Effects of Stratification on Wind-Driven Upwelling Over a Coastal Valley

Abstract

In-situ observations indicate variations in stratified conditions over the coastal valley off the Sansha Bay in the northwestern Taiwan Strait across different years. However, dynamic processes and mechanisms governing the upwelling process over the valley, as influenced by stratification, are still unknown. By employing idealized numerical simulations, we demonstrate that compared to the unstratified case, the surface offshore flow intensifies, upwelling flux and net cross-shore transport are enhanced, upwelling area expands, and the cross-shore velocity structure is modified over the valley under stratified conditions. The primary factor controlling the vertical velocity within the valley is the relative vorticity change along a streamline (RVC). Further decomposition of the RVC reveals that the depth-averaged alongshore velocity ($\bar{v}$) and the depth-averaged alongshore gradient of vorticity ($\partial \bar{\xi }/\partial y$) primarily modulate the magnitude and spatial distribution of the vertical velocity. The negative zone of the $\partial \bar{\xi }/\partial y$ expands in the valley, resulting in a larger upwelling area. The magnitude of the $\bar{v}$ in the upper layer is slightly enhanced. The combined influence of these two factors leads to increased upwelling flux in the valley under stratified conditions. The net upslope motion over the valley is intensified under stratified conditions. The augmented advection of relative potential vorticity, originating from the increased amplitude of coastal trapped lee waves, primarily contributes to the enhanced net cross-shore transport in the valley.