Modeling Studies of the Leeuwin Current off Western and Southern Australia

Abstract

The Leeuwin Current strengthens considerably from February to May each year, following the slackening of southerly coastal winds; strong eddies develop. A high-resolution, multilevel, primitive equation ocean model is used to examine this eddy development in an idealized way, by considering the development of flow from rest when temperatures are initially given the observed longshore gradients. The system is allowed to geostrophically adjust in the absence of longshore winds and of any surface heat flux. Two types of experiments are conducted. The first type uses the Indian Ocean climatological temperature gradient forcing (case 1 and 2), while the second type repeats the first experiment with the added contribution of the North West Shelf (NWS) temperature profile (cases 3 and 4). To investigate the additional effects of coastline irregularities, cases 1 and 3 use an ideal coastline, while cases 2 and 4 use an irregular (realistic) coastline. In all cases, maximum surface velocities occur at Cape Leeuwin, where the Leeuwin Current changes direction, and off Southern Australia. Maximum undercurrent velocities occur off Western Australia. In case 1, Cape Leeuwin and the Western Australian coast are the preferred locations for the development of warm, anticyclonic eddies, which are generated due to a mixed instability mechanism. In case 2, the warm, anticyclonic eddies occur in the vicinity of coastal promontories and at Cape Leeuwin. While advection of warm water is present along the entire coast in case 1, the irregular coastline geometry limits the extent of warm water in case 2. The added contribution from the NWS water in cases 3 and 4 augments the onshore 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 region 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 NWS water and the irregular coastline results in the establishment of a stronger surface current and undercurrent than in the previous cases; however, the 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, cyclonic eddies form from the limbs of the established warm, anticyclonic eddies with 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 effects 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 available observations. These results show that the model successfully captures the qualitative nature of the nonlinear, eddying response of the Leeuwin Current.