A REGIONAL MODEL OF SHELF CIRCULATION NEAR BASS STRAIT - A NEW UPWELLING MECHANISM

A study is made of the low-frequency, three-dimensional circulation an d upwelling for the western region.of Bass Strait, using oceanographic data and the Princeton Ocean Model. A novel radiation condition, whic h 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 wh ereby at the cessation of westward winds within the strait, the residu al poleward flow over the steeply sloping Tasmanian shelf separates fr om a gyre that develops over the more gentry sloping topography near t he mouth of the strait. The resultant divergence of the velocity field leads to large vertical velocities (34 m day(-1)), and a plume of sub thermocline 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 me chanisms is such as to enhance upwelling over the shelf slope. To exam ine the circulation and upwelling under realistic conditions, the mode l is forced by observed winds and the CTW paddle modulated using low-p ass 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 obta ined for the region. Moreover, the combined wind and CTW forcing is pr edicted to lead to upwelling rates of 34 m day(-1) and the growth of a n upwelled plume of subthermocline water that is displaced 120 m in th e vertical and 80 km toward the strait.