Vortical and internal wave shear and strain

Abstract

Depth-time records of isopycnal vertical strain have been collected from intensive CTD profiling programs on the research platform (R/P) Floating Instrument Platform (FLIP). The associated vertical wavenumber frequency spectrum of strain, when viewed in an isopycnal-following frame, displays a clear spectral gap at low vertical wavenumber, separating the quasigeostrophic (vortical) strain field and the superinertial internal wave continuum. This gap enables both model and linear-filter-based methods for separating the submesoscale and internal wave strain fields. These fields are examined independently in six field programs spanning the period 1983-2002. Vortical and internal wave strain variances are often comparable in the upper thermocline, of order 0.2. However, vortical strain tends to decrease with increasing depth (decreasing buoyancy frequency as N2 = -g/ρ(dρ/dz) ~(N2)1/2, while internal wave strain variance increases as ~(N2)1/2, exceeding vortical variance by a factor of 5-10 at depths below 500 m. In contrast to strain, the low-frequency spectral gap in the shear spectrum is largely obscured by Doppler-smeared near-inertial motions. The vertical wavenumber spectrum of anticyclonic shear exceeds the cyclonic shear and strain spectra at all scales greater than 10 m. The frequency spectrum of anticyclonic shear exceeds that of both cyclonic shear and strain to frequencies of 0.5 cph, emphasizing the importance of lateral Doppler shifting of near-inertial shear. The limited Doppler shifting of the vortical strain field implies surprisingly small submesoscale aspect ratios: kH/kz ~ 0.001, Burger numbers Br = kH N/kzf ~ 0.1. Submesoscale potential vorticity is dominated by vertical straining rather than the vertical component of relative vorticity. The inferred rms fluctuation of fluid vorticity is far less for the vortical field than for the internal wavefield. © 2014 American Meteorological Society.