Deep water properties, velocities, and dynamics over ocean trenches

Abstract

Observations of water properties and deep currents over several trenches in the Pacie c Ocean central basins give consistent evidence for recent ventilation of water below the trench sills and cyclonic sense of circulation over the trenches. A dynamical argument for this pattern is advanced. First, a review of previous analyses of hydrographic data shows that the trenches are well ventilated by dense bottom water, that within the trenches this bottom water generally spreads away from its source, and that a cyclonic sense of circulation is suggested over some trenches. Then, this cyclonic sense of circulation over the trenches is further documented using deep current meter and e oat data. Finally, bathymetry is used to motivate a simple dynamical framework for e ow over trenches. If the trench sides are sufficiently steep and the trench is sufficiently removed from the equator to ensure a region of closed geostrophic contours, then any upwelling over that region will drive a strong deep cyclonic recirculation in the weakly-stratie ed abyss through vortex stretching. The magnitude of this recirculation is limited by bottom drag. Ageostrophic e ow in a bottom Ekman layer into the trench balances the water upwelled over the trench. The cyclonic recirculation is much stronger than the upwelling-driven e ow predicted across blocked geostrophic contours by the linear planetary geostrophic balance.