Breaking near-inertial waves in the Gulf Stream: Interpreting observations with theory and models

Theory and numerical simulations are used to interpret observations of banded near-inertial shear and turbulence in the winter Gulf Stream. Passing storms over this region generate inertial oscillations in the boundary layer with frequency near the effective Coriolis frequency F=[f(f+ζ)]^1/2. Cross-stream variations in the vorticity of the geostrophic flow, ζ, cause F to vary as much as ± 0.25 f. As a result, the cross-stream wavenumber of the near-inertial oscillations increases in time, driving a rapid radiation of wave energy into the interior and damping the motions in the boundary layer with an e-folding decay time of 15-60 hours. As waves propagate into the interior they become trapped and amplified where their frequency coincides with the minimum frequency of inertia-gravity waves, which depends on both the baroclinicity and vorticity of the geostrophic flow. Thus, frequency spectra in the interior peak at the local minimum frequency. Vertical variations in the minimum frequency cause the vertical wavenumber to grow linearly at a rate approximately equal to the vertical gradient in minimum frequency. This shrinking of vertical scales occurs in the absence of along-stream variations in the wave field, in contrast to a classic critical level. Combined with wave trapping and amplification and the strong geostrophic shear of the front, the process induces sub-critical Richardson numbers and hence wave breaking.