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An upper-ocean vortex associated with tropical instabilities was observed during fall 1990 at 140ЊW 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 Ϫ1 (0.24Њ day Ϫ1), stationary relative to the mean flow, and less than half the 80 cm s Ϫ1 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 3ЊN to 5ЊN, comparable to the annual-mean net surface heating and evaporation minus precipitation for the region.

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The evolution of an upwelling filament was studied over a 2-week period by using satellite infrared images, and its thermohaline structure was mapped in situ. The surface velocity field consisted of a large meander extending offshore for at least 300 km.

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