On the winter cascade from Bass Strait into the Tasman Sea

Abstract

Evidence is presented confirming an earlier suggestion that in winter, cold, salty (up to 35.7‰) water from Bass Strait moves eastwards and sinks sometimes to a depth of 400 m beneath warmer, fresher water from the Tasman Sea. A sharp surface-temperature front is found in winter across eastern Bass Strait a few kilometres inshore from the 200-m contour. This front probably marks the line at which Bass Strait water sinks beneath Tasman Sea water. A current-meter record at an oil drill site over a period of 8 weeks in winter 1975 shows northward currents with a speed of up to 0.5 m s^-1 near the bottom (380 m), consistent with the idea that a layer of Bass Strait water sinks and turns left (northwards) under the influence of the Coriolis force.

An internal mixed layer, more than 100 m deep, was found on XBT traces over the continental slope near Eden (37º3'S.,149º52'E.) on three occasions between October and December 1978. The temperature of this layer increased from 12 to 15ºC iri this time, closely paralleling the rise in temperature of water in Bass Strait just west of the surface-temperature front.

Where salinity data were available, they clearly showed the internal mixed layers to be saline compared with most water of the Tasman Sea, consistent with an origin in Bass Strait. Nutrient and oxygen data provide a valuable confirmation of the hypothesis that the salinity inversions found in much of the Tasman Sea originate in the Bass Strait cascade.