SEASONAL CYCLE OF THE EAST AUSTRALIAN CURRENT

The seasonal circulation associated with the East Australian Current i s examined using a set of steric heights derived from the historical h ydrology and expendable bathythermograph data collected in the region. The data are separated into a network of regional bins allowing for k nown oceanographic and topographic features and a two-harmonic best fi t to the seasonal cycle is obtained in each bin. Maps of the annual-fr equency component of the surface and depth-integrated steric heights ( h and P) show the development and progression of the EAC flow regime t hrough a complete seasonal cycle. The EAC has a strong seasonal cycle from 25 degrees S to 45 degrees S, with strongest southward flow in au stral summer. The seasonal cycle in surface, flow over the continental shelf is documented by two independent methods, geostrophically, usin g cross shelf sea level gradients derived from coastal tide gauge data and steric heights at the continental shelf edge, and directly from m erchant ship observations. The two estimates are in good agreement. Th e seasonal cycle in the EAC is more pronounced than in other midlatitu de western boundary currents for which data are available. At 28 degre es S, the strength of the total Tasman Sea transport (southward flow) varies between a minimum transport of 7 Sv in winter (July) to a maxim um of 16 Sv in summer. The semiannual frequency components of h and P is important near 30 degrees S near the EAC outflow, but not elsewhere . The seasonal cycle of the EAC is not due to strong seasonal variatio ns in Tasman Sea wind stress curl east of the region of interest. Seas onally reversing zonal flows occur offshore north of 25 degrees S, whi ch are apparently locally forced by reversing wind stress curls; but i f these flows were fed from the south by the EAC current system, the E AC would have to be weaker in summer, not stronger. The Leeuwin Curren t Extension along Australia's west and south coasts may pass up the ea st coast of Australia, providing an important contribution to the enha nced southward flow of the EAC in summer. The vigorous anticyclonic ed dies of the EAC also show a marked seasonal cycle, and this is probabl y an important part of the mechanism for the strong seasonal cycle of the EAC south of 25 degrees S. The location of the strongest anticyclo nic eddy in the EAC moves steadily southward throughout the summer sea son, and the phase of the coastal EAC appears also to move southward, contrary to the expectations of linear theory and to the hypothesis th at the Leeuwin Current Extension is the major cause of the seasonal cy cle.