Editorial Type:
Article
Article Type:
Research Article

Stirring by Small-Scale Vortices Caused by Patchy Mixing

Miles A. Sundermeyer School for Marine Science and Technology, University of Massachusetts—Dartmouth, New Bedford, Massachusetts

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

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Neil S. Oakey Ocean Sciences Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

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Blair J. W. Greenan Ocean Sciences Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

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Print Publication:
01 Jul 2005
DOI:
https://doi.org/10.1175/JPO2713.1
Page(s):
1245–1262
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Abstract

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m2 s−1. Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m2 s−1. The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

Corresponding author address: Dr. Miles A. Sundermeyer, School for Marine Science and Technology, University of Massachusetts—Dartmouth, 706 S. Rodney French Blvd., New Bedford, MA 02744-1221. Email: msundermeyer@umassd.edu

Abstract

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m2 s−1. Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m2 s−1. The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

Corresponding author address: Dr. Miles A. Sundermeyer, School for Marine Science and Technology, University of Massachusetts—Dartmouth, 706 S. Rodney French Blvd., New Bedford, MA 02744-1221. Email: msundermeyer@umassd.edu

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