Ring structure in the poleward boundary current off Western Australia in Summer

Abstract

Data from five summer cruises off Western Australia are examined objectively using structure functions to establish principal length scales and amplitudes of mesoscale fields. Previous estimates of length scales using geopotential anomaly and geomagnetic electrokinetograph vectors as inputs to structure- function analyses gave length scales that differed by a factor of two. The present analysis shows that there are two length scales, which dominate in different parts of the flow, and this reconciles the two previous estimates. The shorter scale is λs = 157�25 km and the longer is λL = 309�28 km. Regions of strong large-scale currents have warm- and cold-core rings and mesoscale waves associated with them that assume the Rossby deformation scale. These are the λS structures. The longer, λL structures are found in regions of weak large-scale currents. Geopotential anomaly amplitudes and currents in the rings are, respectively, about 0.7 m2 s-2 (geopotential relief = 1 4 m2 s-1) and 70 cm s-1. Data from one summer cruise with a station density of approximately 12 per degree square are analysed in detail subjectively and the structure-function analysis is shown to be quantitatively meaningful. This cruise was near the shelf and shows the advection of low-salinity tropical water poleward over the slope in a narrow baroclinic current. Seaward cyclonic rings were associated with the current. The baroclinic structure of the current and of the rings is compatible with the winter behaviour of Lagrangian drifters released into the Leeuwin Current.