Abstract
The Monsoon
Among the first Europeans observing the
Asiatic monsoon was Alexander the
Great during his campaign to the mouth
of the Indus (325 B.C.). The oldest
known records of the Arabian Sea
monsoonal climate, however, are
shipping documents, dated about 2300
B.C., which refer to the use of the
seasonal changing winds by traders
[Warren , 1987]. The monsoon
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has
always played an important role in the
live of the sailors inhabiting the coasts of
the Arabian Sea, because they depended
for their trade on the seasonal change in
wind direction. The Arabic word
'mausim', which means season, is also
the origin of the term monsoon.
The monsoon cycle is caused by the
annual shifting position of the Inter
Tropical Convergence Zone (ITCZ)
which oscillates between approximately
the tropics of Capricorn and Cancer,
following the region of maximum solar
heating. The ITCZ results from the rise
of warm humid air, which cools with
increasing height causing heavy rainfall
in the tropics. The latent heat released
during cloud formation and rainfall
provides an additional heat source for the
ITCZ. The resulting surface air flow
towards the ITCZ is deflected by the
Coriolis force, causing the characteristic
trade wind pattern. The yearly course of
the sun causes large temperature
differences on the continents, whereas
differences in the ocean are relatively
small. This differential heating originates
from the difference in heat capacity
being smaller for land than for sea, and
results in the breakdown of the
subtropical high pressure cell and the
formation of a low pressure cell over the
continent during summer. During winter
the SUbtropical high pressure cell restores
causing a reversal in wind direction.
Thus, continents in the tropical region
and adjacent ocean experience a semiannual
reversal in wind direction, which
is termed monsoon. Strength and
direction of the monsoon is influenced by
the position and size of the continents.
The Indian monsoon is particularly
strong due to height and position of
Himalayas and Tibetan Plateau, and the
supply of latent heat from the Indian
Ocean.
The agriculture in India and China, the
two countries in the world with the
largest population, depends heavily on
the rainfall during the summer
monsooon. During the boreal summer
heavy monsoonal rains fall in India,
southeast Asia and southern China.
Locally they are orographically amplified,
so that, for example in parts of
Assam, in the three monsoon months of
June, July and August, precipitation
averages more than 1200 em. Variability
in the intensity of the monsoonal rains
has resulted in extreme drought, with
failing crops, or extreme rainfall, resulting
in catastrophic floods. For example, the failure in the late Holocene of regular
monsoonal rains about 3,500 yr. BP. related
to astronomical forced climate
change- is thought to have caused the
collapse of the flourishing Harrapan
culture in the Indus valley [Bryson and
Swain, 1981]. Until this decrease in
monsoonal rainfall wheat, barley,
melons, and perhaps cotton were grown
in what is now the Thar desert of
Radjasthan [Lamb, 1995].
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