Science Highlights

Task 4: Coupled Shelf-Bay Circulation Dynamics

We conducted observational and numerical modeling studies to investigate the coupled circulation between Mirs Bay to the east of Hong Kong and the adjacent shelf sea during an upwelling season. We synthesized long- and short-term observations to characterize the circulations in the bay-shelf region and identified their linkage. We utilized a three-dimensional circulation model with realistic topography and forced with time-dependent wind, tides and lateral fluxes from the nearby shelf and Pearl River to identify the processes and physics in the circulation of the coupled bay-shelf regime.

Based on the validated model, we found that a strong northeastward coastal upwelling jet persisted over the shelf with highly variable topography outside the bay, and a strong upslope current occurred where the topography was sharply convex. This upslope current intruded into the bay in the lower layer (>10 m) as a cold-water stream. An anti-cyclonic circulation formed inside the bay with a seaward outflow in the upper layer (<10 m). Our momentum and vorticity analyses showed that a southwestward along-isobath pressure gradient force over the convex isobaths off the bay intensified that bay-ward intrusion. Negative relative vorticity advection from the jet was responsible for this pressure gradient force. The anti-cyclonic circulation and elevation fluctuation inside the bay were determined by the interaction between the intruding shelf current and the trough inside the bay, and they were also baroclinically modulated by the intrusion of denser shelf waters. Wind over Mirs Bay intensified exchange flow across its entrance, but suppressed the inside anti-cyclonic circulation.

Figure1: (a-c) The simulated horizontal velocity (m s^-1) vectors on the shelf neighboring Mirs Bay and averaged over the (a) upper (0-10 m), (b) intermediate (10-20 m), and (c) lower (20 m – bottom) layers. The velocity (m s^-1) component in the cross-isobath direction is illustrated in (d-f). A positive value in (d-f) represents a shoreward velocity component crossing the isobath. The color scale in (a) is larger than that in (b) and (c). The contour lines are the 30 m, 25 m, 20 m, and 15 m isobaths from the shelf towards the bay. The cruise transects and time-series observations for Station MBT are illustrated in (a).​