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Abnormal Upwelling and Concentration of Chlorophyll-a off South Vietnam in August 2007

Liu et al. carried out this study off the coast of South Vietnam to investigate a strong upwelling that was associated with a concurrent bloom of phytoplankton that occurred in August 2007. The southwesterly summer monsoon is indicated by analysis of alongshore wind, offshore Ekman Transport, wind stress and curl and other parameters to play an important role in this peculiar case. Variations of atmospheric circulation on multiple time scales are involved in the change in monsoonal wind. The southwesterly monsoon over the South China Sea (SCS) is enhanced by a positive Indian Ocean Dipole (IOD) in 2007 accompanying a La Niña event, on an interannual timescale. Off the South Vietnam coast the upwelling and phytoplankton bloom is regarded to have been largely promoted by the enhancement of the southwesterly wind. Variations of the sea surface temperature (SST) and sea surface wind at 30-60 day periods, on the interseasonal time scale, reveal that the sea surface temperature cooling develops with the evolution of the southwesterly wind anomalies, with almost 1 week delay, which implies the great significance of the Madden-Julian Oscillation (MJO). The westerly wind and sea surface cooling would also be reinforced by a tropical storm that formed in the South China Sea. The Madden-Julian Oscillation is considered to be the major cause among multiple factors, on different time scales, as it induces the maximum velocity of 4 m per second on the southwesterly wind anomalies. The result is the high chlorophyll-a concentration, >0.56 mg/m³, and low sea surface temperature centre, <27^oC, develops in the region of the South Vietnam coast and advects to the South China Sea by riding on the northern rim of the southern anticyclonic gyre.

Sources & Further reading

1.     Liu, Xin, Jing Wang, Xuhua Cheng, and Yan Du. "Abnormal Upwelling and Chlorophyll-a Concentration Off South Vietnam in Summer 2007." Journal of Geophysical Research: Oceans 117, no. C7 (2012): C07021.