Measurements of vertical shear and strain were acquired from the research platform FLIP during the PATCHEX experiment in October, 1986 (34°N, 127°W). Vertical sheer was shear from two separate Doppler sonar systems. A long-range sonar, with independent estimates every 18 m, sampled from 150–1200 m in depth. A short-range sonar measured fine-scale shear over 150–180 m depth, with 1.5 m vertical resolution. Vertical strain, ∂η/∂z, was estimated from two repeatedly profiling CTDs. These sampled to 560 m once every three minutes. The time variation of the strain field is monitored in both Eulerian (fixed-depth) and semi-Lagrangian (isopycnal-following) reference frames, from 150–406 m depth.
Eulerian vertical wavenumber-frequency (m, ω) spectra of vertical shear and strain exhibit a frequency dependency which is a strong function of wavenumber (ω−2–ω0 for m = 0.01–0.3 cpm). In contrast the semi-Lagrangian strain spectrum is more nearly separable in frequency and wavenumber, in closer agreement with the Garrett–Munk (GM) internal wave spectral model.
When a simulated GM shear field is vertically advected by a GM isopycnal displacement field, the resultant Eulerian vertical wavenumber–frequency spectrum exhibits the same qualitative, nonseparable, form as the PATCHEX shear spectrum: The dominant near-inertial waves are Doppler-shifted across all frequency bands, resulting in a “while” frequency spectrum at high wavenumbers. Measured ratios of Eulerian shear/strain variance support this interpretation. Higher shear-low strain variances (characteristic of near-inertial waves) are seen at high wavenumber, high encounter frequencies. The conclusion is that internal wave vertical self-advection strongly alters the observed frequency at high vertical wavenumbers in an Eulerian reference frame.