Eddy-Induced Modulation of Turbulent Dissipation over Rough Topography in the Southern Ocean

Abstract

Mesoscale eddies are universal features of the ocean circulation, yet the processes by which their energy is dissipated remain poorly understood. One hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves, with the breaking of these waves causing locally elevated dissipation focused near the sea floor. This study uses hydrographic and velocity data from a 1-yr mooring cluster deployment in the Southern Ocean to test this hypothesis. The moorings were located over a small (;10 km) topographic obstacle to the east of Drake Passage in a region of high eddy kinetic energy, and one was equipped with an ADCP at 2800-m depth from which internal wave shear variance and dissipation rates were calculated. Examination of the ADCP time series revealed a predominance of upward-propagating internal wave energy and a significant correlation (r 5 0.45) between shear variance levels and subinertial near-bottom current speeds. Periods of strong near-bottom flow coincided with increased convergence of eddy-induced interfacial form stress in the bottom 1500 m. Predictions of internal wave energy radiation were made from theory using measured near-bottom current speeds, and the mean value of wave radiation (5.3 mW m 22) was sufficient to support the dissipated power calculated from the ADCP. A significant temporal correlation was also observed between radiated and dissipated power. Given the ubiquity of strong eddy flows and rough topography in the Southern Ocean, the transfer from eddy to internal wave energy is likely to be an important term in closing the ocean energy budget.

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

“…The plausibility of this hypothesis is the subject of ongoing work. Moored data do document a tendency of near-inertial waves to enter into such critical layer situations in combination with high-frequency waves exiting them (Polzin 2010), and Brearley et al (2013) report that mooring measurements to the east of Drake Passage show CW polarization in association with near-inertial waves, but that total near-bottom shear variance is dominated by higherfrequency waves. Further, although vertical anisotropy characterizes both the tidal and lee-wave generation problem, in the former m c ffi N/U BT , whereas in this leewave example m c ffi 10N/U g (here U BT and U g are the amplitudes of the barotropic tidal velocity and geostrophic flow velocity, respectively).…”

A Microscale View of Mixing and Overturning across the Antarctic Circumpolar Current

“…Conversely, the upward radiation and breaking of eddy-generated internal lee waves has been found to underpin the intensification of turbulent dissipation within 1-2 km of the seafloor in ACC regions of complex bathymetry, including Drake Passage (Naveira Garabato et al 2004;Nikurashin and Ferrari 2010;St. Laurent et al 2012;Sheen et al 2013;Brearley et al 2013). The significant enhancement of K t › ?…”

Generation of Internal Waves by Eddies Impinging on the Western Boundary of the North Atlantic

“…The process of a wave packet captured by an eddy strain field (Bühler and McIntyre 2005;Polzin 2008Polzin , 2010 can contribute to their energy exchange without any dependence on the topography. Additionally, internal waves may be generated when a geostrophic current flows over rough topography (Liang and Thurnherr 2012;Brearley et al 2013) in a process described by the linear theory of lee waves over small-scale abyssal hills (Bell 1975). For steeper and taller topography, nonlinearities arise (Baines 1995;Klymak et al 2010) that render the energy transfer from geostrophic flow to internal waves less predictable.…”