Abnormal upwelling and chlorophyll‐a concentration off South Vietnam in summer 2007

Abstract

The present study investigates a strong upwelling and concurrent phytoplankton bloom off the South Vietnam coast in August 2007. Analysis of the alongshore wind, offshore Ekman transport, wind stress curl and other parameters indicates the southwesterly summer monsoon plays an important role in this peculiar case. The change of monsoonal wind involves variations of atmospheric circulation on multiple time scales. On the interannual time scale, a positive Indian Ocean Dipole (IOD) in 2007 accompanying with a La Niña event acts to enhance the southwesterly monsoon over the South China Sea (SCS). The enhanced southwesterly wind is regarded as a major factor in promoting the upwelling and the phytoplankton bloom off the South Vietnam coast. On the intraseasonal time scale, variations of the sea surface temperature (SST) and sea surface wind at 30–60 day periods reveal that the SST cooling develops with the evolution of the southwesterly wind anomalies with nearly one week delay, implying the great significance of the Madden‐Julian Oscillation (MJO). Moreover, a tropical storm formed in the western SCS in early August reinforces the westerly wind and SST cooling. Among multiple factors on different time scales, the MJO event is considered as the major cause, since it induces the maximum velocity (4 m/s) of the southwesterly wind anomalies. As a result, the high chlorophyll‐a concentration (>0.56 mg/m3) and low SST center (<27°C) develops in the region off the South Vietnam coast and advects to the central SCS, riding on the northern rim of the southern anticyclonic gyre.