Bathymetric control of the barotropic circulation in the Australian-Antarctic Basin

Altimetry has uncovered several areas that are characterized by excessively high levels of sea-surface height (SSH) variability on intraseasonal time scales. The most notable examples are the Argentine Basin, the Bellingshausen Basin, and the Australian-Antarctic Basin (AAB). In this presentation I will explore the nature of the enhanced SSH variability in the AAB.

I will argue that the enhanced variance is likely caused by two distinct modes of variability that are strongly controlled by details of the local bathymetry. First of all, part of the variability can be ascribed to the resonant excitation of purely free modes. These modes are trapped by contours of potential vorticity (f/H) that are closed onto themselves, specifically on top of the Southeast Indian Ridge, and in the eastern AAB (an area also know as the Wilkes Abyssal Plain). These modes decay slowly on a frictional time scale. Second, most of the SSH variability is caused by an almost-free mode that is captured by contours of potential vorticity surrounding the AAB. This mode decays on a rapid time scale of just a few days.