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TO SEASONAL CHANGES OF MONSOON
In the upper figure, ocean-surface wind vectors, represented by arrows,
are superimposed
on the curl of wind stress (CWS), represented by color image;
both parameters are derived from observations by spaceborne
scatterometers. In
November (climatological mean), South China Sea (SCS) is dominated by
the winter monsoon
blowing from the northeast which causes positive (cyclonic or
anticlockwise) CWS in the central basin,
with a stronger center west of Luzon Island and a weaker center east of
Vietnam. Strong negative
CWS is found along the Chinese coast. In the lower figure, sea level
changes (SLC),
represented by color image,
are shown together with the change of geostrophic surface current,
represented by arrows;
both parameters are derived
from spaceborne altimeters. In following month, December, negative SLC
are observed
in the central basin,
stretching from Luzon to Vietnam, and positive SLC are found in the
coastal regions
in the north and in the south. The SLC lead to
two cyclonic gyres of geostrophic current, one west of Luzon and the
other off the southern tip
of Vietnam. The opposite behavior (winds from the southwest, negative
CWS, and
positive SLC) is observed in summer. The negative correlation between
CWS and SLC
is consistent with the classic Ekman pumping scenario.
Cyclonic CWS drives surface divergence and upwelling in
the ocean; the rise of the thermocline causes lower sea levels.
Anticyclonic CWS causes higher
sea level.
Monsoons are the seasonal changes of winds forced by continent-ocean
temperature contrast. A
large percentage of the world's population and their agrarian economy
must endure the vagaries
of these monsoons. Besides bringing rain to land, monsoons also change
ocean currents and
upwelling. Over land the consequences of monsoon are, perhaps, well
observed, but the oceanic
responses have not been sufficiently monitored. The lack of observations
is particularly evident in
the SCS, where, until recently, conflicting territorial claims by
neighboring countries
have made research expedition extremely difficult. SCS is a semi-closed
ocean basin whose circulation is
largely driven by the seasonal change of wind.
SCS is located at the cross-road of major climate
systems. It is situated between the land masses of Asia and Australia
and between the warm pool
of the western Pacific and Indian ocean. The interaction between the
ocean and atmosphere in this
area has significant climate impact, and spacebased observation is the
ocean and atmosphere in this
area has significant climate impact, and spacebased observation is the
best, if not the only,
mean of revealing processes illustrated in the figure.
[from Liu, W.T. and X. Xie, 1999: Spacebased observations of the
seasonal changes of South
Asian Monsoons and oceanic responses. Geophys. Res. Lett., 26,
1473-1476].
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