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

Abstract

Abstract 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 (anti-cyclonic) 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.

Affiliated Institutions

• Wuhan University CN
• Shenzhen Polytechnic CN
• Hong Kong Polytechnic University HK

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