Modeling Study of Variable Upwelling Circulation in the East China Sea: Response to a Coastal Promontory

A three-dimensional, high-resolution numerical model is used to investigate processes and dynamics of an intensified upwelling that is induced by a coastal promontory over the East China Sea (ECS) shelf. The center of the intensified upwelling around the promontory has been constantly observed, but, so far, it has been dynamically unexplained. Forced by an idealized southeasterly wind stress, the model results well capture the observed upwelling at the lee of the coastal promontory. The intensified upwelling is formed by a strengthened shoreward transport downstream of the promontory as the upwelling jet veers shoreward. The jet is mainly controlled by a cross-shore geostrophic balance and is largely modulated by both centrifugal acceleration associated with nonlinear advection and by bottom stress. The strengthened shoreward transport is mainly attributed to the cross-shore geostrophic current that is induced by a countercurrent (negative) pressure gradient force (PGF) and partly attributed to the bottom Ekman transport. Based on the analyses of the momentum balance and depth-integrated vorticity dynamics, the authors provide a new explanation for the origin of negative PGF. It is found that the countercurrent PGF is generated by negative bottom stress curl and strengthened by negative vorticity advection downstream of the promontory. While the negative bottom stress curl arises from bottom shear vorticity, the source of negative advection downstream of the promontory is the negative shear vorticity on the seaside of the shoreward-bent jet. Nevertheless, cyclonic curvature vorticity at the bottom and positive vorticity advection in the water column at the promontory weakens the negative PGF. Although nonlinear advection strengthens vorticity advection, it weakens bottom stress curl and has little net effect on the countercurrent PGF