Convective mixing by internal waves in the Puerto Rico Trench

Abstract

A2.4 km long deep-sea mooringwas deployed for 14 months in the Puerto Rico Trench, the deepestpart of the Atlantic Ocean. Below its top buoyancy package, the mooring line held a 200 m long stringof high-resolution temperature sensors and a current meter. Over the instrumented range between6,004 and 6,204 m, waters are very weakly stratified, with local buoyancy frequency equaling about1.9 times the semidiurnal tidal frequency. Besides quiescent waters, the detailed observations showregular vertical turbulent overturning associated with periodic warming and driven by internal tidaland inertial waves. During such episodes, the 4-day and 200 m vertically averaged dissipation rateincreases from approximately 10−11 up to 10−9 m2 s−3, and eddy diffusivity increases from 5 ×10−5 up to 4 × 10−3 m2 s−1. No large-scale shear-driven Kelvin-Helmholtz billows are observed,and free convection seems the dominant primary turbulent overturning mechanism affecting materialredistribution and life in the weakly stratified environment.