Origin and circulation of water types of the 25.00 sigma-t surface of the South-west Pacific

Abstract

The salinity and oxygen of 25.00 sigma-t waters of the Tasman and Coral Seas have been averaged within 5º squares. From these average values four water types were identified by their salinity-oxygen relations. These were: a tropical high salinity (mean salinity 36.00‰, mean oxygen 3.50 ml/l.), a tropical low salinity (mean salinity 35.25‰, mean oxygen 3.10 ml/l.), a subtropical high salinity (mean salinity 35.75‰, mean oxygen 5.15 ml/l.), and a subtropical low salinity (mean salinity 35.46‰, mean oxygen 5.15 ml/l.). Waters north of 15ºS. were all mixtures of the two tropical water types, and those south of 35º S. were all mixtures of the two subtropical water types. Topography of the 25.00 sigma-t surface showed easterly flow at around 15º S. and, as far as could be interpreted, easterly flow at 35º S. Between these two zones of tropical and subtropical water types there occurred a region of mixed transition waters in which large meridional gradients of oxygen but small gradients of salinity were found. This transition region occupied a much larger area off east Australia than elsewhere, mainly because there was greater meridional exchange of the tropical and subtropical water types off east Australia. North of 20º S. the high salinity subtropical water type spreads northward in the central Tasman and Coral Seas in summer and the low salinity tropical water type spreads southward along the Queensland continental margin in winter. South of 20º S. it was impossible to separate changes due to circulation from those caused by vertical mixing and winter formation of 25.00 sigma-t water at the surface.