OBSERVATIONS OF VARIABILITY IN THE SOUTH-PACIFIC SUBTROPICAL GYRE

Variability of the subtropical gyre in the South Pacific Ocean was inv estigated using high-resolution expendable bathythermograph sections a long a repeated track between New Zealand and Hawaii. The southern par t of the section sampled most of the zonal flow in the subtropical gyr e with the eastward flowing branch between New Zealand and Fiji and th e westward branch extending north of Fiji to approximately 10 degrees S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was cal culated, relative to 800 m, with the aid of a mean T-S relationship. V ariability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conc lude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre scale c irculation along the transect. The shape and intensity of the gyre var ied seasonally throughout the water column (0-800 m). Geostrophic tran sport was most intense (15 Sv, where Sv=10(6)m(3)s(-1)) in November. A t this time, the northern edges of eastward dow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximatel y two to three months after the basinwide seasonal peak in Ekman pumpi ng. Transport was weakest(ll Sv) in May and was associated with an inc rease in the poleward slant of the gyre center with depth and a decrea se in the ratio of thermocline to surface transport. Seasonal wind for cing was considered as a possible mechanism for the observed annual in tensification 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 how (-1 cm s(-1) at 800 m), consistent with direct measurements in the re gion. Interannual variability, possibly related to the El Nino-Souther n 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 waven umber spectral energy to higher wavenumbers.