Inter-annual variation of chlorophyll in the northern South China Sea observed at the SEATS Station and its asymmetric responses to climate oscillation

It is widely recognised that the variation of average surface chlorophyll a concentration (Chl) in the South China Sea (SCS) is closely related to wind forcing, especially during the intense winter monsoon. In this study, we demonstrate that after removal of the seasonal cycles, the variation of Chl showed strong asymmetric responses to wind speed under El Nino or La Nina conditions. The analysis was based on a time-series of Chl in the study area (115-117 degrees E, 17-19 degrees N) around the SEATS (South-East Asian Time-series Study) station located in the central northern SCS from September 1997 to the end of 2011, which was constructed by merging the SeaWiFS data (1997-2006) and MODIS data (2003-2011). The merged daily data were validated by shipboard observations at the SEATS station. The non-seasonal variations of monthly mean Chl, wind speed, sea surface height (SSH) and sea surface temperature (SST) were examined against the multivariate ENSO index (MEI). The analysis reveals strongly asymmetric correlations of Chl and SST with positive MEI (El Nino) or negative MEI (La Nina). Under El Nino conditions, both showed significant correlations with MEI or wind speed; under La Nina conditions, both showed weak or insignificant correlations. The contrast was more pronounced for Chl than for SST. The subdued responses of Chl to wind forcing under La Nina conditions were attributable to a deepened thermocline, for which wind driven nutrient pumping is less efficient. A deeper thermocline, which was observed during the 1999-2000 La Nina event and inferred by positive SSH anomalies during other La Nina events, was probably caused by reduced SCS throughflow under La Nina conditions. Intrusion of the nutrient-depleted Kuroshio water in the surface layer as observed during the 1999-2000 La Nina could be partially responsible for the suppressed Chl response.