Dynamics of cross-shelf water exchanges off Pearl River Estuary in summer. (November 2020)

• Record Type: Journal Article
• Authors: Liu, Zhiqiang; Zu, Tingting; Gan, Jianping
• Is Part Of: Progress in oceanography. Volume 189(2020)
• Publication Date: 2020-11
• Subjects: Exchange flow -- Shelf circulation -- Northern South China Sea -- Cross-isobath Transport -- Depth-integrated Potential Vorticity Dynamics; Oceanography -- Periodicals; 551.4605
• DOI: 10.1016/j.pocean.2020.102465
• Languages: English
• ISSNs: 0079-6611

Abstract

Highlights: Meander of isobaths modulate the direction and intensity of cross-shore flows. Winds and exchange flow in Taiwan Strait drive the cross-shore water exchanges. Bottom stress curl and relative vorticity advection regulate the cross-shore flows. Abstract: Dynamics regulating the cross-isobath momentum and water exchanges over the shelf in the vicinity of the Pearl River Estuary (PRE) in July 2015 and 2017 are examined using comprehensive observations and multiple-level nested high-resolution numerical simulations. Although driven by the upwelling- and downwelling-favorable winds in July 2015 and 2017, respectively, the shelf currents and the cross-isobath exchanges illustrate great similarity during these two periods. This study finds that the southwesterly-induced flow downstream in the Taiwan Strait maintained the northeastward upwelling currents in the vicinity of the PRE that caused the extensive upslope invasion of the deep shelf waters. The cross-shelf exchanges of the shelf and coastal waters mainly occur over the irregular topography off the entrance of the HuangMaoHai estuary, to the west of the Dangan Island, and over a shallow trough to the east of Hong Kong. It is also rationalized that the intensified exchange flow is established by an along-shelf pressure gradient force that notably exceeds the contributions from the bottom and surface Ekman transport as well as the nonlinearity of the shelf current in the water column. The baroclinicity of the shelf and coastal waters plays minimal roles in altering this along-shore pressure gradient force, especially when upwelling-favorable winds prevail. The exchange flows in the water column thereby show great similarity with those in the bottom layer, where the bottom along-shore pressure gradient force is jointly imposed by the nonlinear advection of relative vorticity and frictional stress curl in both the surface and bottom layers. The spatial variation of these terms is regulated by the intensity of the shelf current and along-shelf variation of bottom topography.