Seasonal Geostrophic Velocity Anomalies and Eddy Kinetic Energy in the South China Sea: Perspectives from Steric and Manometric Observations

The sea level anomaly (SLA) has been accurately tracked by satellite altimetry, yet its barotropic and depth-integrated baroclinic components are routinely interpreted using theoretical or modeled vertical structures. In this study, we utilized manometric SLA from the Gravity Recovery and Climate Experiment (GRACE) and steric SLA from observational and reanalysis temperature/salinity databases to evaluate their roles in seasonal variations of geostrophic velocity anomalies and eddy kinetic energy (EKE) in the South China Sea (SCS). Through the empirical orthogonal function analysis, we found that the manometric component of geostrophic velocity anomalies is closely associated with the western boundary current, reflecting a barotropic response to seasonally reversed wind stress in summer and winter. The steric component, primarily driven by baroclinic instability, shapes two large cyclonic (anticyclonic) gyres (Luzon and Nansha Gyres) in the northeastern and southern SCS during summer (winter), as well as small mesoscale anomalies in the northwestern SCS during spring and autumn. The cross-correlation analysis demonstrates considerable influence of wind stress on the surface dynamics throughout SCS, while wind stress curl predominantly contributes to the gyres and dipole system off the Vietnamese coasts. Opposing covariances between manometric and steric EKE along the eastern deep and western shelf sides of the southwestern continental slope were investigated via vertical density, temperature, and salinity anomalies along three transects. These patterns arise from seasonally distinct horizontal and vertical mixing structure in upper-layer and near-bottom cross-shelf currents, providing observational evidence for significant interactions between baroclinic and barotropic instabilities in coastal regions. SIGNIFICANCE STATEMENT: This study uses in situ, satellite, and reanalysis data to reveal different contributions of mass-induced and temperature-/salinity-induced sea level changes on seasonal ocean dynamics in the South China Sea. By decomposing geostrophic velocity anomalies and eddy kinetic energy, we distinguished their respective responses to the wind stress and wind stress curl in the deep and shelf ocean, revealing the wind–topography–flow structure near the shelf boundaries. Our studies provide an observational perspective on how barotropic and baroclinic processes respond separately to external forces such as wind and seafloor features, advancing our understanding of regional ocean circulation.