ENSO-Driven Variability of Shallow Water Masses in the South China Sea between 2005 and 2023

Abstract The shallow water masses of the South China Sea (SCS) play a critical role in regulating regional climate and sustaining marine ecosystems. Using Argo float observations, we examine SCS water mass properties and eddy structures on isopycnal surfaces. By analyzing data in density space, we separate true water mass changes from vertical displacement of isopycnals. This approach helps reveal previously undocumented interannual variability in temperature and salinity together. We find that during the study period (2005–23), the shallow waters of the SCS, including within eddies, are strongly linked to El Niño–Southern Oscillation (ENSO) variability. During El Niño events, SCS waters become warmer and saltier, and during La Niña, they become colder and fresher. We show that the water mass variability can be explained by anomalies in local surface fluxes—with enhanced heating and reduced precipitation during El Niño and weaker heating with increased precipitation during La Niña. The water mass response is observed throughout the SCS but appears amplified within cyclonic eddies, likely due to their shallower mixed layer and greater sensitivity to surface fluxes. Our results demonstrate the sensitivity of SCS water masses to large-scale climate variability and provide new insight into the sensitivity of the SCS to ENSO. Significance Statement Using Argo data analyzed on isopycnal surfaces, we identify coherent temperature–salinity changes in South China Sea water masses that are not evident on fixed depth levels. The upper ocean becomes warmer and saltier during El Niño and cooler and fresher during La Niña in response to anomalous surface heat and freshwater fluxes associated with large-scale ENSO-related variability. Analyses on depth levels conflate true water mass changes with vertical isopycnal heave, masking this coupled signal.