Wintertime circulation off southeast Australia: Strong forcing by the East Australian Current

Abstract

Numerical results and observations are used to examine the sub–weather band wintertime circulation off the eastern shelf slope region between Tasmania and Cape Howe. The numerical model is forced using wintertime-averaged atmospheric fluxes of momentum, heat, and freshwater and using transports along open boundaries that are obtained from a global ocean model. The boundary to the north of Cape Howe has a prescribed transport of 17 Sv, which from available observations represents strong forcing by the East Australian Current (EAC). Nonetheless, the model results are in qualitative (and quantitative) agreement with several observational studies. In accord with observations, water within Bass Strait is found to be driven to the east and north with speeds of up to 30 cm s−1 off the southeast Victorian coast. The water is colder and denser than that found farther offshore, which is associated with the southward flowing (warm) EAC. In agreement with observations the density difference between the two water masses is about 0.6 kg m−3 and leads to a cascade of dense Bass Strait water to depths of 300 m and as a plume that extends 5–10 km in the cross-shelf direction. Typical vertical velocities of the model cascade are 20 m d−1. Along-isobath currents become bottom intensified with speeds of 20 cm s−1 or so at depths of 200 m. The EAC is found to flow along isobaths and to the south and then west past Tasmania with speeds of up to 20 cm s−1. Mesoscale eddies grow to diameters of 100 km (speeds <40 cm s−1), and in accord with observations a semipermanent anticyclonic eddy is found to the east of Cape Howe.