Observations of a Tropical Instability Vortex

Abstract

An upper-ocean vortex associated with tropical instabilities was observed during fall 1990 at 140°W in the shear region between the Pacific South Equatorial Current and the North Equatorial Counter current. The velocity and thermohaline structures of the vortex were mapped in three dimensions using hydrography, acoustic Doppler current measurements, drifters, and satellite infrared images. The vortex translated westward at 30 cm s?1 (0.24° day?1), stationary relative to the mean flow, and less than half the 80 cm s?1 speed of contemporaneous meridional oscillations of the Equatorial Undercurrent and South Equatorial Current. The coherent flow pattern was restricted to above the thermocline. Convergence at the North Equatorial Front and divergence near the vortex center occurred in a dipole pattern similar to those predicted by various numerical models. The convergence and the anticyclonic vorticity were of the same magnitudes as the local inertial frequency, suggesting that the feature was a fully nonlinear, large Rossby number vortex, and may have been subject to centrifugal instability. The anticyclonic flow was associated with a thermocline depression of 30 m and a deformation of the North Equatorial Front. Northward advection of cold, saline, equatorial water and southward advection of warmer, fresher, tropical water yielded the cusplike surface temperature pattern commonly associated with tropical instabilities. Equatorward heat and freshwater fluxes implied cooling and freshening from 3°N to 5°N, comparable to the annual-mean net surface heating and evaporation minus precipitation for the region.