Deep water properties, velocities, and dynamics over ocean trenches

Observations of water properties and deep currents over several trenches in the Pacific 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 float data. Finally, bathymetry is used to motivate a simple dynamical framework for flow 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-stratified abyss through vortex stretching. The magnitude of this recirculation is limited by bottom drag. Ageostrophic flow 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 flow predicted across blocked geostrophic contours by the Linear planetary geostrophic balance.