Estimating Suppression of Eddy Mixing by Mean Flows

Andreas Klocker Massachusetts Institute of Technology, Cambridge, Massachusetts

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Raffaele Ferrari Massachusetts Institute of Technology, Cambridge, Massachusetts

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Joseph H. LaCasce Department of Geosciences, University of Oslo, Oslo, Norway

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Abstract

Particle- and tracer-based estimates of lateral diffusivities are used to estimate the suppression of eddy mixing across strong currents. Particles and tracers are advected using a velocity field derived from sea surface height measurements from the South Pacific, in a region west of Drake Passage. This velocity field has been used in a companion paper to show that both particle- and tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. These estimates of eddy diffusivities are here analyzed to show 1) that the degree of suppression of mixing across the strong Antarctic Circumpolar Current is correctly predicted by mixing length theory modified to include eddy propagation along the mean flow and 2) that the suppression can be inferred from particle trajectories by studying the structure of the autocorrelation function of the particle velocities beyond the first zero crossing. These results are then used to discuss how to compute lateral and vertical variations in eddy diffusivities using floats and drifters in the real ocean.

Current affiliation: Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.

Corresponding author address: Andreas Klocker, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia. E-mail: andreas.klocker@anu.edu.au

Abstract

Particle- and tracer-based estimates of lateral diffusivities are used to estimate the suppression of eddy mixing across strong currents. Particles and tracers are advected using a velocity field derived from sea surface height measurements from the South Pacific, in a region west of Drake Passage. This velocity field has been used in a companion paper to show that both particle- and tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. These estimates of eddy diffusivities are here analyzed to show 1) that the degree of suppression of mixing across the strong Antarctic Circumpolar Current is correctly predicted by mixing length theory modified to include eddy propagation along the mean flow and 2) that the suppression can be inferred from particle trajectories by studying the structure of the autocorrelation function of the particle velocities beyond the first zero crossing. These results are then used to discuss how to compute lateral and vertical variations in eddy diffusivities using floats and drifters in the real ocean.

Current affiliation: Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.

Corresponding author address: Andreas Klocker, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia. E-mail: andreas.klocker@anu.edu.au
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