Variation of Sea Surface Height in the South China Sea
Yinglai Jia(1) and Qinyu Liu(2)
Physical Oceanography Lab.,
Ocean University of China, No.5 Yushan RD.,
(2) physical oceanography Lab., Ocean University of China,No.5 Yushan RD., 266003,Qingdao, China
The dominant forcing mechanisms of Sea Surface Height (SSH) variations in the South China Sea are the variations of the sea surface wind and bouyancy flux as well as the impacts of the Kuroshio.
Analysis of the seasonal variation of the SSH using TOPEX/POSEIDON-ERS altimeter data along with Levitus steric height, COADS wind stress curl, and POM model result indicates that the pattern of the SSH anomaly in winter is dominated by a negative SSH anomaly with two centers in the deep water basin. The north center (16°N,115°E) corresponds to the Luzon Cold Eddy, while the south one (10°N,112.5°E) corresponds to the cyclonic eddy east of Vietnam coast. Geostrophic current calculated from the SSH anomaly indicates a basin wide cyclonic gyre with strong southward western boundary current. The latter leads to the cold tongue of the SST on the Sunda Slope. In summer, the SSH pattern changes to positive SSH anomaly with two centers in the deep water, located in the northwest of the Philippines and the southeast of the Vietnam coast, respectively. Between both positive centers, there is a negative SSH anomaly due to the upwelling induced by the southwestly monsoon and the cyclonic wind stress curl east of Saigon. Dynamical analysis show that the seasonal variation of the SSH mainly depends on the wind stress curl of monsoon in the south of the 18°N, because the annual baroclinic Rossby waves cross the basin in less than a few months and the upper ocean is in a quasi-steady Sverdrup balance in seasonal or longer time scale.
Unlike the forced variation of the SSH by the wind in central SCS, the SSH variation north of 18°N is more complicated in the SCS. About 30% or more of the variation of the SSH in the north SCS is forced by the Kuroshio. The Kuroshio bends into the SCS through Luzon Strait, forming anticyclonic eddies quasi-periodically around 119.5°E and 120°E with a frequency of 70-90 days.. The eddy shedding is not a product of the local wind stress curl but rather the intrinsic dynamics of the Kuroshio bending. The frontal instability in the south of the Kuroshio bend fosters the growing up of a cyclonic eddy which cleaves Kuroshio bend and triggers the separation of the anticyclonic eddy. As there are generally four or five eddies shed within a year, the mass or momentum transport into the SCS by these eddies may play a very important role in the intraseasonal, seasonal, or even interannual variation of the SSH in north SCS.. Future studies are needed to assess these impacts.
With the accumulation of the TOPEX/POSEIDON, ERS-1 and Jason-1 altimeters’ observation, inter-annual, even decadal variations of the SSH in the SCS can be studied in the future. With the improved observation of the Geoid, the mean structure of the circulation in the SCS can also be obtained, that will help the study of the Kuroshio and the western boundary current in the SCS.