IDENTIFICATION OF SPATIAL VARIABILITY AND EDDIES IN THE CIRCULATION OF THE SOUTH CHINA SEA

Abstract

A method of using a velocity Jacobian (J(u, v)) to identify the spatial variability of the two-dimensional flow field has been delineated in detail. Positive and negative J(u, v) distinguish the vorticity-dominated and straindominated flows embedded in the mean general circulation, respectively. Normalized by the divergence of velocity, the non-dimensional J(u, v) becomes insensitive to the magnitude of the flow, illustrating the contrast between the strain- and vorticity-dominated flows. An index obtained from tracking the rotating angle of the fluid in a given point is employed to locate the eddy centers, saddle, and non-stationary points in the flow field. Both J(u, v )a nd the index are applied to the seasonal geostrophic circulation field in the South China Sea (SCS). They capture well the spatial flow variability and eddy centers or saddle points. Strong spatially variable flow fields, with alternating vorticity- and strain-dominated flows, are found in the basin scale flow along the continental margin. The index method, with a threshold of selected J(u, v), clearly pinpoints seasonal eddies embedded in the mean circulation associated with the distinct regional forcing in the southwest and northeast SCS.