Vertical Heat-Flux Measurements from a Neutrally Buoyant Float

Abstract

A neutrally buoyant float instrumented to measure 1?5 m shear and stratification was deployed for ten days in a near-inertial critical layer at the base of a warm-core ring. Vertical velocity and temperature data, from which large-scale (>5 m) subinertial fluctuations have been removed, are used to estimate the vertical heat flux ?w?T??. The resulting directly measured net heat flux is significantly nonzero and consistent with that inferred from microstructure measurements of turbulent dissipation rates ? and ?T. The w, T cospectra tends to be negative at low encounter frequencies (f< wE<1.6N) and positive at higher encounter frequencies. The low frequency of the negative heat flux appears to be due to the intermittent co-occurrence of shear instability and wave-intensified stratification. The positive heat flux is associated with smaller scales (high Doppler frequencies) associated with secondary gravitational instability, fully three-dimensional turbulence, and restratification.