Temporal and spatial evolution of a deep-reaching anticyclonic eddy in the South China Sea

Abstract

The temporal and spatial evolution of a deep-reaching anticyclonic eddy (AE) is studied using a combination of satellite measurements, moored observations and ocean model reanalysis data in the South China Sea (SCS). Three evolutionary stages in eddy’s lifecycle are identified from changes in eddy dynamical characteristics estimated from satellite altimetry: birth (22 days), growth (64 days), and decay (47 days). Similar patterns are also distinguished from dynamic signals in HYCOM. Further, flows reversal and upwelling of cold water below 1500 m were captured by the in-situ records when this energetic, highly nonlinear and long-lived (over 19 weeks) AE passed by our mooring position. Its detailed vertical structure is examined through temperature anomalies, vertical shear of horizontal velocities, and horizontal streamlines estimated from ocean model reanalysis data. Results from the model reveal a mesoscale AE with first-mode baroclinic structure: a bowl-shaped anticyclonic flow in the upper ocean connected to a slant-cylinder cyclonic flow at depth, with a transition layer at depths between 400 and 700 m. It is in good agreement with moored observations but showing a shallower transition depth, suggesting a slight deficiency in the model due to limited deep-sea observations. Last, we estimate eddy heat transport at different depths and stages along the AE’s path based on the model data. The result reveals that pronounced heat fluxes occur during growth stage (depths 400 m), counting for 73.03% of the total value. In the decay stage, major heat transport occurs at deeper depth (depths > 700–1500 m). Dynamical characteristics suggest that the vertical structure and temporal evolution of the eddy play significant roles in basin-scale movement and heat transferring. Considering that mesoscale eddies are ubiquitous in the SCS, our results support a recently-proposed mechanism, whereby upper ocean flows produce changes in the deep-sea circulation, potentially influencing boundary layer dynamics. For the first time to track and link an individual AE observed by satellite altimetry and ocean model, comparisons indicate that assimilative HYCOM outputs may be useful for examining the deep ocean properties within the SCS, especially under the impact of such an intensified surface-detected eddy.