The Vertical Structure of Currents on the North West Shelf of Australia at Subtidal Frequencies

Abstract

An EOF analysis of current meter data collected on the North West Shelf of Australia (NWS) suggested that the major part of the low-frequency (subinertial) circulation on the shelf was wind force. The circulation appeared to be dominantly barotropic, but significant variations in the flow with depth were observed. In this paper, the vertical structure of the currents on the NWS is investigated using a rotary EOF analysis of the vertical currents. Use analysis is performed in the frequency domain over a frequency range of 0.03 to 0.78 cpd including the local inertial frequency of 0.69 cpd. The anticlockwise (cum sole) EOFs are found to be significantly coherent with the wind stress in the four frequency bands considered in the analysis, whereas significant coherences occurred in only two of the clockwise bands. To study the origin of the vertical structure in the currents, the observations are compared to the results of a model of the frictional boundary layers. The model incorporates surface and bottom boundary layers of relatively high eddy viscosity separated by a zone of relatively low eddy viscosity. The parameterization of eddy viscosity as a function of depth requires specification of the rms wind stress, the rms bottom stress and internal dissipation. With reasonable choices of these parameters, the model reproduces the vertical structures in the anticlockwise bands fairly well, but performs poorly in the clockwise bands. It is suggested that the source of much of the structure in the clockwise bands is due to baroclinicity in the flow.