VERTICAL HEAT-FLUX MEASUREMENTS FROM A NEUTRALLY BUOYANT FLOAT

A neutrally buoyant float instrumented to measure 1-5 m shear and stra tification was deployed for ten days in a near-inertial critical layer at the base of a warm-core ring. Vertical velocity and temperature da ta, from which large-scale (much greater than m) subinertial fluctuati ons have been removed, are used to estimate the vertical heat flux [w' T']. The resulting directly measured net heat flux is significantly no nzero and consistent with that inferred from microstructure measuremen ts of turbulent dissipation rates epsilon and chi(T). The w, T cospect ra tends to be negative at low encounter frequencies (f < omega(E) < 1 .6N) and positive at higher encounter frequencies. The low frequency o f the negative heat flux appears to be due to the intermittent co-occu rrence of shear instability and wave-intensified stratification. The p ositive heat flux is associated with smaller scales (high Doppler freq uencies) associated with secondary gravitational instability, fully th ree-dimensional turbulence, and restratification.