SUMMER UPWELLING ON THE SYDNEY INNER CONTINENTAL-SHELF - THE RELATIVE ROLES OF LOCAL WIND FORCING AND MESOSCALE EDDY ENCROACHMENT

The inner shelf and surf-zone near Sydney experience several major upwelling events each summer, with a typical event being several days long. Two potential causes of these events are local wind forcing and cross-shelf advection related to the presence of mesoscale eddies over the continental slope. The relative roles of these processes are assessed from cross-shelf temperature transects, weekly hydrographic stations in 50 and 100 m of water, and daily surf-zone temperatures. Local wind forcing affects the vertical motion of the pycnocline over the stratified inner shelf, with typical vertical velocities of about 10 m day-1 for a coast-parallel wind stress of 0.2 N m-2. The hydrographic response lags the wind stress by between 1 and 2 days. About half of the major upwelling events in the surf-zone and over the inner shelf can be identified with periods of strong southward wind stress. Cross-shelf advection associated with the presence of anticyclonic mesoscale eddies near the coast accounts for most of the remaining major upwelling events. The strong southward flow on the inshore side of these eddies results in an onshore mass flux in the bottom boundary layer, which displaces warm surface water seaward over the inner shelf. The currents may also be sufficiently strong for bottom friction to totally homogenize the water column at the 50 m isobath. Both cross-shelf advection and vertical mixing then lead to cooler water in the surf-zone.