1074 / 2024-09-20 10:56:27

Variability of the bottom boundary layer induced by the dynamics of the cross-isobath transport over a variable shelf

Turbulent boundary layer,northern SCS shelf,circulation

The bottom boundary layer (BBL) dynamics play an important role in regulating the energy, momentum balance, and circulation in the shallow shelf areas. Unlike previous studies that disconnect BBL with background variable shelf circulation, we investigate the dynamic connection between the wind-driven shelf circulation and BBL dynamics, and indicate the spatial characteristics of BBL dynamics in response to three-dimensional heterogeneous transport over the highly variable shelf topography. Our process-oriented modeling study demonstrates that the mixing dynamics and upslope buoyancy transport over varying shelf topography alter the spatial variability of BBL dynamics in the Northern South China Sea. Driven by southwesterly upwelling-favorable winds, the along-shelf current generated a frictional upslope Ekman transport. The along-isobath pressure gradient force (PGFx*) arise from the flow-topography interactions over the meandering shelf induced the geostrophic cross-isobath transport (GCT). The downwave (upwave) PGFx* enhanced (offset) the frictional upslope transport over the east (west) of the shelf that had a concaving (uniform) bottom topography. Over the eastern shelf with concave isobaths, the intensified PGFx*and upslope cross-isobath dense water transport strengthened stratification and weakened the effect of bottom stress-induced mixing, limiting the development of the BBL. The antithesis occurred over the western shelf, where a small bottom stress controlled the BBL. River discharge and the tidal current modulated the alongshore current, upslope transport, bottom stress intensity, and BBL development. We modeled the trajectory of seabed particles as they responded to the BBL dynamic regimes, and we found that high (low) concentration, short (long) suspension time, and strong (weak) shoreward transport occurred over the eastern (western) shelf, respectively.