Spatial structure of thermocline and abyssal internal waves in the Sargasso Sea

Vertical and horizontal spatial structures are analyzed for the steady and internal wave velocity contributions in one hundred full-water-depth velocity profiles collected in the Sargasso Sea in water depths between 4500 and 5500 m. Temporal decompositions into subinertial, near-inertial, and high-frequency velocity contributions are obtained from multiple, but brief time series at several locations. Horizontal spatial variability is evaluated from two simultaneous velocity profiles at separations ranging from 15 m to 12.5 km. The total internal wave field exhibits equipartition between east and north velocity components, a decrease in energy density at the smallest vertical wavenumbers, and an overall dependence for kinetic energy (KE) on vertical wavenumber as m−2.5. Most of the internal wave energy is in near-inertial motions and, of this, most occurs at Wentzel–Kramers–Brillouin (WKB) normalized vertical wavelengths of 150–800 stretched-m (i.e., sm) with a spectral peak at 500-sm wavelength (for No=3 cph) and average surrounding the peak of 3 c/skm (330 sm). Near-inertial contributions exhibit a power law of m−3, while higher-frequency internal waves (ω>2f) a slope of m−2. There is strong vertical polarization (clockwise>anticlockwise) (CW>ACW) of the near-inertial contribution but little or none for higher-frequency motions. There is more WKB normalized near-inertial KE in the lower than in the upper half of the WKB-scaled water column while high-frequency internal waves have comparable upper and lower halves energies. The upper half shows a deficit compared to the Garrett and Munk model spectrum at vertical wavelengths shorter than 100 sm. Time-mean shear is largest in the upper half, so critical-layer processes may play a role. The internal wave KE of simultaneous but spatially separated profiles has a zero-correlation scale of 15–20 km, dominantly due to near-inertial waves. Thus, deep near-inertial motions exhibit wavelengths of 60–80 km in contrast to longer scales reported in the surface mixed layer and upper pycnocline. The aspect ratio k/m (330 sm/70 km) corresponds to a wave frequency of 1.05f. The downward group velocity is 0.6 mm s−1, with a vertical energy flux for the near-inertial motions of 0.6 mW m−2.