A numerical study of the eddying characteristics of the Leeuwin Current System

Abstract

The Regional Ocean Modelling System (ROMS v2.1) was used to investigate the physical processes that sustained the mesoscale eddy fields. The model was able to mimic the spatial, temporal, and migratory scales of the Leeuwin Current (LC) System's mean flow and eddying characteristics. Importantly, eddy generation sites were consistent with those reported previously observed from satellite imagery, and two main regions were identified: site 1, adjacent to the Abrolhos Islands’ western edge (latitude 29°S); and site 2, adjacent to the shelf break (latitude 27.8°S). The description of the eddying dynamics suggested the eddy field evolution followed a similar pathway at both sites. An offshore flow impinged on the continental shelf and was redirected southward, intensifying the LC's southward alongshore flow on the shelf. This southward redirection formed a current loop, which closed and formed a distinct clockwise eddy. The eddy remained near stationary and attached to the bottom topography, but it redirected LC and continental shelf waters offshore to dominate the local dynamics. The resulting offshore meander grew laterally, shallowed, and closed to form an anticlockwise eddy to the original clockwise eddy's south, forming a characteristic LC eddy pair (dipole). The model demonstrated that the LC and Leeuwin Undercurrent coupling played an important role in the onset of eddies at both sites. An energy diagnostic scheme was used, and the dominant instability process linked to the developing anticlockwise eddy at site 1 was mixed mode barotropic and baroclinic instability. The baroclinic instability source was the available potential energy stored within the mean lateral density gradient between the eddy dipole. The LC's meandering southward flow interacting with the undercurrent's northward subsurface flow generated horizontal shear, which was the source of barotropic instability. The dominant instability process at site 2 was baroclinic in origin. At both sites, the baroclinic mechanism followed a series of phases coinciding with the physical description of the evolving eddies.