Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea
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Authors
Chang, C.-P.
Chen, George T.-J.
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1995-11
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Abstract
The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea.
This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over
southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated
with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF
data for 1981-86 are used to study the tropical circulations associated with the development of low-level
westerlies during both events.
Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and
nine lune surges, respectively, indicate that both events occur with the approach of a midlatitude trough-front
system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may
explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring,
this is the only Asian monsoon region where midlatitude fronts can move into the Tropics without having to
overcome significant terrain barriers.
Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres.
During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension
of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over the
Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection
is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling
the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection
and propagates westward to dominate the flow changes over the Bay of Bengal and the southern Indian Ocean.
The steady 4-5 m S-I westward speed and near-perfect symmetry with respect to the equator indicate the
possibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northern
South China Sea.
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This research was supported by the National Science Foundation under Grant ATM- 9106495 and the Office of Naval Research under Contract N0001495WR30039AOl. The second author was also supported by ROC National Science Council Grant NSC84-2111M002021AP2.
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Monthly Weather Review, Vol. 123, pp. 3254, November 1995
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.