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

Abstract

It is widely recognized that the variation of average surface chlorophyll <i>a</i> 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 probably attributed 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

Links Between the Coral δ13C Record of Primary Productivity Variations in the Northern South China Sea and the East Asian Winter Monsoon

“…The physical‐biogeochemical conditions of the SCS are controlled mainly by the East Asian monsoon (Liu et al, ; Ning et al, ; Shaw & Chao, ). Seasonally, winter is the most productive season in the SCS due to the strong wind forcing of the winter monsoon during that period (supporting information Figures S1–S4) (Liu et al, , ; Palacz et al, ); this wind forcing enhances vertical mixing and introduces nutrients into the euphotic zone (Liu et al, ; Tseng et al, ). Short‐term satellite observations suggest that the PP in the SCS increased during the period from 1998 to 2010 (Liu et al, ; Palacz et al, ), when most of the tropics experienced a reduction in PP due to the strengthened thermal stratification associated with ocean warming (Behrenfeld et al, ; Dave & Lozier, ).…”

“…Seasonally, winter is the most productive season in the SCS due to the strong wind forcing of the winter monsoon during that period (supporting information Figures S1–S4) (Liu et al, , ; Palacz et al, ); this wind forcing enhances vertical mixing and introduces nutrients into the euphotic zone (Liu et al, ; Tseng et al, ). Short‐term satellite observations suggest that the PP in the SCS increased during the period from 1998 to 2010 (Liu et al, ; Palacz et al, ), when most of the tropics experienced a reduction in PP due to the strengthened thermal stratification associated with ocean warming (Behrenfeld et al, ; Dave & Lozier, ). Based on the observed “wind‐nutrients‐productivity” mechanism on the seasonal timescale, previous work attributed the increase in the PP in the SCS from 1998 to 2010 to the corresponding enhancement of the wind speed, especially those of the winter monsoon (Palacz et al, ).…”

Enhancement of Mesopelagic Sinking Particle Fluxes Due to Upwelling, Aerosol Deposition, and Monsoonal Influences in the Northwestern South China Sea

“…Intraseasonal events are also important. Cyclonic eddies (Tang et al, ; Wang et al, ), typhoons (Lin et al, ; K. K Liu, Wang, et al, ), and internal waves (Pan et al, 2012) may all serve to upwell nutrient‐rich deep water to the upper ocean, thereby stimulating phytoplankton growth and increasing primary production. Model results for the SCS indicate a 41% increase in export production due to the effects of cyclonic (cold, upwelling) eddies and a 31% decrease due to anticyclonic (warm, downwelling) eddies (Xiu & Chai, ).…”

On contributions by wind‐induced mixing and eddy pumping to interannual chlorophyll variability during different ENSO phases in the northern South China Sea

“…Note that the decrease in heat transport across 15 N is caused by more heat being transported from the south into the NSCS area, because the southward western boundary current is strengthened during La Niña, which also suggests that more WPAC water recirculates in the SCS instead of exiting via the Karimata Strait. We further investigated interannual variability of the factors that may affect the thermocline depth, including surface buoyancy flux with both thermal and haline contributions, wind stress curl, and heat advection flux in the NSCS (e.g., Hao et al 2012;Liu et al 2013;Peng et al 2017). The negative correlation between the thermocline depth and MEI index confirms that the La Niña condition is generally associated with deepened thermocline in the NSCS (Fig.…”