Global Observations of Rotary-With-Depth Shear Spectra

Abstract

Abstract Internal waves are predominantly generated by winds, tide/topography interactions and balanced flow/topography interactions. Observations of vertical shear of horizontal velocity (uz, vz) from 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 [λz ~ 𝒪(100 m)] internal waves is inferred in this study from rotary-with-depth shears. Total shear variance and vertical asymmetry ratio (Ω), i.e. the normalized difference between downward- and upward-propagating intermediate wavenumber shear variance, where Ω > 0 (< 0) indicates excess downgoing (upgoing) shear variance, are calculated for three depth ranges: 200-600 m, 600 m to 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 = 0.13), with a near-equal distribution below 600-m depth ( ~ 0). Downgoing shear variance in the upper water column dominates at all latitudes. There is no statistically significant correlation between the global distribution of Ω and internal wave generation, pointing to an important role for processes that re-distribute energy within the internal wave continuum on wavelengths of 𝒪(100 m).