The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Variabilities of the ecosystem of the South China Sea (SCS) are driven by atmospheric drivers such as the Madden-Julian Oscillation (MJO) and tropical cyclones (TCs). Previous studies have revealed the effects of the MJO and TCs on surface chlorophyll concentrations, respectively. However, the effects of the MJO and TCs, separately or concurrently, on ecosystem structure and functionality remain unclear. Here, we utilized a well-calibrated physical-biogeochemical model (ROMS-CoSiNE) to quantify these responses. For MJO, the idealized MJO-forced model experiment exhibited a substantial enhancement in primary production (56%) and export production (23%). This increase was attributed to the increase of nanophytoplankton (15%) induced by MJO, which was consistent with phytoplankton pigment data collected in cruises. In addition, composite analysis with a realistic model run revealed that MJO and TCs impacted different regions in SCS, with TCs mainly acting in northern SCS, while MJO mainly acted in southern SCS. TCs had weaker impacts than the MJO, although weak wind-driven mixing can also lead to tiny increases in production and carbon export (<5%). The coupling of MJO and TCs significantly increased primary production, but had less effect on carbon export, which could be explained by the degree of the decoupling between production and carbon export, forced by non-steady dynamic forcing. From this perspective, this study improved our comprehension of the responses of the SCS ecosystem to dynamics at multiple scales.
 

Madden-Julian Oscillation, tropical cyclones, South China Sea, pelagic ecosystem, biological carbon pump