Numerical modelling of the mean flow characteristics of the Leeuwin Current System

The oceanic circulation of the region between Carnarvon (25°S) and Jurien Bay (31°S) was examined using observational and remotely sensed data in conjunction with a detailed numerical modelling study. The model was validated using in situ ADCP and CTD data, and the horizontal eddy viscosity parameterization was tested against field observations. The results generated using the Regional Ocean Model (ROMS v1.8) simulated the observed mean surface flow's spatial and temporal scales, particularly those of the eddy features. The modelled results also provided insight into the Leeuwin Current (LC)'s annually averaged alongshore transport and the Leeuwin Undercurrent (LUC)'s mean flow characteristics. Analysis of the model results, between 26.8°S and 30.8°S, revealed the annually averaged alongshore transport across the LC's southern boundary was 5.7 Sv, of which the northern boundary contributed 3.60 Sv or 63%. The inflow of water through the western boundary due to geostrophic balance was estimated at 2.1 Sv or 37%, and found to vary meridionally. A geopotential gradient, estimated at 1.9×10−7 and acting towards the equator, drives the LUC. Located at depths between 250 and 600 m, the modelled undercurrent is characterized as an energetic flow with mesoscale variability, that is strongly influenced by the more energetic surface current. Coupling was evident in the region adjacent to the Abrolhos Islands' western boundary (29°S) where the offshore movement of LC waters by a surface meander caused the vertical scale of the undercurrent to increase, inducing an anticlockwise vorticity response at a depth of ∼180 m. The resulting subsurface eddy grew preferentially downwards before a surface response was observed. The model showed that the eddy increased its vertical scale before splitting into two forming a surface and subsurface eddy with a vertical scale characteristic of the two currents. The surface eddy intensified, detached, and moved offshore, while the subsurface eddy remained attached to the topography.