Suppression of Internal Wave Breaking in the Antarctic Circumpolar Current near Topography

Abstract

Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure-and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…Turbulent quantities are important to the energy balance of the global ocean and are prevalently parameterized in largescale circulation models. Results from previous work using finestructure methods to interpret mixing rates have recently been shown to be biased high (Frants et al (2013), Mater et al (2015), Waterman et al (2014)), often by up to two orders of magnitude. This work has revisted the values and patterns of mid-depth mixing in this region to inform modeling efforts and large-scale estimates of vertical diffusivity.…”

“…Both Sheen et al (2014) and Waterman et al (2014) showed that the shear based parameterization overestimates observed mixing rates in near-bottom regions of strong internal wave generation. The parameterization relies on the assumption that the background internal-wave field is well characterized by the Garrett-Munk (GM) spectrum.…”

“…Since the advent of acoustic Doppler current profiling, measurements in the Southern Ocean have demonstrated internal wave velocity finestructure in regions of rough topography. This includes the Kerguelen Plateau (Polzin and Firing (1997), Waterman et al (2014)) and the Scotia Sea (Naveira Garabato et al (2004)). The presence of velocity, and in some cases, density finestructure has been widely interpreted as signatures of enhanced turbulence and mixing levels.…”

“…Recent studies have shown discrepancies between dissipation and diffusivity estimates from microstructure measurements and finestructure parameterizations , Waterman et al (2014)) in the Southern Ocean. Both Sheen et al (2014) and Waterman et al (2014) showed that the shear based parameterization overestimates observed mixing rates in near-bottom regions of strong internal wave generation.…”

“…The frontal region of the Southern Ocean fails this characterization because it is forced by energetic internal waves that originate as lee waves in the deep flow over topography. As discussed by Waterman et al (2014), the extent to which the internal wave environment of the Southern Ocean can be modeled by the GM spectrum is unclear.…”