Sundermeyer Miles A.

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The LatMix summer campaign : submesoscale stirring in the upper ocean

(American Meteorological Society, 2015-08) Shcherbina, Andrey Y. ; Sundermeyer, Miles A. ; Kunze, Eric ; D'Asaro, Eric A. ; Badin, Gualtiero ; Birch, Daniel ; Brunner-Suzuki, Anne-Marie E. G. ; Callies, Joern ; Cervantes, Brandy T. Kuebel ; Claret, Mariona ; Concannon, Brian ; Early, Jeffrey ; Ferrari, Raffaele ; Goodman, Louis ; Harcourt, Ramsey R. ; Klymak, Jody M. ; Lee, Craig M. ; Lelong, M.-Pascale ; Levine, Murray D. ; Lien, Ren-Chieh ; Mahadevan, Amala ; McWilliams, James C. ; Molemaker, M. Jeroen ; Mukherjee, Sonaljit ; Nash, Jonathan D. ; Ozgokmen, Tamay M. ; Pierce, Stephen D. ; Ramachandran, Sanjiv ; Samelson, Roger M. ; Sanford, Thomas B. ; Shearman, R. Kipp ; Skyllingstad, Eric D. ; Smith, K. Shafer ; Tandon, Amit ; Taylor, John R. ; Terray, Eugene A. ; Thomas, Leif N. ; Ledwell, James R.

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.
Investigating the eddy diffusivity concept in the coastal ocean

(American Meteorological Society, 2016-06-29) Rypina, Irina I. ; Kirincich, Anthony R. ; Lentz, Steven J. ; Sundermeyer, Miles A.

This paper aims to test the validity, utility, and limitations of the lateral eddy diffusivity concept in a coastal environment through analyzing data from coupled drifter and dye releases within the footprint of a high-resolution (800 m) high-frequency radar south of Martha’s Vineyard, Massachusetts. Specifically, this study investigates how well a combination of radar-based velocities and drifter-derived diffusivities can reproduce observed dye spreading over an 8-h time interval. A drifter-based estimate of an anisotropic diffusivity tensor is used to parameterize small-scale motions that are unresolved and underresolved by the radar system. This leads to a significant improvement in the ability of the radar to reproduce the observed dye spreading.
Mixing in a coastal environment : 1. A view from dye dispersion

(American Geophysical Union, 2004-10-26) Ledwell, James R. ; Duda, Timothy F. ; Sundermeyer, Miles A. ; Seim, Harvey E.

Dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of New England. The experiments were done in 1996 and 1997 as part of the Coastal Mixing and Optics Experiment. During the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 [ Sundermeyer and Ledwell, 2001 ]. Diapycnal diffusivities inferred from dye dispersion range from 10−6 to 10−5 m2/s at buoyancy frequencies from 9 to 28 cycles/hour. Diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by Oakey and Greenan [2004] agree closely in most cases. Estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by Duda and Rehmann [2002] and Rehmann and Duda [2000] are fairly consistent with the dye diffusivities. The dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by MacKinnon and Gregg [2003] from profiling microstructure measurements obtained at the same site in August 1996. All of the measurements support the general conclusion that the diffusivity, averaged over several days, is seldom greater than 10−5 m2/s in the stratified waters at the site, and usually not much greater than 10−6 m2/s. Severe storms, such as a hurricane that passed over the CMO site in 1996, can dramatically increase the mixing at the site, however.
Dispersion in the open ocean seasonal pycnocline at scales of 1-10 km and 1-6 days

(American Meteorological Society, 2020-02-06) Sundermeyer, Miles A. ; Birch, Daniel ; Ledwell, James R. ; Levine, Murray D. ; Pierce, Stephen D. ; Cervantes, Brandy T. Kuebel

Results are presented from two dye release experiments conducted in the seasonal thermocline of the Sargasso Sea, one in a region of low horizontal strain rate (~10−6 s−1), the second in a region of intermediate horizontal strain rate (~10−5 s−1). Both experiments lasted ~6 days, covering spatial scales of 1–10 and 1–50 km for the low and intermediate strain rate regimes, respectively. Diapycnal diffusivities estimated from the two experiments were κz = (2–5) × 10−6 m2 s−1, while isopycnal diffusivities were κH = (0.2–3) m2 s−1, with the range in κH being less a reflection of site-to-site variability, and more due to uncertainties in the background strain rate acting on the patch combined with uncertain time dependence. The Site I (low strain) experiment exhibited minimal stretching, elongating to approximately 10 km over 6 days while maintaining a width of ~5 km, and with a notable vertical tilt in the meridional direction. By contrast, the Site II (intermediate strain) experiment exhibited significant stretching, elongating to more than 50 km in length and advecting more than 150 km while still maintaining a width of order 3–5 km. Early surveys from both experiments showed patchy distributions indicative of small-scale stirring at scales of order a few hundred meters. Later surveys show relatively smooth, coherent distributions with only occasional patchiness, suggestive of a diffusive rather than stirring process at the scales of the now larger patches. Together the two experiments provide important clues as to the rates and underlying processes driving diapycnal and isopycnal mixing at these scales.
Stirring by small-scale vortices caused by patchy mixing

(American Meteorological Society, 2005-07) Sundermeyer, Miles A. ; Ledwell, James R. ; Oakey, Neil S. ; Greenan, Blair J. W.

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 m2s−1. Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m2s−1. The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.
Three-dimensional mapping of fluorescent dye using a scanning, depth-resolving airborne lidar

(American Meteorological Society, 2007-06) Sundermeyer, Miles A. ; Terray, Eugene A. ; Ledwell, James R. ; Cunningham, A. G. ; LaRocque, P. E. ; Banic, J. ; Lillycrop, W. J.

Results are presented from a pilot study using a fluorescent dye tracer imaged by airborne lidar in the ocean surface layer on spatial scales of meters to kilometers and temporal scales of minutes to hours. The lidar used here employs a scanning, frequency-doubled Nd:YAG laser to emit an infrared (1064 nm) and green (532 nm) pulse 6 ns in duration at a rate of 1 kHz. The received signal is split to infrared, green, and fluorescent (nominally 580–600 nm) channels, the latter two of which are used to compute absolute dye concentration as a function of depth and horizontal position. Comparison of dye concentrations inferred from the lidar with in situ fluorometry measurements made by ship shows good agreement both qualitatively and quantitatively for absolute dye concentrations ranging from 1 to >10 ppb. Uncertainties associated with horizontal variations in the natural seawater attenuation are approximately 1 ppb. The results demonstrate the ability of airborne lidar to capture high-resolution three-dimensional “snapshots” of the distribution of the tracer as it evolves over very short time and space scales. Such measurements offer a powerful observational tool for studies of transport and mixing on these scales.
Observations and numerical simulations of large-eddy circulation in the ocean surface mixed layer

(John Wiley & Sons, 2014-11-06) Sundermeyer, Miles A. ; Skyllingstad, Eric D. ; Ledwell, James R. ; Concannon, Brian ; Terray, Eugene A. ; Birch, Daniel ; Pierce, Stephen D. ; Cervantes, Brandy T. Kuebel

Two near-surface dye releases were mapped on scales of minutes to hours temporally, meters to order 1 km horizontally, and 1–20 m vertically using a scanning, depth-resolving airborne lidar. In both cases, dye evolved into a series of rolls with their major axes approximately aligned with the wind and/or near-surface current. In both cases, roll spacing was also of order 5–10 times the mixed layer depth, considerably larger than the 1–2 aspect ratio expected for Langmuir cells. Numerical large-eddy simulations under similar forcing showed similar features, even without Stokes drift forcing. In one case, inertial shear driven by light winds induced large aspect ratio large-eddy circulation. In the second, a preexisting lateral mixed layer density gradient provided the dominant forcing. In both cases, the growth of the large-eddy structures and the strength of the resulting dispersion were highly dependent on the type of forcing.
Studies of lateral dispersion in the ocean

(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1998-09) Sundermeyer, Miles A.

This thesis is written in two parts. The first part deals with the problem oflateral dispersion due to mesoscale eddies in the open ocean, and the interaction between the mesoscale strain and horizontal diffusion on spatial scales less than 10 km. The second and major part examines lateral dispersion over the continental shelf on scales of 100 m to 10 km and over time scales of 1- 5 days. PART I: Lateral Dispersion and the North Atlantic Tracer Release Experiment Mixing and stirring of Lagrangian particles and a passive tracer were studied by comparison of float and tracer observations from the North Atlantic Tracer Release Experiment. Statistics computed from the NATRE floats were found to be similar to those estimated by Ledwell et al. (1998) from the tracer dispersion. Mean velocities computed from the floats were (u, v) = ( -1.2±0.3, -0.9±0.2) em s-1 for the (zonal, meridional) components, and large-scale effective eddy diffusivities were (KP. 11 , K:e 22 ) = (1.5±0. 7, 0. 7±0.4) x 103 m2 s-1 . The NATRE observations were used to evaluate theoretical models of tracer and particle dispersal. The tracer dispersion observed by Ledwell et al. (1998) was consistent with an exponential growth phase for about the first 6 months and a linear growth at larger times. A numerical model of mesoscale turbulence that was calibrated with float statistics also showed an exponential growth phase of tracer and a reduced growth for longer times. Numerical results further show that Garrett's (1983) theory, relating the effective small-scale diffusivity to the rms strain rate and tracer streak width, requires a scale factor of 2 when the observed growth rate of streak length is used as a measure of the strain rate. This scale factor will be different for different measures of the strain rate, and may also be affected by temporal and spatial variations in the mesoscale strain field. PART II: Lateral Dispersion over the New England Continental Shelf Lateral dispersion over the continental shelf was examined using dye studies of the Coastal Mixing and Optics (CMO) program. Four experiments performed at intermediate depths and lasting 3 to 5 days were examined. In some cases, the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment. In other cases, significant patchiness was observed on scales ranging from 2- 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations also showed that the dye distributions were significantly influenced by shearing and straining on scales of 5- 10 m in the vertical and 1- 10 km in the horizontal. Superimposed on these larger-scale distortions were simultaneous increases in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3 to 4.9 m2 s-1 . Analysis of the dye data in concert with shear estimates from shipboard ADCP observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water-masses can not account for the observed diffusive spreading of the dye patches. This result suggests that some other mechanisms provided an additional diffusivity of order 0.15 to 4.0 m2 s-1 . An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, was proposed which could explain the observed dispersion in some cases. Order-of-magnitude estimates of the effective lateral dispersion due to vortical motions showed that this mechanism could account for effective horizontal diffusivities of order 0.01 to 1.1 m2 s-1 . The upper range of these estimates were within the range required by the observations for two of the four experiments examined.
Mixing in the North Atlantic Tracer Release Experiment : observations and numerical simulations of Lagrangian particles and passive tracer

(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1995-09) Sundermeyer, Miles A.

Mixing and stirring of passive tracer and Lagrangian particles in the open ocean was studied through comparison of observations from the North Atlantic Tracer Release Experiment, a numerical model, and existing theory. Based on the observed distribution of tracer during the first six months of the NATRE field experiment, Ledwell et al. (1993) estimated that on scales of 1 to 10 km small-scale diffusivity κs ≈ 3 m2s-1 and rms strain rate γ ≈ 3 X 10-7 s-1 . From the observed tracer distribution after one year, Ledwell (personal communication) further estimated that on scales greater than the mesoscale the effective eddy diffusivity κe ≈ 1 x 103 m2s-1. In the present study, statistics of the NATRE float data and numerical simulations of Lagrangian particles and passive tracer were used to determine the biases and uncertainties associated with these estimates. The numerical model was calibrated so that the statistics of model floats agreed as closely as possible with the NATRE floats. It is found that observations of a passive tracer such as were made during the NATRE experiment may be used to determine the rms streak width, δs, and the rms strain rate and hence to estimate the effective small-scale diffusivity. However, caution must be exercised when estimating κs from the theoretical balance, δs = square root κs/γ, as this may introduce a bias which would lead to the over-estimation of κs. Of particular relevance to NATRE is that observations of δs may be biased toward larger estimated rms streak width due to the inability of the observer to distinguish individual streaks from those which have resulted from a recent merger of streaks. Numerical experiments show that such a bias could lead to the over-estimation of κs by up to a factor of 2 to 4, suggesting that the estimate of κs made by Ledwell et al., (1993) from the NATRE tracer observations has an associated uncertainty of similar magnitude. Analysis of NATRE float data indicates that the estimate κe ≈ 1 x 103 m2s-1 inferred from the tracer distribution in Spring, 1993 and Fall, 1994 is accurate to within a factor of 2.

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