Global Observations of Rotary-with-Depth Shear Spectra

Internal waves are predominantly generated by winds, tide-topography interactions, and balanced flow-topography interactions. Observations of vertical shear of horizontal velocity (u(z), y(z)) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [lambda(z) similar to O(100) m] internal waves is inferred in this study from rotary-with-depth shears. Total shear variance and vertical asymmetry ratio (Omega), i.e., the normalized difference between downward- and upward-propagating intermediate wavenumber shear variance, where Omega > 0 (<0) indicates excess downgoing (upgoing) shear variance, are calculated for three depth ranges: 200-600 m, 600 m- 1000 mab (meters above bottom), and below 1000 mab. Globally, downgoing (clockwise-with-depth in the Northern Hemisphere) exceeds upgoing ( counterclockwise-with-depth in the Northern Hemisphere) shear variance by 30% in the upper 600 m of the water column (corresponding to the globally averaged asymmetry ratio of <(Omega)over bar> = 0.13), with a near-equal distribution below 600-m depth ((Omega) over bar similar to 0). Downgoing shear variance in the upper water column dominates at all latitudes. There is no statistically significant correlation between the global distribution of Omega and internal wave generation, pointing to an important role for processes that redistribute energy within the internal wave continuum on wavelengths of O(100) m.

Automated summary

Internal waves are mainly generated by wind, tide-topography interactions, and balanced flow-topography interactions. Observations of vertical shear and calculations of total shear variance and vertical asymmetry ratio indicate that downgoing shear variance dominates in the upper 600 m of the water column globally, while the distribution is almost equal below 600 m depth.