Simulation of boundary currents around Australia

Charitha B Pattiaratchi, University of Western Australia, Crawley, WA, Australia, Sarath Wijeratne, The University of Western Australia, School of Civil, Environmental and Mining Engineering and The UWA Oceans Institute, Nedlands, Australia and Roger Proctor, University of Tasmania, Integrated Marine Observing System, Hobart, TAS, Australia
Abstract:
Australia is an island continent surrounded by boundary currents which include (clockwise from north) the: Hiri (HiC), East Australian (EAC), Zeehan (ZC), Flinders (FC), Leeuwin (LC) and Holloway (HC) currents. The EAC is the major western boundary current transporting 25 Sv southwards and consists of a large meso-scale eddy field. The LC is a poleward flowing eastern boundary current with a connection to the north of Australia through the HC and to the south via ZC. A high resolution (2-4 km) ROMS (Regional Ocean Modelling System) model, Oz-ROMS, was configured for the entire Australian continental shelf and slope together with the adjacent deep ocean region. Multi-year simulations were performed using atmospheric forcing (wind, pressure, air-sea fluxes) using ECMWF ERA interim data. The open boundary tracers (salinity and temperature) and transport (barotropic and 3D velocity components) were specified using the global Hybrid Coordinate Ocean Model outputs. The tidal forcing was specified using a global inverse barotropic tidal model (Oregon State University TPXO 7.2). The model was able to reproduce the spatial and temporal scales of all the boundary currents, and their connectivity with predicted transports and seasonal and inter-annual variability compared well with observations. We show that subsurface flow which originates as the Tasman Outflow, extends westward along the southern slopes to become the Flinders (FU) which separates into two sections: (1) flowing westward into the southern Indian Ocean; and, (2) flows northward at Cape Leeuwin to become the Leeuwin Undercurrent (LUC) reaching the north-west of Australia. The model results have revealed the existence of a previously undescribed subsurface north-eastward flow on the north-western shelf/slopes and through Timor Passage (TP). Eastward flowing South Indian Countercurrent (SICC) is one of the major inputs to Leeuwin Current (LC) and LUC, where surface SICC feeds the LC whilst southern mid-depth SICC feeds the LUC. The East Australian Current (EAC) originates from northern and southern branches of South Equatorial Current (SEC), where the southern branch of SEC (South Caledonia Current, SCJ) input is mainly confined to the subsurface. The model results also indicate a strong direct link between Indonesian Throughflow and Tasman Outflow.
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