The Depth Dependence of Shear Finestructure off Point Arena and near Pioneer Seamount

Abstract

High-resolution velocity profiles to depths of 1600 m were collected off Point Arena and near Pioneer Seamount, California. They reveal shear increasing with depth relative to the GM76 model spectrum. Using an empirical parameterization found to collapse microstructure data the ?dissipation rate? and ?eddy diffusivity? are estimated from these shears. Away from the seamount, dissipation rates are depth invariant at 3?6(?10?10 W kg?1). As a result, the eddy diffusivity increases with depth, approaching 0.2?0.3(?10?4 m2 s?1) below 1200-m depth. This may be a result of the proximity of the continental rise and sloping topography, but there is evidence that it is a general result for the abyssal ocean. Immediately above the seamount, there is a 300?400-m thick layer of elevated shear, corresponding to an eddy diffusivity of ? 10?4 m2 s?1. If this localized enhancement is typical of seamounts, topographically induced mixing is insufficient to significantly modify global average mixing.