Upwelling in the continental shelf of northern South China Sea associated with 1997-1998 El Nio

Abstract

A strong upwelling off the continental shelf of the northern South China Sea (SCS) in 1998 summer is reinvestigated using a suite of new satellite measurements and numerical modeling. Previous studies indicate that the upwelling in the western SCS, especially off the Vietnam coast, almost disappears during 1998 summer following the El Nio because of the weakened southwest monsoon. This study identifies that the coastal upwelling in the adjacent northern SCS (NSCS) is significantly strengthened during 1998 summer, and the alongshore wind stress is dramatically enhanced over the region. As a result, the offshore Ekman transport in 1998 summer is the strongest, almost twice the average of the other 17 years during 1997-2007. The Chl a concentrations in the representative upwelling regions are much higher than any in other years. Further analysis suggests that two adjacent basin-scale upwellings in the SCS have different responses and maintaining mechanisms because of the anticyclonic atmospheric circulation anomaly over the SCS and northwest Pacific. The northern flank of the atmospheric circulation anomaly intensifies the monsoonal winds off the NSCS coast, while the southern flank suppresses the southwesterly winds along the Vietnam coast. Copyright 2011 by the American Geophysical Union.