It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997-1998 El Nino event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18 degrees N, 116 degrees E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997-1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 degrees C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll-a dropped from 0.2 to 0.1 mg/m(3). The monthly area-averaged integrated primary production estimated for the northern SCS area (112-119 degrees E, 15-21 degrees N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997-1998 El Nino event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics. (C) 2009 Elsevier Ltd. All rights reserved.
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