The southward transport of sub‐mesoscale lenses of Bass Strait Water in the centre of anti‐cyclonic mesoscale eddies

Abstract

Dense shelf water from Bass Strait, southeast Australia, is presently understood to travel northward along the continental shelf, and disperse eastward into the Tasman Sea. Here we report the unexpected discovery by autonomous gliders of lenses of shelf water ∼40 km in diameter and 200–300 m tall at depth in the center of three ∼200 km diameter anti‐cyclonic eddies. Reanalysis of 2420 vertical profiles off the continental slope in the western Tasman Sea since 1982 found only 3 distinct patches of Bass Strait Water (BSW), all with positive dynamic height anomalies indicative of anti‐cyclones. Through a yet to be understood process, BSW separates from the continental slope and forms a mid‐depth lens that aligns vertically with the larger anti‐cyclonic mesoscale eddy; and remains at the center of the eddy for 6+ months as it is advected 700 km southward. This pathway subducts shelf water into the ocean interior, and provides a link between mesoscale circulation and shelf water transport. The BSW that is captured in anti‐cyclones advects south past the east coast of Tasmania, with some moving into the eastern Indian Ocean.