Reversing process of the South China Sea western boundary current in autumn 2011

Using the merged sea level anomaly and the absolute geostrophic velocity products obtained from the satellite altimetry data as well as the Argos drifter data, we analyzed the reversing process of the South China Sea (SCS) western boundary current (SCSwbc) from summer pattern to winter pattern in 2011 and the important oceanic phenomena occurring during this process. Results show that outbreak time of the northeast monsoon over the southern SCS lagged about one month behind that over the northern SCS. In the SCS monsoon reversing period, the SCSwbc reversed also rapidly into the winter pattern at the Guangdong continental slope in late September, and following this, the southward Vietnam coastal boundary current strengthened. However, the northward Natuna current still kept a summer state until the middle of October. Thus the balance between the two southward and northward currents lost when they meet, their meeting position went gradually toward the south. However, a loop current formed southeast of the Vietnam because the main stream of the Vietnam Off-shore Current (VOC) kept basically near the original latitude. Meanwhile, the VOC and its associated dipole circulation system became strong. After the middle of October, the northward Natuna Current started to weaken, the loop current shed at last and becam a cool ring. The VOC and its associated dipole sub-basin circulation system also weakened gradually, until disappeared. Analyzing results indicate that the responses of the SCSwbc in the northern, middle and southern segment regions to the northeast monsoon were different.

Key words: South China Sea, western boundary current, reversing process, northeast monsoon