Influences of mean shear in Florida current on turbulent production by internal waves

Abstract

Observations of shear finestructure and turbulence in the Florida Current are analyzed to assess whether internal wave parameterizations can predict viscous dissipation rates in a vertically sheared background. Measurements by the Multi-Scale Profiler (MSP) at seven stations spanning the Straits of Florida characterize levels and patterns of internal wave activity and mixing.Turbulent diffusivity over the section is moderate, at a decade above oceanic background levels. Strong mixing occurs within 100 m of the channel bottom, in turbulent stratified boundary layers. Weak background-level mixing occupies the high velocity core of the current. Mean shear is highest west of the core, but it is stabilized by strong stratification. Moderately strong mean shear found deep at midchannel weakens toward the surface. Fluctuating shear spectra are 1 to 4 times the oceanic reference, and typically appear anisotropic and asymmetric. Distortion of the mean shear by internal waves contributes to clockwise variance, and there are signs of near-critical reflection off the east channel wall.By using total rather than fluctuating shear variance or spectra in existing finescale parameterizations, observed dissipation is predicted within a factor of two for most of this dataset. This accounts for the mean shear contribution, as it supplements large-scale internal waves in setting the cutoff past which instability transfers energy toward turbulence. Overall, interactions affecting smaller waves seem to proceed at anticipated rates, regardless of how much asymmetry, anisotropy, or subinertial vs internal wave shear is present at larger scales, even for flow with squared mean Froude up to 0.5.For regions of strong turbulence or high mean shear that are limited in extent, finescale shear variance for use in the parameterizations should be estimated spatially rather than spectrally. Spectral windows that isolate such features may yield inadequate resolution or representation of important contributions of larger-scale shear. The cutoff wavenumber corresponding to the onset of instability is then overestimated, leading to underestimated dissipation rates.