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.