Vorticity Dynamics of Cross-isobath Geostrophic Transport in the Stratified Steep and Concave Shelves

We investigated the variability and the physics of cross-isobath transport that lead to the formation of prominent upwelling centers in the steep shelf to the east of Hainan Island (EHI) and in the ambient shallow Gulf of Zhanjiang (GOZ). In situ measurements and a three-dimensional modeling showed that strengthened upslope transport of cold deep waters occurred in these two shelves, which are characterized by steep slope and concaving isobaths, respectively. The major driving force for these shoreward cross-isobath transports were not from the bottom frictional dynamics, but from the along–isobath pressure gradient force (PGF) as a result of the flow response to the variable shelf topography. The physical origins of the PGF of these prominent upwelling centers, however, were dynamically different. We found that the sources of the PGF were the Modified Joint Effect Baroclinicity and Relief (MJEBAR) due to importance of baroclinicity in the steep EHI and the net water-column stress curl in the concaving GOZ. The along-shelf geostrophic current that maintains the flow-topography interaction for the formation of the PGF was determined by the competing slope and baroclinic effects in EHI and by the cross-isobath changing bottom pressure in GOZ. Based on depth-integrated vorticity dynamics for a stratified and free-surface sea, this study illustrates the contrasting forcing functions of the three-dimensional circulation over the steep and shallow concaving shelves.