A Regional Model of Shelf Circulation near Bass Strait: A New Upwelling Mechanism

Abstract

A study is made of the low-frequency, three-dimensional circulation and upwelling for the western region of Bass Strait, using oceanographic data and the Princeton Ocean Model. A novel radiation condition, which simulates the effect of an eastern shelf on Kelvin wave scattering, is successfully tested against analytic solutions for coastal-trapped wave (CTW) scattering and generation. Using a realistic bathymetry for the Bass Strait region, the model was forced with 8-day period zonal winds within the strait and a mode 1 CTW at the northwestern boundary. Results demonstrate the existence of a new mechanism for upwelling whereby at the cessation of westward winds within the strait, the residual poleward flow over the steeply sloping Tasmanian shelf separates from a gyre that develops over the more gently sloping topography near the mouth of the strait. The resultant divergence of the velocity field leads to large vertical velocities (34 m day?1), and a plume of subthermocline water that is upwelled by 80 m and drawn 60 km toward the strait. While upwelling driven by the CTW paddle is less significant, data from the region suggest that the relative phase of the forcing mechanisms is such as to enhance upwelling over the shelf slope. To examine the circulation and upwelling under realistic conditions, the model is forced by observed winds and the CTW paddle modulated using low-pass filtered sea-level data. The results reproduce some of the 30 cm s?1 and 20-cm variability apparent in current and sea level data obtained for the region. Moreover, the combined wind and CTW forcing is predicted to lead to upwelling rates of 34 m day?1 and the growth of an upwelled plume of subthermocline water that is displaced 120 m in the vertical and 80 km toward the strait.