Buoyancy Effect on the Winter South China Sea Western Boundary Current


The month-to-month variation of the winter South China Sea (SCS) western boundary current (WBC) along the western slope is examined using drifter observations, satellite altimetry data and an ocean reanalysis. The most surprising phenomenon is that the WBC velocity at the sea surface reaches the maxima in November-December, which cannot be explained by wind forcing and Kuroshio intrusion alone. Analysis results demonstrate that buoyancy effect should be considered to explain the month-to-month variation besides wind-Kuroshio effects. In winter, cold-and-salty advection by the WBC from the north decreases/reverses the zonal density gradient in the seasonal pycnocline induced by wind forcing and Kuroshio intrusion, and therefore weakens wind-Kuroshio-induced WBC. Buoyancy effect on the winter SCS WBC is opposite to wind-Kuroshio effects. In addition, buoyancy effect reaches the maximum in January, which is concurrent with wind-Kuroshio effects. As a result of their competition, the zonal density gradient in the seasonal pycnocline is maximum in November-December, resulting in the maximum surface velocity along the western slope occurring in November-December. This study demonstrates the importance of buoyancy forcing to the winter SCS WBC.


Figure 1. Monthly climatology of wind stress averaged over the western slope (magenta curve, N/m2 ), wind stress curl averaged over the South China Sea (blue curve, N/m2 per 104 km), and volume transport of the Luzon Strait in the upper 500 m (Kuroshio intrusion, black curve, Sv)


Figure 2. Sea surface (a) temperature (°C), (b) salinity (PSU), and (c) density σθ (kg/m3 ) in January


Figure 3.  (a) Vertical and (b) zonal gradients of potential density (kg/m3 per 100 km) in January along 9.0°N. Contours in (a) and (b) are potential density (kg/m3 ).