On the anomalous structure of the Southeast Vietnam Offshore Current during 1994 to 2015

Abstract

The summer circulation in the South China Sea (SCS) often has a dipole structure associated with the eastward Southeast Vietnam Offshore Current (SVOC). The dipole has an anti-cyclone to the south and a cyclone to the north. A 22-year (1994–2015) HYbrid Coordinate Ocean Model (HYCOM) global reanalysis is used to study why the SVOC in the SCS is anomalous in summer of 1995, 1998, 2007, 2010, and 2015. The normal SVOC Sea Surface Height (SSH) pattern is defined based on previous observations of the dipole associated with it. An Empirical Orthogonal Function (EOF) analysis is performed on the monthly mean SSH and wind stress curl to seek the answer. SSH EOF1 and EOF2 explain 43.3% and 18.9% of the total SSH variance, respectively. Wind stress curl EOF1 and EOF2 explain 63.4% and 14.9% of the total variance, respectively. SSH EOF1 is a basin-scale gyre mode that represents the barotropic response to the basin-scale monsoon wind stress curl EOF1. SSH EOF2 is a dipole mode that represents a baroclinic response to both wind stress curl EOF1 and EOF2 (western North Pacific anticyclone mode) via forced baroclinic Rossby waves. SSH EOF2 explains 39.6% of the SSH variance in the SVOC dipole and is more important for the SVOC formation than SSH EOF1, which explains 28.2% of the SVOC dipole SSH variance. SSH EOF2 principal component time series indicate that the summer dipole mode in 1995, 1998, 2010, and 2015 is associated with an anti-cyclone to the north and a cyclone to the south, opposite to the normal SVOC dipole. In 2007, the SSH EOF2 is very weak with its principal component close to zero.