Patterns of Upper Layer Circulation Variability in the South China Sea from Satellite Altimetry Using the Self-Organizing Map

Abstract

Patterns of the South China Sea (SCS) circulation variability are extracted from merged satellite altimetry data from October 1992 through August 2004 by using the self-organizing map (SOM). The annual cycle, seasonal and inter-annual variations of the SCS surface circulation are identified through the evolution of the characteristic circulation patterns. The annual cycle of the SCS general circulation patterns is described as a change between two opposite basin-scale SW-NE oriented gyres embedded with eddies: low sea surface height anomaly (SSHA) (cyclonic) in winter and high SSHA (anticyclonic) in summer half year. The transition starts from July - August (January - February) with a high (low) SSHA tongue east of Vietnam around 12°-14° N, which develops into a big anticyclonic (cyclonic) gyre while moving eastward to the deep basin. During the transitions, a dipole structure, cyclonic (anticyclonic) in the north and anticyclonic (cyclonic) in the south, may be formed southeast off Vietnam with a strong zonal jet around 10° - 12° N. The seasonal variation is modulated by the interannual-variations. Besides the strong 1997/1998 event in response to the peak Pacific El Niño in 1997, the overall SCS sea level is found to have a significant rise during 1999-2001, however, in summer 2004 the overall SCS sea level is lower and the basin-wide anticyclonic gyre becomes weaker than the other years