A 3D Circulation Model of the South China Sea

 

Huijie Xue, Fei Chai, Neal Pettigrew

School of Marine Sciences, University of Maine, Orono, ME 04469-5741, USA

 

Danya Xu

South China Sea Institute of Oceanography, Guangzhou, China

 

Maochong Shi

Ocean University of Qingdao, Qingdao, China

 

1. Introduction

The Princeton Ocean Model is used to study the circulation in the South China Sea (SCS) and its seasonal transition in response to alternating monsoon winds, Kuroshio intrusion and river runoffs. The model has an orthogonal curvilinear grid in the horizontal with a minimum grid spacing of 10 km in the northern shelf of SCS, and 22 levels in the vertical. The model domain includes the Sulu Sea and a small area of the western Pacific east of Taiwan and Philippines. The North Equatorial Current, Kuroshio and Mindanad Current, as well as the inflows and outflows through Gasper and Karimata Strait, Sulu Archipelago and Taiwan Strait, are prescribed as the open boundary forcing.

This paper presents the results from a 15-year model integration. Time series of the transport across the Bachi Channel, Taiwan Strait, on the Guangdong Shelf, and off the coast of Vietnum suggest that the upper ocean has reached an equilibrium state. Net transport through the Bashi Channel is inward for most part of the year and reaches maximum in early winter. Strong coastal currents are found on the northern shelf and off the coast of Vietnum. The coastal current is counter-clockwise in winter, and turns clockwise in summer. Upwelling occurs off the coast of Vietnum and Fujian and East Guangdong coast in summer. The direction of the river plume is controlled by wind. In winter the Pearl River diluted water moves westward and reaches as far as Hainan, whereas in summer it moves eastward and reaches Taiwan Bank. In addition, the model generates semi-permanent eddies southeast of Hainan, near Dongsha, and south of Taiwan Bank.


2. Model description

A 3-D prognostic ocean circulation model ( the Princeton Ocean Model, POM ) is applied to the South China Sea and the neighboring regions.

2.1 Model grid

The model has 151 x 101 horizontal grid points and 22 sigma levels to map the South China Sea with realistic topography (fig.1 , fig.2 ) to 6000 metes. The maximum horizontal grid space is 50 km and the minimum is 10 km.

2.2 Initial condition and Integration

Initial condition comes from the Navy Postgraduate School 1/4 degree global model (POCM) 10 year averaged (1988-1997) January velocity u and v, temperature and salinity.


The model is started from a "hot status" without spin up. The internal time step is 720s and the external time step is 20s. The model is integrated on a DEC alpha workstation, and the result is saved every 5 days.

3. Boundary forcing

The boundary forcing data set consists of the lateral open boundary forcing , surface meteorological forcing, and river outflows.

3.1 Lateral Open Boundary Forcing

Monthly u, v, el, S and T from the POCM have been interpolated to our model grid as dynamic forcing at the open boundary. Here only the open boundary condition in January is shown.  Temperature and salinity distribution at the eastern boundary (fig.1) shows the western Pacific surface water (<10m) with lower salinity because of precipitation and subsurface water with higher temperature and higher salinity. fig.2 shows the u velocity at the eastern boundary. It is clear that in the Taiwan Strait, flows are northeastward from the surface to the bottom. Kuroshio flows out of the domain with a speed greater than 0.8 m/s above 200 meter depth southeast of Taiwan. The North Equatorial Current (NEC) is the major source of inflow. fig.3 and  fig.4 are similar but for the southen open boundary. Gasper Strait and Sulu Archipalego are shallow, whereas east of Mindanad water is very deep where Mindanad Current is the major outflow on the southearn boundary. We calculate the total transport across the open boundaries and find: the NEC varies from 30 to 45 Sv, Kuroshio is about 25 Sv, Flows in Taiwan Strait is about 1-3 sev, Mindanad Current is between 10 and 20 Sv. All these values are reasonable and they keep the total inflow and outflow in balance over a whole year.

The following boundary condition was found to work properly for the present work. Gravity wave radiation condition was applied to the velocity component perpendicular to open boundaries. An upwind-advection scheme was applied to temperature, salinity, and velocity component parallel to the boundary so that in case of inflow the boundary conditions derived from the global model were imported by inward velocities. A flow relaxation zone is applied at the eastern open boundary.

Tide is not considered in this experiment.

3.2 Surface Meteorological Forcing

Previous studies show that the circulation in the SCS is driven mainly by monsoon winds and Kuroshio intrusion, and secondly by surface heat flux. At the surface, the model is forced by monthly climatological winds, heat flux, shortwave radiation and fresh water flux( evaporation minus precipitation) from the Comprehensive Ocean and Atmosphere Data Set (COADS). The South China Sea experiences alternating summer and winter monsoon every year. During the winter monsoon (from November to March), cold northeasterly winds blow over the region as a result of the Siberian high pressure system located over the east Asian continent. Wind over the western part of the SCS is stronger. In December, northeasterly wind reaches its maximum, exceeding 10 m/s. Radiative cooling and persistent cold air advection result in a negative net downward flux between December and February. From May to September southwest  monsoon winds dominate. Warm and weaker winds blow over the region. Wind speed is about 5 m/s. Ocean absorbs heat form the atmosphere and the net downward heat flux increases form north to south.

3.3 River Runoff

There are two major rivers (Mekong and Pearl) that bring large amount of fresh water into the South China Sea. The river plume can affect the circulation on the shelf. In summer, forced by the southwesterly monsoon, the Pearl River diluted water flows eastward passing the Dongsha Island and sometimes reaches Taiwan Bank. During winter, the Pearl River plume turns westward and reaches Hainan Island. In the model, the annual mean discharge rate for Mekong and Pearl River are prescibed at15,000 m3/s and 10,000 m3/s (Perry et al.,1996). Table 1 shows the seasonal variation in the river discharge.

                                    Table 1. monthly discharge of the Mekong and the Pearl River, as a percentage to the annual discharge.
 

month

   Jan

   Feb

  Mar

  Apr

  May

   Jun

    Jul

  Aug

   Sep

  Oct

  Nov

  Dec

    %

 2.162 

 2.425 

 3.372 

 7.700 

 14.63 

 19.23 

 13.62 

 13.05 

 10.37 

 6.000 

 4.062 

 2.900 


 

4. Simulation results

The model has been integrated for 15 years and the upper ocean (above 500 m) appears to have reached equilibrium. Here are some results.

4.1  salinity,temperature and surface circulation
 
 January-0m-S
April-0m-S
July-0m-S
October-0m-S
January-0m-T
April-0m-T
July-0m-T
October-0m-T

4.2  salinity,temperature and 50m circulation
 
 January-50m-S
April-50m-S
July-50m-S
October-50m-S
January-50m-T
April-50m-T
July-50m-T
October-50m-T

4.3  salinity,temperature and 200m circulation
 
 January-200m-S
April-200m-S
July-200m-S
October-200m-S
January-200m-T
April-200m-T
July-200m-T
October-200m-T

4.4 cross sections

In summer, there are strong upwelling off the coast of Vietnam and along Fujian and Guangdong coast.

temperature and salinity from Vietnam to Dongsha

Fujian coast to Taiwan

4.5 time series

Volume transport is monitored throughout the integration.

 

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