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

[摘要] It is widely recognised that the variation of average surface chlorophyll aconcentration (Chl) in the South China Sea (SCS) is closely related to windforcing, especially during the intense winter monsoon. In this study, wedemonstrate that after removal of the seasonal cycles, the variation of Chlshowed strong asymmetric responses to wind speed under El Niño or LaNiña conditions. The analysis was based on a time-series of Chl in thestudy area (115–117° E, 17–19° N) around the SEATS(South-East Asian Time-series Study) station located in the central northernSCS from September 1997 to the end of 2011, which was constructed by mergingthe SeaWiFS data (1997–2006) and MODIS data (2003–2011). The merged dailydata were validated by shipboard observations at the SEATS station. Thenon-seasonal variations of monthly mean Chl, wind speed, sea surface height(SSH) and sea surface temperature (SST) were examined against themultivariate ENSO index (MEI). The analysis reveals strongly asymmetriccorrelations of Chl and SST with positive MEI (El Niño) or negative MEI(La Niña). Under El Niño conditions, both showed significantcorrelations with MEI or wind speed; under La Niña conditions, bothshowed weak or insignificant correlations. The contrast was more pronouncedfor Chl than for SST. The subdued responses of Chl to wind forcing under LaNiña conditions were attributable to a deepened thermocline, for whichwind driven nutrient pumping is less efficient. A deeper thermocline, whichwas observed during the 1999–2000 La Niña event and inferred by positiveSSH anomalies during other La Niña events, was probably caused by reducedSCS throughflow under La Niña conditions. Intrusion of thenutrient-depleted Kuroshio water in the surface layer as observed during the1999–2000 La Niña could be partially responsible for the suppressed Chlresponse.

[学科分类] 地球化学与岩石