Ocean Mixing and Stirring  
  Principal Investigator: Dr. Miles A. Sundermeyer
msundermeyer@umassd.edu • 508-999-8892
University of Massachusetts Dartmouth
School for Marine Science and Technology
 
 

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Research:

* Lidar Studies of Small- Scale Lateral Dispersion - LatMix
* 3D Dye Mapping using Airborne Lidar - Florida 2004 Pilot
* Lab Studies of Stirring by Small-Scale Geostrophic Motions
* Numerical Simulations of Vortical Mode Stirring
* Coastal Mixing & Optics (CMO)
(... More links coming soon!)
* The North Atlantic Tracer Release Experiment (NATRE)
(Links coming soon!)

Additional Links
* M. Sundermeyer's CV (PDF)

 

© 2005 Miles A. Sundermeyer (msundermeyer@umassd.edu)
Note: Please do not use the data, text, or images contained on this site without prior permission.

Lab Studies of Stirring by Small-Scale Geostrophic Motions
PIs: M. A. Sundermeyer and D. L. Hebert
Grad Students: G. A. Stuart (Masters Degree June 2007)
A.-M. Suzuki-Brunner (Ph.D. expected June 2010)
Grant Title: Laboratory Studies of Stirring by Small-Scale Geostrophic Motions
Funding Agency: The National Science Foundation
Award: $571,352

Objectives

We use laboratory experiments to study lateral stirring by small-scale geostrophic motions formed by the adjustment of diapycnal mixing events. Experiments build on analytical and numerical studies by Sundermeyer et al. (2005) and Sundermeyer and Lelong (in press), which predict the amount of lateral dispersion caused by vortical mode stirring. A major advantage of the laboratory studies over previous analytical and numerical studies, however, is that here diapycnal mixing events will ultimately be driven by internal wave breaking rather than artificially imposed mixing. Two major goals of this work are 1) to test theoretical predictions for vortical mode stirring when internal wave forcing and breaking are present, and 2) to provide a basis for parameterizing horizontal dispersion rates by vortical mode stirring in the ocean. The proposed work will help provide a quantitative description of vortical mode stirring on scales of 1-10 km in the ocean.

 
Rotating tank facility at the University of Rhode Island's Graduate School of Occeanography.

Description of Lab Experiments

Experiments are conducted at the University of Rhode Island's Graduate School of Oceanography rotating tank facility. A 1 m diameter, 30 cm deep uniformly stratified rotating tank is used to model conditions in the ocean's stratified interior. Two methods will be used to generate diapycnal mixing events: 1) mechanical stirring in the form of localized grid-forced turbulence, and 2) a quasi-random field of internal waves and wave breaking generated by near resonant forcing of a mode-1 internal wave, and wave-wave interactions to scatter energy into higher modes. The formation of vortical modes and their effects on lateral stirring of a passive fluorescent dye will be examined using a combination of Particle Imaging Velocimetry (PIV), Laser Induced Fluorescence (LIF), and digital video analysis. Results will be compared with theoretical and numerical predictions from previous studies.

Results to Date

 
(Left) Laboratory setup for single mixer experiments. (Middle) Side (above) and top (below) views of a single grid mixer. (Right) Side (above) and top (below) views after mixing dye within the mixed patch.

Single Mixer Experiments - In the first phase of our study we analyze the case of a single eddy in order to confirm the dynamics of the geostrophic adjustment problem, and to obtain relevant time and space scales for the multi-eddy case. For these experiments, isolated mixed patches are generated mechanically using grid-forced turbulence. A schematic of the laboratory configuration for the single mixed patch case is shown below. Also shown is a pair of photographs of the tank before and after a mixed patch is generated. The mixed patch is created by vertically oscillating a small disk-shaped grid for a brief time, and allowing the resulting mixed patch to adjust under the influence of rotation. In the right-most image, a green dye is injected into the center of the mixed patch as it is being generated. The dye clearly shows both the vertical and horizontal extent of the mixed patch.

The same single mixer experiments can also be visualized using PIV. The sequence of images below show the adjustment and gradual decay of the vortex that ensues from the mixed patch adjustment. These images provide a means of quantitatively assessing the strength of the resulting eddy, its size, and its decay time, all of which have significant implacations to the overall lateral dispersion that results.

A series of PIV images (horizontal velocity shown by colored arrows) of the adjustment of a single mixed patch. Times are shown in each image in terms of number of seconds since the cessation of mixing, and in terms of the number of inertial periods. Noteworthy is the rapid organization of the adjustment into a coherent eddy after approximately 1 inertial period, and the subsequent persistance of the eddy.
 
Schematic of laboratory setup for multi-mixer experiments. Configuration is similar to single mixer case, except that multiple mixers are activated in turn at quasi-random locations in the tank.

Multi-Mixer Experiments - Next we analyze the case of multiple eddies and their effect on lateral dispersion. Again we begin by generating isolated mixed patches, this time at regular time intervals, but at random locations in the tank. A schematic of the multi-eddy laboratory set-up is shown to the right.

Dye is released at the center of the tank, and tracked for many tens of inertial periods in order to visualize the dispersion. The sequence of images below shows an example of one of these experiments. Times in seconds and inertial periods are given in each frame. Still images of the dye are analyzed to estimate the second moment of the dye patch as it evolves in time. The second moment of the patch is then used to infer the effective eddy diffusivity of the patch.

Sequence of images from multi-eddy experiment showing eddy dispersion of a dye released at the center of the tank. Times are shown in each image in terms of number of seconds since the cessation of mixing, and in terms of the number of inertial periods.
(Left) Horizontal second moments inferred at a single snapshot in time from one of the tracer tracer images shown above, and (Right) growth of the second moments in each direction over the course of the experiment.

Ongoing Analysis

As a next step to the above analysis, we will repeat the above experiments for varying experimental parameters, including the Coriolis frequency, time between mixed patches, and stratification. For the single mixer experiments, this will enable us to verify the dynamics of the geostrophic adjustment problem, while for the multi-eddy case, it will enable us to begin to test the theoretical scaling prediction of Sundermeyer et al. (2005).

Once we have completed this initial phase of the experiments, we will turn to phase two of this project in which the mixed patches will be generated by internal wave breaking forced by large-scale internal waves. This will be done using a large-scale wave generator, again on a rotating tank. Here we will explicitly explore the dynamics of the wave breaking itself, and how the wave field effects the subsequent development of, and stirring by, small-scale eddies.

... Stay tuned for more results to follow ...

Presentations

Stuart, G. A., M. A. Sundermeyer, and D. L. Hebert. Lab Studies of Small-Scale Geostrophic Eddies. Middle Atlantic Bight Physical Oceanography and Meteorology Meeting, November, 2005, Stony Brook, NY.
 

Prof. Miles A. Sundermeyer
The School for Marine Science and Technology
706 South Rodney French Blvd., New Bedford, MA 02744-1221
voice: 508.999.8892 fax: 508-910-6371 e-mail: msundermeyer@umassd.edu
www.smast.umassd.edu/msundermeyer