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.
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
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.
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