Wind-Forced Downwelling Slope Currents: A Numerical Study

Abstract

A study is made of the dynamics of slope currents that arise from a steady, constant alongshore wind over a uniform shelf. Over the first 10?20 days, the evolution of the downwelled system on an f plane is qualitatively described by linear coastal-trapped wave dynamics. After this time the thermal wind shear associated with the bottom mixed layer becomes important in the evolution of the undercurrent (UC), which is shown to be driven by the alongshore pressure gradient due to sea level. As the UC over the slope evolves, the bottom Ekman transport becomes small and negative, leading to the detachment of flow near the shelf break, localized spreading of isopycnals, and further intensification of the UC. In contrast to results obtained without bottom drag, bottom friction and boundary layer transport are shown to lead to a two- to threefold increase in cross-shelf interior transport, downwelling, and thermal wind shear. By day 60, the resultant UC has speeds of up to 15 cm s?1 and a net transport of 0.7 Sv (Sv ≡ 106 m3 s?1), or ? of the surface Ekman transport. The alongshore transport associated with the UC is shown to be fed and drained by on- and offshore transports of comparable magnitude to the UC. The reduction in bottom stress over the shelf also leads to an alongshore current and density field that change little over many hundreds of kilometers. The sensitivity of results to the stratification, bathymetry, and wind stress curl is outlined and some suggestions are made regarding the shelf circulation forced by winds within the Great Australian Bight.