Webster, P.J., "Monsoons" in "the Enigma of Weather" (Scientific American, 1994). 

Zhu, Y., & Houghton, D.D., "The Impact of Indian Ocean SST on the Large-Scale Asian Summer Monsoon and the Hydrological Cycle", International Journal of  Climatology, v.17, 96, p. 617-632 (1996).


     

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    Updated May 22, 2000