Model diagnostic analysis of cross-shelf flow in the northern South China Sea

Abstract

To explore cross-shelf flow in the northern South China Sea, model diagnostic calculation was conducted using a regional, three-dimensional, primitive equation model (POM) and the climatological temperature and salinity fields supplied by the Generalized Digital Environment Model (version 3.0). Using the model calculations, we systematically evaluated the characteristics of cross-shelf flow and estimated its volume, heat, and salt transports in different sections along the continental slope. The dynamic process of cross-shelf flow was further analyzed using the vorticity balance. Results revealed the spatial distribution of the cross-shelf flow in the northern South China Sea. Between the 200– 2000 m isobaths, the horizontal distribution of cross-shelf flow was uniform, and strong cross-shelf flow was present in areas of the continental slope with complex topography. We selected three sections (S1, S2, and S3 along the 200, 500, and 1000 m isobaths, respectively) to characterize the cross-shelf flow. Three local maximal cross-shelf velocities in the on-shelf direction were detected in sections S1 and S2, but this feature was not clear in section S3. Strong cross-shelf flow (>2 cm/s) was present at about 115°E and 118°E of section S1 in winter. The annual mean cross-shelf volume transports in sections S1, S2, and S3 were 9.4, 15.8, and - 2.1 Sv, respectively. The heat content transports were 0.78, 0.84, and - 0.22 PW, respectively, and the salt content transports were 342.5, 575.0, and - 76.0 Gg s - 1, respectively. The cross-shelf flow was towards the shelf in the 200–500 m isobaths region and in the opposite direction in the 500–2000 m isobaths region. Cross-shelf transport was stronger in winter and weaker in summer in sections S1 and S2, but no seasonal difference was found in section S3. These cross-shelf transports of sea water, heat, and salt along the continental shelf reflect the main characteristics of cross-shelf flow. According to the volume transport between the three sections, flow convergence arose between sections S1 and S2 and divergence occurred between sections S2 and S3. In terms of volume conservation, these spatial distributions of flow convergence and divergence resulted in the cross-shelf flow. The along-shelf flow was the primary component of slope current, whereas cross-shelf flow contributed only about 15%–20%. Although cross-shelf flow was not as strong as along-shelf flow, its influence must not be ignored due to its huge volume, heat, and salt transports. Analysis of momentum balance revealed that the slope current was mainly controlled by the geostrophic equilibrium, in which the pressure gradient and Coriolis force played a dominated role. The cross-shelf flow was mainly barotropic in most regions, but it exhibited a baroclinic feature in the region east of 118°E. The analysis of barotropic vorticity balance indicated that the cross-shelf flow was mainly controlled by the joint effect of baroclinicity and bottom relief term (JEBAR), advection of the geostrophic potential vorticity term (APV), and the advection and diffusion term. The JEBAR term mainly controlled the cross-shelf flow along the continental slope due to the positive potential vorticity produced by the interaction between the baroclinic sea water and topography. To the east of 118°E, the instrusive Kuroshio Current led to interaction between baroclinic sea water and the continental slope, which increased the JEBAR term and therefore strengthened the cross-shelf flow.