Seasonal cycle of the East Australian Current

Abstract

The seasonal circulation associated with the East Australian Current is examined using a set of steric heights derived from the historical hydrology and expendable bathythermograph data collected in the region. The data are separated into a network of regional bins allowing for known oceanographic and topographic features and a two‐harmonic best fit to the seasonal cycle is obtained in each bin. Maps of the annual‐frequency component of the surface and depth‐integrated steric heights (h and P) show the development and progression of the EAC flow regime through a complete seasonal cycle. The EAC has a strong seasonal cycle from 25°S to 45°S, with strongest southward flow in austral summer. The seasonal cycle in surface flow over the continental shelf is documented by two independent methods, geostrophically, using cross shelf sea level gradients derived from coastal tide gauge data and steric heights at the continental shelf edge, and directly from merchant ship observations. The two estimates are in good agreement. The seasonal cycle in the EAC is more pronounced than in other midlatitude western boundary currents for which data are available. At 28°S, the strength of the total Tasman Sea transport (southward flow) varies between a minimum transport of 7 Sv in winter (July) to a maximum of 16 Sv in summer. The semiannual frequency components of h and P is important near 30°S near the EAC outflow, but not elsewhere. The seasonal cycle of the EAC is not due to strong seasonal variations in Tasman Sea wind stress curl east of the region of interest. Seasonally reversing zonal flows occur offshore north of 25°S, which are apparently locally forced by reversing wind stress curls; but if these flows were fed from the south by the EAC current system, the EAC would have to be weaker in summer, not stronger. The Leeuwin Current Extension along Australia's west and south coasts may pass up the east coast of Australia, providing an important contribution to the enhanced southward flow of the EAC in summer. The vigorous anticyclonic eddies of the EAC also show a marked seasonal cycle, and this is probably an important part of the mechanism for the strong seasonal cycle of the EAC south of 25°S. The location of the strongest anticyclonic eddy in the EAC moves steadily southward throughout the summer season, and the phase of the coastal EAC appears also to move southward, contrary to the expectations of linear theory and to the hypothesis that the Leeuwin Current Extension is the major cause of the seasonal cycle.