Deep tracer and dynamical plumes in the tropical Pacific Ocean

Anomalous middepth plumes in potential temperature‐salinity, θ‐ S , and buoyancy frequency squared, N 2 , originate east of the East Pacific Rise Crest and decay toward the west. Conductivity‐temperature‐depth (CTD) data from recent hydrographic sections at 15°S and 10°N are used together with meridional sections at 110°, 135°, and 151°W to map these structures. Warm salty plumes west of the rise crest have maxima centered at 2700 m, 10°S and 8°N, and are interrupted by a cold, fresh tongue centered at 2900 m, 2°S. The θ‐ S anomalies decay to half their peak strength 2800 km to the west of the rise crest, ±300 km in the meridional, and ±0.4 km in the vertical. Vertical N 2 minima occur within the plumes, regions of reduced vertical gradients in θ and S . These minima are underlain by maxima near the depth of the rise crest, about 3200 m. The N 2 plumes decay more rapidly to the west of the rise crest than do the θ‐ S plumes. The N 2 structure is consistent with a pair of stacked gyres in each hemisphere. There are at least three possible mechanisms consistent with some aspects of these features. First, a deep maximum in upwelling somewhere below 2700 m would result in equatorward and westward interior flow at 2700 m, advecting these plumes along with it. Second, rapid upwelling of warm, salty, unstratified water in the eastern basins could result in westward overflows over the rise crest. Third, upwelling and associated entrainment processes owing to hydrothermal venting could result in stacked counter‐rotating gyres west of the rise crest.