The effect of wind direction and mixing upon the spreading of a buoyant plume in a non-tidal regime

A three-dimensional baroclinic model, incorporating an accurate density advection scheme, and a range of turbulence closure models which can account for wind-wave turbulence at the sea surface, is used to examine the horizontal spreading and vertical mixing of a surface buoyant plume under a range of idealized wind stresses and bottom topography variations. Initial calculations show that the spread of the plume and its vertical mixing are very sensitive to the values of vertical viscosity and diffusivity. These coefficients are initially determined with a two-equation turbulence energy model, although subsequent calculations show that similar results can be obtained using a one equation model provided suitable stability functions are chosen for viscosity and diffusivity. Calculations assuming a constant water depth show that the offshore spread of the plume is greatest from an along shore upwelling favourable wind which gives rise to an offshore spread as a surface buoyant jet. An along shore downwelling favourable wind slightly reduces the offshore spread. In the case of a sloping bottom the offshore extent is reduced by the bottom slope, with the surface wind driven flow being a maximum in the near shore region. The magnitude of the surface current is sensitive to assumed values of the surface roughness length, and in the region of the plume shows a significant spatial variability due to a combination of a surface wind driven flow and that produced by the plume outflow. Suggestions as to how a proposed experiment involving a shore based HF Radar and a salinity survey in the region of the Ebro plume may be used to validate a physically realistic model of the region are made.