A Numerical Study of Wind- and Thermal-Forcing Effects on the Ocean Circulation off Western Australia

Abstract

A high-resolution, multilevel, primitive equation model is initialized with climatological data to investigate the combined effects of wind and thermal forcing on the ocean circulation off Western Australia during the austral fall and winter, corresponding to the period of strongest flow for the anomalous Leeuwin Current. This process-oriented study builds on previous modeling studies, which have elucidated the role of thermal forcing in the generation of the Leeuwin Current and eddies, by including the additional effects of wind forcing for the eastern boundary current region off Western Australia. The ocean circulation is generated by the model using a combination of density forcing from the climatological Indian Ocean thermal structure, the influx of warm low-salinity waters from the North West (NW) Shelf, and the climatological wind stress. In the first experiment (case I), forcing by the Indian Ocean and wind stress are imposed, while in the second experiment (case 2), the additional effects of the North West (NW) Shelf waters are considered. In the absence of the NW Shelf waters (case I), geostrophic flow, driven by the Indian Ocean thermal gradient, dominates the wind forcing at the poleward end of the domain and establishes an equatorward undercurrent and a poleward surface current (the Leeuwin Current), which accelerates poleward into the prevailing wind. Wind-forcing effects are discernible only offshore at the equatorward end of the region. The inclusion of NW Shelf waters (case 2) completely dominates the wind forcing at the equatorward end of the model. The effects of the NW Shelf waters weaken away from the source region but they continue to augment the Indian Ocean forcing, resulting in a stronger flow along the entire coastal boundary. The ocean circulation also has significant mesoscale variability. In the first experiment, both the Indian Ocean thermal structure and wind forcing lead to the dominance of barotropic (horizontal shear) instability over baroclinic (vertical shear) instability. In the second ~xperiment, the NW Shelf waters add baroclinicity, which weakens poleward, to the Leeuwin Current and locally increase the barotropic instability near their source. Away from the source waters, where there is a mixed instability, the combined effect of the Indian Ocean thermal structure and wind forcing is stronger than the NW Shelf waters and leads to a dominance of barotropic over baroclinic instability. Several scales of eddies are found to be dominant. The forcing by the Indian Ocean and wind stress (case I ) leads to an eddy wavelength of -330 km. With the inclusion of the NW Shelf waters (case 2), the wavelengths associated with mesoscale variability are -150 and 330 km, consistent with observed. eddy length scales.