A Numerical Study of Ocean Circulation in the South China Sea

               

Principal Investigator

XH Wang

Geography & Oceanography

Australian Defence Force Academy

University of New South Wales

Projects

h12 - VPP

The South China Sea (SCS) is the largest marginal
sea in Southeast Asia which covers an area of 3.5
million square kilometres. The semi-enclosed ocean basin is bordered by eight countries of significance to our region, including China, Vietnam, Cambodia, Thailand, Malaysia, Brunei, The Philippines and Taiwan. Supported by the available data sets of the Southern China Sea, this project aims to develop a high resolution three-dimensional ocean model to study dynamics of the SCS, and its exchange with the waters of western Pacific Ocean. This project is considered to be important as it would establish a milestone in understanding and predicting the natural variability of the SCS, its impact on local climate of the Southeast Asia and the interations of the two, if the project is successful. This project is also considered to be important as the Southeast Asia and its marginal sea is a strategic region to Australia.
   
         
               

     
               

What are the results to date and the future of this work?

A three-dimensional, sigma-coordinate primitive equation model (Princeton Ocean Model) with a free surface is used to simulate the monthly mean circulation in the South China Sea(SCS). The model has a resolution of 0.5 deg in the horizontal and 10 sigma layers in the vertical in a region from equator to 27 deg N and from 98 deg E to 125 deg E. There are four openings in the model region, where the inflow and outflow in the Kuroshio, through the Taiwan Strait and Karimata Strait are obtained from the model output of the Parallel Ocean Climate Model(POCM). At the sea surface, the model is forced by monthly averaged climatological wind stress, heat and salinity flux.

Several important features are reproduced in the model results.

· First, for the vertical integrated circulation(VIC), there is a large cyclonic gyre dominating the whole basin in winter, in which a meso-scale cyclonic eddy named Luzon cold eddy(LCE) appears to the northwest of the Luzon island. In spring the large cyclonic gyre becomes weak due to weak wind stress forcing and the LCE moves northwestward. A weak anticyclonic gyre begins to appear in the central SCS(10 deg N-14 deg N) in March. In summer, the VIC is cyclonic in the north half basin and anticyclonic in the south half and the LCE disappears. In fall, the cyclonic gyre expands southward and then occupies the whole basin.

               
- Appendix B

 
               

       

· Second, the surface current represents the Ekman transport, the direction of which is nearly perpendicular to that of surface wind. From the sea surface temperature (SST) distribution, there is upwelling between 11 deg N to 13deg N off Vietnam in summertime. Third, in the lower layer, the thermal-wind relation is nearly met between the temperature distribution and the circulation pattern due to relatively weak influence of the surface wind and solar radiation. For example, the LCE corresponds to a strong upwelling whose center temperature is lower than that of the surroundings by about 2 to 3 degrees. In winter and spring, there is warm water appearing in the central SCS, which corresponds to an anticyclonic gyre there. While in the south of the SCS, there is a cyclonic gyre corresponding to cold water.

· Finally, the LCE can reach at depth of 200m. Its generation and maintenance should be mainly due to the local wind stress curl. The Kuroshio can not flow directly into the SCS interior but forms an anticyclonic loop in the Luzon Strait, where a small part of Kuroshio water can flow out of the Taiwan Strait along the west coast of the Taiwan island.

Future work

· In order to resolve small scale eddies and examine the effect of ocean bottom topography, a higher resolution SCS model is needed. We intend to improve the exisiting model resolution to 1/4 deg in horizontal and 21 sigma levels in vertical. Higher resolution models will also enable us to investigate the driving mechanism of SCS Warm Current and the Vietnam Cold Eddy.

· We intend to couple ocean model with an atmospheric model in order to study the variability of the SCS and its impact on South East Asia local climate.

What computational techniques are used?

Our ocean model is based on the Princeton Ocean Model which is a 3-dimensional numerical hydrodynamic ocean model. The model is coded in FORTRAN, and vectorisation of the source code of the program reaches over 80%.

Publications

Qinyu Liu, XH Wang and Haijun Yang, A numerical study of ocean circulation in the South China Sea. 6th National Australian Meteorological and Oceanographic Society Conference, Canberra February, 1999.

       
Appendix B -