Abstract
Abstract. 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 Niño or La Niña conditions. The analysis was based on a time-series of Chl in the study area (115–117° E, 17–19° 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 Niño) or negative MEI (La Niña). Under El Niño conditions, both showed significant correlations with MEI or wind speed; under La Niña 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 Niña 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 Niña event and inferred by positive SSH anomalies during other La Niña events, was probably caused by reduced SCS throughflow under La Niña conditions. Intrusion of the nutrient-depleted Kuroshio water in the surface layer as observed during the 1999–2000 La Niña could be partially responsible for the suppressed Chl response.
Highlights
The physical-biogeochemical condition of the South China Sea (SCS) is sensitive to climate variation because of the strong forcing of alternating monsoons; the sensitivity is attributable to its location situated between the Eurasia supercontinent and the vast Pacific Ocean to the east and the Indian Ocean to the south (e.g. Sun et al, 2005; Tseng et al, 2009a)
Separate sub-samples for chlorophyll a analysis were filtered onboard ship. (The sampling dates and the cruise names are listed in Table 1.) The filters were stored at −20 ◦C and returned to the
It is noted that the weakest wind always occurred during the inter-monsoon periods in spring or autumn with noticeable secondary maxima occurring in summer
Summary
The physical-biogeochemical condition of the South China Sea (SCS) is sensitive to climate variation because of the strong forcing of alternating monsoons; the sensitivity is attributable to its location situated between the Eurasia supercontinent and the vast Pacific Ocean to the east and the Indian Ocean to the south (e.g. Sun et al, 2005; Tseng et al, 2009a). The physical-biogeochemical condition of the South China Sea (SCS) is sensitive to climate variation because of the strong forcing of alternating monsoons; the sensitivity is attributable to its location situated between the Eurasia supercontinent and the vast Pacific Ocean to the east and the Indian Ocean to the south Liu et al.: Inter-annual variation of chlorophyll in the northern South China Sea to the north of the flow path (Shaw and Chao, 1994; Chen et al, 2012a). The wind stress curls in the West Philippine Sea and off southwestern Taiwan, as well as that around the northern Luzon Strait in winter may enhance the intrusion (Chang and Oey, 2012; Wu and Hsin, 2012). The circulation in the SCS is subject to modulation by climate oscillation via wind variation (Chao et al, 1996b), which closely follows the multivariate ENSO index (MEI) (Palacz et al, 2011)
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