Anomalous biogeochemical conditions in the northern South China Sea during the El‐Niño events between 1997 and 2003

Abstract

Anomalous biogeochemical conditions were observed at the SouthEast Asian Time‐series Study (SEATS) station in the northern South China Sea (SCS) during the 1997–98 and 2002–03 El Niño events. The time‐series records showed decreases of monthly mean sea surface chlorophyll‐a (S‐chl) (and integrated primary production, IPP) by 42% (and 42%) and 13% (and 10%), respectively, below the climatological mean in the winter months (DJF) of the two events. The negative anomalies in S‐chl and IPP corresponded to elevated sea surface temperature by 1.2°C and 0.4°C, respectively, above the climatological mean, while the mean wind speed was reduced by about 20% and 11%, respectively. Statistical analysis demonstrated the reduction in S‐chl and IPP during El Niño events was caused by the diminished vertical mixing and strengthened stratification. Regional anomalies in hydrographic and biological conditions in the northern SCS (15–21°N and 112–119°E) were consistent with those found at the SEATS site.