Abnormal upwelling off the southeast of Vietnam in summer 2016

Abstract

The upwelling appears generally off the southeast of Vietnam coast in summer. Previous studies have shown that under the influence of El Nino events, the upwelling would be weakened, charactering with high sea surface temperature (SST) and low Chlorophyll-a concentration (Chl-a). However, a different pattern of upwelling appeared in summer 2016, a decaying period of a strong El Nino event. There was a high SST and low Chl-a in June and July, which were similar with that of 1998 and 2010, another two El Nino years. However, we found a strong upwelling together with a moderate phytoplankton bloom off the southeast of Vietnam in August 2016. The analysis of the wind data and SST data indicated that the southwesterly summer monsoon played an important role on this particular case. The abrupt intensification of the southwest wind in late July of 2016 resulted in the SST cooling with nearly one-week delay, which meant that the great significance of the Madden-Julian Oscillation (MJO) occurred. The continuous intensification of the southwest wind enhanced the upwelling associated with Ekman pumping and offshore Ekman transport. As a result, the high-nutrient water of the subsurface was brought into the upper layer, which induced the high Chl-a, and cold-water mass spreading northeastward offshore.