Stratified inertial subrange inferred from in situ measurements in the bottom boundary layer of the rockall channel

Abstract

Deep-ocean high-resolution moored temperature data are analyzed with a focus on superbuoyant frequencies. Alocal Taylor hypothesis based on the horizontal velocity averaged over 2 h is used to infer horizontal wavenumber spectra of temperature variance. The inertial subrange extends over fairly low horizontal wavenumbers, typically within 2 × 10-3 and 2 × 10-1 cycles per minute (cpm). It is therefore interpreted as a stratified inertial subrange for most of this wavenumber interval, whereas in some cases the convective inertial subrange is resolved as well. Kinetic energy dissipation rate ∈ is inferred using theoretical expressions for the stratified inertial subrange.Awide range of values within 10-9 and 4×10-7 m2 s-3 is obtained for time periods either dominated by semidiurnal tides or by significant subinertial variability.Ascaling for ∈ that depends on the potential energy within the inertio-gravity waves (IGW) frequency band PEIGW and the buoyancy frequency N is proposed for these two cases.When semidiurnal tides dominate, ∈~'(PEIGWN)3/2, whereas ∈~ 'PEIGWNin the presence of significant subinertial variability. This result is obtained for energy levels ranging from1 to 30 times theGarrett-Munk energy level and is in contrastwith classical finescale parameterization inwhich ∈~;(PEIGW)2 that applies far from energy sources. The specificities of the stratified bottom boundary layer, namely a weak stratification, may account for this difference. © 2010 American Meteorological Society.