Pacific Equatorial Turbulence

Abstract

In January and February 1979 a free-fall vehicle was deployed at 150°W near the equator to sample velocity and temperature microstructure. Airfoil shear probes on the vehicle are able to resolve the peak of the velocity gradient spectrum, and permit computation of the rate of dissipation of turbulent energy (??). These measurements indicate maximum values of ?? of 10?7 W kg?1 (10?3 Cm2 s?3) to be within 0.5° of the equator, above the velocity core of the undercurrent, similar in location to the observed distribution in the Atlantic (Crawford and Osborn, 1979a). However, a secondary maximum, below the core of the undercurrent found in the Atlantic, was not observed in the Pacific. A comparison of profile-averaged values of &?? with the zonal current and density profiles suggests that temporal variations in &?? are related to the strength of the zonal shear and the magnitude of the stable stratification above the 150 m deep core of the undercurrent. The vertical eddy viscosity due to turbulence (KM) computed from these measurements is of order 0.001 m2 s?1 for the region between 20 and 140 m depth, with larger values neat 20 m, smaller values near 140 m. Upper limits to the vertical turbulent heat flux computed from the formula of Osborn (1980) yield a broad maximum of downward diffusion of heat of as much as 20 W m?2 between 40 and 120 m depth, and an average value for the turbulent eddy diffusivity for heat (KH) of <0.001 m2 s?1. The distribution of both KH and &?? are close to log-normal for data near the equator above the core, and show log-normal standard deviations of 1.1 and 1.0, respectively.