Long-term biogeochemical variations in the southern South China Sea and adjacent seas: A model data analysis

The southern portion of the South China Sea (SCS), which constitutes the largest semi-enclosed marginal sea in Southeast Asia, is currently undergoing unusual surface warming, which may significantly impact its marine ecosystems. This study examines the long-term variations of biogeochemistry in the southern SCS and its adjacent seas from 1993 to 2020, using data from the Copernicus Marine Environment Monitoring Service (CMEMS) model to better understand the implications of these warming trends on nutrient dynamics and overall marine health. Analysis of interannual variations highlights a strong relationship between sea surface temperature (SST) anomalies and the occurrence of El Niño-Southern Oscillation and Indian Ocean Dipole events, leading to significant alterations in biogeochemistry in the southern SCS. These findings emphasize the sensitivity of biogeochemistry to climatic variability. Over the study period, a region-wide increasing SST trend (0.2 °C per decade) indicates the influence of climate change, while surface Chlorophyll concentration and Phytoplankton biomass show decreasing trends, suggesting potential shifts in primary productivity. Additionally, surface Dissolved Oxygen and pH levels have significantly declined, pointing to broader impacts on ocean health. These observed declining trends in biogeochemical variables are particularly pronounced in the southeastern Vietnam, East Coast of Peninsular Malaysia, and Java Sea regions, which are primarily driven by significant changes in SST and related wind patterns. The weakening of summer southwesterly winds and winter northeasterly winds, evidenced by linear trends in wind stress, has reduced nutrient supply through upwelling and decreased ocean heat loss, respectively. This disruption exacerbates surface ocean warming and limits the replenishment of essential nutrients, leading to diminished primary productivity and altering marine ecosystem dynamics. This study provides valuable insights into present biogeochemical trends under climate change, forming a crucial basis for future projection studies in this region.