Modeling studies of the Leeuwin Current off Western and Southern Australia

The Leeuwin Current strengthens considerably from February to May each year , following the slackening of southerly coastal winds; strong eddies develo p. A high-resolution, multilevel, primitive equation ocean model is used to examine this eddy development in an idealized way, by considering the deve lopment of how from rest when temperatures are initially given the observed longshore gradients. The system is allowed to geostrophically adjust in th e absence of longshore winds and of any surface heat flux. Two types: of ex periments are conducted. The first type uses the Indian Ocean climatologica l temperature gradient forcing (case 1 and 2), while the second type repeat s the first experiment with the added contribution of the North West Shelf (NWS) temperature profile (cases 3 and 4). To investigate the additional ef fects of coastline irregularities, cases 1 and 3 use an ideal coastline, wh ile cases 2 and use an irregular (realistic) coastline. In all cases, maximum surface velocities occur at Cape Leeuwin, where the L eeuwin Current changes direction and off Southern Australia. Maximum underc urrent velocities occur off Western Australia. in case 1, Cape Leeuwin and the Western Australian coast are the preferred locations for the developmen t of warm, anticyclonic eddies, which are generated due to a mixed instabil ity mechanism. In case 2, the warm, anticyclonic eddies occur in the vicini ty of coastal promontories and at Cape Leeuwin. While advection of warm wat er is present along the entire coast in case 1, the irregular coastline geo metry limits the extent of warm water in case 2. The added contribution from the NWS water in cases 3 and 3 augments the ons hore geostrophic inflow to produce a model Leeuwin Current and undercurrent that are more vigorous and unstable than in the previous cases. In case 3, the NWS water adds strong horizontal shear to the coastal equatorial regio n of the domain and vertical shear to the inshore current. It also advects warmer water along the entire coast. In case 4, the addition of both the NW S water and the irregular coastline results in the establishment of a stron ger surface current and undercurrent than in the previous cases; however, t he irregular coastline limits the extent of the advection of the NWS warmer water along the Australian coast. In all cases, warm, anticyclonic eddies develop at the coast. Cold, cycloni c eddies form from the limbs of the established warm, anticyclonic eddies w ith the result that two counterrotating cells are developed. Once the eddy pairs begin their westward propagation, the Leeuwin Current intensifies as nonlinear effects result in a jet between the eddies and the coast. These e ffects translate downstream to augment the current velocities at the coast, which, due to a mixed instability mechanism, result in the development of new anticyclonic eddies at the coast. The results from case 4, which has the most realistic features, highlights the major characteristics of the Leeuwin Current and agrees well with avail able observations. These results show that the model successfully captures the qualitative nature of the nonlinear, eddying response of the Leeuwin Cu rrent.