Summertime Internal Wave Properties along the SR1b Section across Drake Passage

Abstract The Antarctic Circumpolar Current (ACC) connects all major oceans with a set of deep-reaching eastward frontal jets. When crossing large-amplitude topography, the jets drive energetic internal waves, turbulence, and mixing. Here, we present an analysis of internal wave observations from 19 occupations of the SR1b repeat hydrography section across the ACC in the Scotia Sea. Internal wave strength is quantified with vertical straining of the density stratification (strain), vertical shear of horizontal velocity (shear), vertical divergence of vertical velocity (divergence), and vertical kinetic energy (VKE). When temporally averaged, all four parameters show a consistent pattern with bottom intensification over steep and rough topography but not elsewhere. Both the ACC jets and internal waves are anchored to seafloor topography. The internal waves are weak in the topographically smooth Ona Basin, even though the near-bottom currents there are stronger than where the ACC crosses the Scotia Ridge. Since vertical divergence, which has not been used in this context before, is both highly correlated with but also easier to measure than internal wave shear, we propose the divergence-to-strain variance ratio as an alternative to the shear-to-strain variance ratio for quantifying the frequency content of the internal wave field. Along SR1b, both variance ratios, as well as the wavenumber spectral slopes of shear and VKE, show a consistent spatial pattern. Low values in the upper ocean along the entire section are associated with longer and higher-frequency waves. Highly variable and energetic waves are found in the deep ACC jets. Unexpectedly high variance ratios and spectral slopes, indicating a preponderance of short, near-inertial waves in the deep Ona Basin, are attributed to critical layer absorption.