The Evolution of Density-Driven Circulation Over Sloping Bottom Topography

The short timescale temporal ev~lution of buoyancy-driven coastal flow over sloping bottom topography is examined using a two-dimensional, vertically averaged numerical model. Winter shelf circulation driven by a coastal "point source" buoyancy flux is modeled by initiating a coastal outflow with density anomaly t into well-mixed shelf water. The nonlinear interaction between the time-varying velocity and density field is represented by an advection-diffusion equation. Three cases are discu~: that of a buoyant ( t < 0) outflow, a neutral ( E = 0) outflow, and a dense (t > 0) outflow. Results are similar to observations from well-mixed shelf areas and show that density-topography interactions are capable of substantially influencing coastal circulation. A negative (buoyant) coastal buoyancy flux is shown to generate alongshore motion with relatively small cross-shelf transport. Conversely, positive (dense) coastal buoyancy flux is shown to generate flow that travels across isobaths to initiate an offshore cyclonic gyre, which is then advected in the direction of propagation of a right-bounded wave. A vortiG.ity analysis shows that local circulation is controlled by the interaction of vortex stretching, JEBAR, and the time change of vorticity; the residual of which is roughly balanced by bottom dissipation.