Time-dependent upwelling and downwelling over the shelf and slope in the Northern South China Sea (NSCS) off Hong Kong is studied using the Regional Ocean Modelling System (ROMS) with primitive equation dynamics. The model is forced by prevailing monsoon winds over shelf-slope topography. This study identifies the process of current variability and the associated dynamical mechanisms in the response of shelf-slope circulation to prevailing monsoon wind forcing, variable topography, and other dynamic forcing elements. Over the shelf, a wind-driven coastal jet is accompanied by cross-shelf transport in the surface and bottom frictional layers, creating an upslope/downslope tilt of isopycnals during upwelling/downwelling favourable conditions. The slope response is weaker. Shelf-sl...[ Read more ]

Time-dependent upwelling and downwelling over the shelf and slope in the Northern South China Sea (NSCS) off Hong Kong is studied using the Regional Ocean Modelling System (ROMS) with primitive equation dynamics. The model is forced by prevailing monsoon winds over shelf-slope topography. This study identifies the process of current variability and the associated dynamical mechanisms in the response of shelf-slope circulation to prevailing monsoon wind forcing, variable topography, and other dynamic forcing elements. Over the shelf, a wind-driven coastal jet is accompanied by cross-shelf transport in the surface and bottom frictional layers, creating an upslope/downslope tilt of isopycnals during upwelling/downwelling favourable conditions. The slope response is weaker. Shelf-slope exchange occurs between the upper slope and outer-middle shelf region in both seasons. In the summer, middle shelf and upper slope waters are connected by isopycnal surfaces, facilitating the exchange of shelf and slope water. Such a connection does not occur in the winter. Sensitivity experiments find that along- and cross-shelf circulation is proportional to stratication and wind forcing, while wind magnitude has a larger effect than wind stress curl. Steep topography results in increased shelf circulation. Shelf-slope exchange is similarly sensitive. During the seasonal transition, summer winds result in upwelling conditions over the shelf within ~ 17 days. Along- and cross-slope circulation is proportional to stratification and wind forcing, while steep topography results in decreased slope circulation. Even after ~ 45 days of summer monsoon wind forcing, the slope retains its winter response during the seasonal transition. The dynamical analyses show that in the two-dimensional topography cases, cross-isobath transport is Ekman, and along-isobath transport is geostrophic. Bottom pressure gradient force (PGF) drives along-shelf transport, while along-slope transport is driven largely by baroclinicity/steepness in the winter/summer. In the three-dimensional topography cases, shelf and slope dynamics differ. Enhanced upslope/downslope cross-slope transport occurs, formed by cross-isobath geostrophic current due to negative/positive along-isobath PGF approaching concave/convex isobaths. This intensified slope upwelling/downwelling does not affect conditions over the shelf. From the depth-integrated vorticity equation, it is found that the source of this PGF is the Modified Joint Effect of Baroclinicity and Relief in both seasons. This study provides new insight into shelf circulation and continental slope dynamics in the NSCS.