DYNAMICS RESPONSE OF SOUTH CHINA SEA

TO SEASONAL CHANGES OF MONSOON image

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].