Topographically Induced Upwelling off Eastern Australia

A high-resolution, numerical study of an idealized western boundary current flow over variable topography is presented, with application to the East Australian Current (EAC). The results indicate that alongshelf topographic variations off Australia’s east coast cause the EAC to accelerate over the narrowing continental shelf near Cape Byron. This acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer (BBL), which would usually cause the EAC-driven BBL to shut down. Consequently , a region of persistent, high bottom stress was established off Cape Byron, which was responsible for driving an upwelling BBL. It is shown that the enhanced vertical mixing, associated with a low Richardson number flow beneath the EAC, reduced the local stratification. Consequently , the Burger number is decreased resulting in a long shutdown timescale of the BBL, which enables a nearshore thermal front to be established and maintained. Such fronts are commonly observed in the region. As a part of the analysis the term balances of the model equations are presented, comparing the dynamical balances at locations along the domain that exhibit varying degrees of topographic variability. The results indicate that the BBL dynamics were not purely geostrophic, further explaining why BBL shutdown was not prevailing. Moreover, it is shown that the formation of the thermal front was dependent on the magnitude of the EAC’s southward transport, explaining why the occurrence of thermal fronts is greater during the spring and summer periods.