Eddies off southeastern Australia

Abstract

Satellite infrared images and satellite drifters, together with some supporting ship data, have been used to follow complex and rapidly evolving events in the East Australian Current system. The episodic extension of a geopotential ridge into the southern Tasman Sea was seen to be arrested and diverted by a substantial anticyclonic eddy. Later the ridge and the eddy partially coalesced, and this provided a pathway for the East Australian Current to travel southward and encircle the eddy. A new eddy was formed in summer when this eddy coalesced with another. The new eddy initially had two subsurface thermostads with temperatures of 18.8 and 17.0°C, respectively, that were the result of mixing within the two component eddies the previous winter. It was kept under continuous surveillance by drifters for 8 months and by occasional ship surveys for a further 5 months. The drifters showed the motion of the eddy center to comprise an overall northward component upon which were occasionally superimposed anticlockwise loops of 100 km diameter, which were described in about 30 days. The maximum speed of translation of the eddy center exceeded 0.3 m s −1. The loops brought the eddy in near the continental shelf edge, with the result that it was transiently distorted into a precessing ellipse. The unusual northward move of the eddy took place over the winter, during which a new mixed layer cooled and deepened until it incorporated the upper thermostad in June and the deeper thermostad in September; no further cooling occurred. The motion of the drifters relative to the eddy center was used to calculate angular momentum, taken to be twice the rate at which the area was swept by the radius to a drifter. In the short term, angular momentum was found to be quasi-conservative around individual elliptical loops and overall showed roughly a r 1.8 dependence. It was used to calculate the radial distribution of angular velocity, azimuthal velocity, and relative vorticity for the eddy in its undisturbed circular form.