Origin and circulation of water types on the 26.00 sigma-t surface of the south-west Pacific

Abstract

CSIRO hydrological data from the south-west Pacific (0-45ºS., 140°E.-160º W.) during 1960-67 have been used to calculate annual means of salinity and oxygen, within 5º squares, on the 26.00 sigma-t surface. In addition, for some squares it was possible to calculate mean summer (December-February) and mean winter (July- September) values of salinity and oxygen.

From the annual means the salinity-oxygen relations were used to characterize four water types. Water type 1 of tropical origin had a salinity of 35.03‰ and an oxygen content of 3.20 ml/l. Water type 2, also of tropical origin, had a salinity of 35.23‰ and an oxygen content of 2.30 ml/l. Water type 3 of subtropical origin had a salinity of 35 +66‰ and an oxygen content of 5.45 ml/l. Water type 4 of Subantarctic origin had a salinity of 34.80‰ and an oxygen content of 6.00 ml/l.

The ultimate origins of water types 1 and 2 are thought to be at considerable distances from the south-west Pacific region. Water type 1 is suggested as a mixture of waters of the North Equatorial Pacific, to the west of about 160°W., and waters of type 3 originating in the south Tasman Sea and other regions of the south Pacific. Water type 2 forms at around 16O°W., by mixing of eastern tropical Pacific waters and water type 3. It is thought that water types 1 and 2 form at different times of the year depending upon the extent of meridional or zonal flow in the central Equatorial region. Water types 3 and 4, however, are formed by southward spreading and winter cooling at the surface of subtropical waters, and by northward spreading and summer warming at the surface of Subantarctic waters, respectively. These two water types are therefore of south-west Pacific origin. Generally in the Tasman Sea (south of 25ºS.), the concentration of water type 1 is very low (less than 10%); of water type 2 only a little higher (20-30%), but that of water type 3 is high (around 60-75%). The concentration of water type 4 was much greater (40%) off the west coast of South Island, New Zealand, than off the east coast of Tasmania (15 %). Seasonal pulses in the concentrations of water types 1 and 2 along 170°E., between 0 and 15ºS., are in phase with seasonal changes in the concentration of these two water types in the Tasman and Coral Seas, if these water types spread southward at about 10 cm/sec. Summer increases in the concentration of water type 3 in the Tasman Sea off New Caledonia have been explained by the northward spreading of the previous winter's accumulation of this water type in the central Tasman Sea.