Eddy structure and the West Australian current

Abstract

Abstract Four cruises in the summers of 1972 and 1973 in an 8 × 8° area were planned to investigate mesoscale and large-scale circulation off the southwest Australian coast in the West Australian Current. A trough with its base centered over the Naturaliste Plateau extends 800 km northeast to the coast; this constitutes the West Australian Current which is a 100- to 200-km wide cyclonic stream turning poleward as a coastal southerly. The transport of about 10 7 m 3 s −1 is confined near the surface. Currents are on average 0.5 m s −1 and have a scale depth of about 370 m; speeds up to 1.5 m s −1 are observed. Far off shore, near-zonal Rossby waves coexist with the stream giving a meandering surface effect. Near the shelf the southerly also meanders, giving a coastal mesoscale planetary wave with a meridional wavevector. Both systems have a wavelength of about 300 to 380 km. Surface samples have a T–S signature which identifies with the subtropical high salinity surface water in the central Indian Ocean. Advection of heat in the surface mixed layer reflects the pattern of the trough and of the Rossby waves. Mixed layer depth patterns show the meridional coastal wave in general but during high winds and surface cooling they also show the Rossby waves and the large scale cyclonic current. The current is unlike other eastern boundary currents and dynamically has some features of western boundary currents in addition to the meandering poleward flow: lateral shear vorticity and inertial curvature vorticity are less important than the change in planetary vorticity, but only marginally so.