Observations of Low Richardson Numbers and Undercurrents near a Front in the East Australian Current

Abstract

A surface current jet with speeds in excess of 1.5 m s−1 was found flowing southward and out to sea, off the northern New South Wales coast. Estimates of surface velocity, from ship's drift and GEK, combined with thermal winds from three closely spaced XBT sections, were used to construct vertical sections of longshore velocity. Data suggest that a cold current flows northward along the bottom of the continental shelf and slope. This undercurrent appears to sink beneath the main southward flow; the shear layer between current and undercurrent is sufficiently strong that Richardson numbers calculated from thermal winds are less than 0.25 in some places. It is estimated that the turbulent drag across such shear layers may be large enough to be a major mechanism for frictional loss in the East Australian Current.