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

Abstract

The chlorophyll in the northern South China Sea (NSCS) shows strong interannual variability between different phases of the El Niño‐Southern Oscillation (ENSO), primarily due to the influence from Kuroshio intrusion. Chlorophyll observations also reveal that significant year‐to‐year variation remains in the same ENSO phase, but its controlling mechanism is unknown. In this study, we examined driving mechanisms for such regional ecosystem variability on a year‐to‐year timescale. Using observational data and modeling results, we found that both cyclonic eddies (CEs) and wind‐induced mixing affect phytoplankton variability, but the former is the dominant factor regulating the interannual variability of chlorophyll during La Niña years, while the latter becomes the dominant one during El Niño years. The underlying mechanisms leading to the contrast in biological responses are attributable to changes in background nutricline depth and mixed‐layer depth (MLD) during different ENSO phases. During La Niña, both thermocline and nutricline are deepened in the NSCS. Thus, wind mixing is less effective in non‐eddy areas due to the deepened nutricline, while the CE‐induced subsurface nutrient anomaly associated with increased MLD is able to control the interannual variability of chlorophyll. During El Niño, wind‐induced mixing is more effective than CEs because the surface wind can influence a larger area.

Marine Environmental Regionalization for the Beibu Gulf Based on a Physical-Biological Model

“…Therefore, the distribution of PHY in 2011 reflected that the range and the value of high PHY areas in the northern gulf was greater than in 2011 and 2015, indicating that the lower sea temperature in the La Niña year was more conducive to PHY growth in the Beibu Gulf [61]. In the La Niña phase, the wind strengthened, resulting in an increase in CV and seawater mixing, so the concentration of PHY increased [62,63]. From the results of the model simulated outputs (Figures 9-11), we found that the La Niña year of 2011 had lower TEM in general, especially in the northern gulf, and the area of high PHY was larger than that of 2013 and 2015 in the northeastern gulf, but that it moved to south.…”

Composite of Typhoon‐Induced Sea Surface Temperature and Chlorophyll‐a Responses in the South China Sea

“…The impact of a single typhoon on the ocean is quite different from the accumulated changes, and large responses are mainly identified in coastal regions (Figures 8b and 8e). Coastal regions are characterized by a weak stratification and small vertical difference (Xiu et al, 2018); thus, the typhoon-driven mixing is expected to induce small changes. However, the thermocline and nutricline near the coast are shallow (Wang et al, 2020), and the water column can be easily mixed by typhoons such that even a weak and fast-moving typhoon can induce a prominent change at the ocean surface (Zhao et al, 2017).…”

Asymmetric Response of the Biological Carbon Pump to the ENSO in the South China Sea

“…The LST varies as the bifurcation latitude of the North Equatorial Current shifts, which is strongly correlated with the ENSO (Nan et al., 2015; Yaremchuk & Qu, 2004). On the other hand, a shift in the scope of the intertropical convergence zone could directly impact the monsoon intensity and subsequent biological responses in the SCS (Tseng et al., 2009; C. Wang et al., 2006; Xiu et al., 2019).…”

“…On the other hand, a shift in the scope of the intertropical convergence zone could directly impact the monsoon intensity and subsequent biological responses in the SCS (Tseng et al, 2009;C. Wang et al, 2006;Xiu et al, 2019).…”