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  • Author or Editor: Amala Mahadevan x
  • LatMix: Studies of Submesoscale Stirring and Mixing x
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Gualtiero Badin
,
Amit Tandon
, and
Amala Mahadevan

Abstract

Using a process study model, the effect of mixed layer submesoscale instabilities on the lateral mixing of passive tracers in the pycnocline is explored. Mixed layer eddies that are generated from the baroclinic instability of a front within the mixed layer are found to penetrate into the pycnocline leading to an eddying flow field that acts to mix properties laterally along isopycnal surfaces. The mixing of passive tracers released on such isopycnal surfaces is quantified by estimating the variance of the tracer distribution over time. The evolution of the tracer variance reveals that the flow undergoes three different turbulent regimes. The first regime, lasting about 3–4 days (about 5 inertial periods) exhibits near-diffusive behavior; dispersion of the tracer grows nearly linearly with time. In the second regime, which lasts for about 10 days (about 14 inertial periods), tracer dispersion exhibits exponential growth because of the integrated action of high strain rates created by the instabilities. In the third regime, tracer dispersion follows Richardson’s power law. The Nakamura effective diffusivity is used to study the role of individual dynamical filaments in lateral mixing. The filaments, which carry a high concentration of tracer, are characterized by the coincidence of large horizontal strain rate with large vertical vorticity. Within filaments, tracer is sheared without being dispersed, and consequently the effective diffusivity is small in filaments. While the filament centers act as barriers to transport, eddy fluxes are enhanced at the filament edges where gradients are large.

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Andrey Y. Shcherbina
,
Miles A. Sundermeyer
,
Eric Kunze
,
Eric D’Asaro
,
Gualtiero Badin
,
Daniel Birch
,
Anne-Marie E. G. Brunner-Suzuki
,
Jörn Callies
,
Brandy T. Kuebel Cervantes
,
Mariona Claret
,
Brian Concannon
,
Jeffrey Early
,
Raffaele Ferrari
,
Louis Goodman
,
Ramsey R. Harcourt
,
Jody M. Klymak
,
Craig M. Lee
,
M.-Pascale Lelong
,
Murray D. Levine
,
Ren-Chieh Lien
,
Amala Mahadevan
,
James C. McWilliams
,
M. Jeroen Molemaker
,
Sonaljit Mukherjee
,
Jonathan D. Nash
,
Tamay Özgökmen
,
Stephen D. Pierce
,
Sanjiv Ramachandran
,
Roger M. Samelson
,
Thomas B. Sanford
,
R. Kipp Shearman
,
Eric D. Skyllingstad
,
K. Shafer Smith
,
Amit Tandon
,
John R. Taylor
,
Eugene A. Terray
,
Leif N. Thomas
, and
James R. Ledwell

Abstract

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

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