East Australian Current volume transports at 30°S: Estimates from the World Ocean Circulation Experiment hydrographic sections PR11/P6 and the PCM3 current meter array

Abstract

Between September 1991 and March 1994 the oceanic region off the east coast of Australia at 30°S was the subject of an intense observational program Part of the World Ocean Circulation Experiment, the objective was to improve our understanding of the dynamics of the East Australian Current (EAC) and to measure its volume and energy flux. The two main components were the Pacific Current Meter Array 3 (PCM3) with six moorings spanning a total distance of 120 km, and repeat, high‐resolution hydrographic surveys. The long‐term average fromPCM3 shows the EAC as a narrow, swift, and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0.6 m s−1 in the core. The measurements also revealed a thick countercurrent underneath the poleward flow. The mean volume transport for the entire section covered by the array (between the coast and 154.4°E) was 22.1±4.6 Sv toward the south with an rms variability of 30 Sv. Combining the data from the PCM3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field. The snapshots were used to evaluate the reliability of the PCM3 data for determining volume transports. The rms difference between transports derived from direct current observations and from hydrographie data was 3.8 Sv, indicating that the PCM3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements. Transport variability was dominated by periods between 90 and 140 days, which are attributed to eddy shedding in the region. Occasional intense northward flow events were observed where total transports reached up to 50 Sv northward. On those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic, related to the northward migration past the array of the EAC coastal separation point.