Turbulence and Diapycnal Mixing in Drake Passage

L. St. Laurent * Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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A. C. Naveira Garabato National Oceanography Centre, University of Southampton, Southampton, United Kingdom

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J. R. Ledwell Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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A. M. Thurnherr Division of Ocean and Climate Physics, Lamont-Doherty Earth Observatory, Palisades, New York

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J. M. Toole * Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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A. J. Watson School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom

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Abstract

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.

Corresponding author address: Louis St. Laurent, Woods Hole Oceanographic Institution, 266 Woods Hole Rd., MS29, Woods Hole, MA 02543. E-mail: lstlaurent@whoi.edu

Abstract

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10−8 W kg−1 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 × 10−4 m2 s−1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10−10) W kg−1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.

Corresponding author address: Louis St. Laurent, Woods Hole Oceanographic Institution, 266 Woods Hole Rd., MS29, Woods Hole, MA 02543. E-mail: lstlaurent@whoi.edu
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