Circulation patterns from seabed-drifter studies, Western Port and inner Bass Strait, Australia

Abstract

Seabed drifters were used to investigate bottom-water movements in Western Port, Victoria, Australia and in nearby parts of Bass Strait. The objective was to document circulation in and around the entrance areas and also throughout the bay, placing emphasis on smaller-scale patterns associated with net flood or ebb movements. Approximately 2000 seabed drifters were released in close-spaced arrays, half between November and December, 1974, during the summer wind regime, and the other half in September–October, 1975, before the onset of the summer wind regime. The 1974 deployment was made along 26 transects located throughout the major and minor channels of the bay and its entrances, and in Bass Strait. Groups of 5–15 drifters were released at any station. The 1975 deployment was based on the 1974 results and release stations were chosen to elucidate specific characteristics previously observed and also to document the characteristics of bottom-water movement associated with spring and neap tides. Drifter returns were 37% for the 1974 releases and 16% for 1975. These numbers were adequate to clearly define the patterns of bottom-water movement in and around the bay. In some cases drifters were retrieved by fishing boats so that drift rates could be computed. Results show an eastward movement of bottom waters in Bass Strait with net speeds of 2.3 cm/sec (1.9 km/day). The main net inward movement of water to the bay is through the western side of the Western Entrance with an ebb-dominant zone existing on the eastern side. At the Eastern Entrance, evidence of net movement is inconclusive. Detailed circulation patterns for eight smaller-scale features have been recognised and these are often closely related to the morphology of various segments of the bay. The net movement within the bay is clockwise around French Island.