The Subtropical Mode Waters of the South Pacific Ocean

Abstract

The subtropical mode waters (STMW) of the southwestern Pacific Ocean are described, including their physical characteristics, spatial distribution, and temporal variability. STMW is a thermostad, or minimum in stratification, having temperatures of about 15°?19°C and vertical temperature gradient less than about 2°C per 100 m. Typical salinity is 35.5 psu at 16.5°C. The STMW layer is formed by deep mixing and cooling in the eastward-flowing waters of the separated East Australia Current. Surface mixed layers are observed as deep as 300 m north of New Zealand in winter, in the center of a recurring anticyclonic eddy. The STMW thermostad in the South Pacific is considerably weaker than its counterparts in the North Atlantic and North Pacific, a contrast that may help to discriminate between physical processes contributing to its formation. A quarterly time series of expendable bathythermograph transects between New Zealand and Fiji is used to study the temporal variability of STMW. Large fluctuations are observed at both annual and subannual periods. Based on the quarterly census of STMW volume, the lifetime of the thermostad is estimated to be of order 1 year. During the years 1986?91 wintertime sea surface and air temperature minima warmed by about 1.5°C. The volume of STMW decreased dramatically during that period, with the 1989?91 census showing only a small fraction of the 1986?87 STMW volume. The observed fluctuations may be due either to long-period change in air?sea heat exchange or to fluctuations in heat transport by ocean currents.