Observations of Variability in the South Pacific Subtropical Gyre

Abstract

Variability of the subtropical gyre in the South Pacific Ocean was investigated using high-resolution expendable bathythermograph sections along a repeated track between New Zealand and Hawaii. The southern part of the section sampled most of the zonal flow in the subtropical gyre with the eastward flowing branch between New Zealand and Fiji and the westward branch extending north of Fiji to approximately 10°S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was calculated, relative to 800 m, with the aid of a mean T ? S relationship. Variability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conclude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre-scale circulation along the transect. The shape and intensity of the gyre varied seasonally throughout the water column (0?800 m). Geostrophic transport was most intense (15 Sv, where Sv ? 106m3s?1) in November. At this time, the northern edges of eastward flow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximately two to three months after the basinwide seasonal peak in Ekman pumping. Transport was weakest (11 Sv) in May and was associated with an increase in the poleward slant of the gyre center with depth and a decrease in the ratio of thermocline to surface transport. Seasonal wind forcing was considered as a possible mechanism for the observed annual intensification of the gyre-scale circulation. A simple linear model of thermocline response to local changes in wind stress curl explained a significant fraction of the observed annual variability. Conservation of potential vorticity q yielded an estimate for the absolute mean flow (?1 cm s?1 at 800 m), consistent with direct measurements in the region. Interannual variability, possibly related to the El Niño-Southern Oscillation cycle, was observed. The cold event of 1988/89 appeared to be associated with relatively weak gyre-scale transport. After 1991, gyre-scale transport was more intense and a prominent change in the small-scale circulation occurred, with a shift in the alongtrack wavenumber spectral energy to higher wavenumbers.