Observing the Fluctuations of Gyre-Scale Ocean Circulation: A Study of the Subtropical South Pacific

Abstract

Seasonal and interannual variability of the subtropical gyre in the South Pacific Ocean are investigated by means of a time series of expendable bathythermograph (XBT) sections between New Zealand (36°S, 175°E) and Fiji (18°S, 178°E). The experiment spans much of the subtropical gyre and is a prototype for future basin-scale observations. Eddy-resolving transects along the precisely repeating ship track, spanning four years, are used to estimate the mean field and fluctuations of temperature and geostrophic velocity. The mean field dominates on very large spatial scales while the fluctuations dominate on small scales. Mean and fluctuations have equal energy at a horizontal wavelength of about 2000 km. The study region contains three recurring small-scale features. These are the East Auckland Current, flowing eastward along the New Zealand continental slope, a front at about 29°S which is likely an extension of the Tasman Front, and a weaker feature, the Tropical Convergence at about 22°S. At lower latitudes in the study region, the entire thermocline migrates vertically at annual period. This annual oscillation ends near the front at 29°S. Farther poleward, the only substantial subsurface annual variation is in the strength of the East Auckland Current. Interannual variability of circulation during 1986?90 consisted of rapid transitions between two rather steady states. In one state, which persisted through 1987?88 and from mid-1989 to the present (mid-1990), the eastward flowing limb of the gyre was relatively strong and narrow, with a reversal in velocity at the ocean surface south of Fiji.