Rapid response of the East Australian Current to remote wind forcing: The role of barotropic-baroclinic interactions

The strength of the East Australian Current (EAC) is observed to vary in response to changes in basin-scale winds in the South Pacific, with a time lag of three years. First mode baroclinic Rossby waves would take 10-15 years to reach the western boundary from the center of the South Pacific, so a faster mechanism is needed to explain this link. We use an ocean general circulation model forced with idealized anomalies of wind stress curl to examine the mechanism responsible for the rapid response of the EAC. A curl perturbation in the central South Pacific produces baroclinic and barotropic Rossby waves. The barotropic waves propagate quickly to the western boundary at New Zealand (NZ), where they scatter into a coastal Kelvin wave that travels anti-cyclonically around the coast of NZ. In the Tasman Sea, the height anomaly associated with the Kelvin wave spawns first mode baroclinic waves that take about three years to propagate across the Tasman Sea to influence the EAC. The model suggests that the rapid response of the EAC to changes in wind forcing can be explained by a combination of barotropic and baroclinic Rossby waves with conversion between modes facilitated by topography.