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E. A. D'Asaro
and
Henry Perkins

Abstract

Independent estimates of the frequency–wavenumber spectrum of near-inertial waves for the Sargasso Sea in late summer were made using 8 time series of horizontal velocity from a single moored vertical array and 58 vertical profiles of horizontal velocity from a horizontal array of expendable velocity profilers. The profiler data were analyzed to produce an internal wave frequency-wavenumber spectrum with sufficient resolution to resolve the details of the inertial peak and compute the vertical energy flux. Comparison with the lower resolution spectrum from the moored array shows qualitative agreement; the differences are most likely due to biases in both techniques and to intermittency of the internal wave field.

These data reveal a marked asymmetry of the near-inertial internal wave field, with a net downward enemy flux of 0.12±0.12 ergs cm−2 s−1. The downward propagating waves have more energy, a larger horizontal scale and a lower frequency than the upward propagating waves. The magnitude of the downward energy flux is comparable to the net input of energy into surface inertial currants, confirming the likely importance of the wind as an energy source for near-inertial internal waves.

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R-C. Lien
,
E. A. D'Asaro
, and
M. J. McPhaden

Abstract

In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ϵ and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ϵ, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.

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M. C. Gregg
,
E. A. D'Asaro
,
T. J. Shay
, and
N. Larson

Abstract

Repeated profiles of microstructure and shear alongside a drogued buoy show a 10 m thick mixing zone at the same depth as a near-inertial feature. Because the profile was diffusively stable and free of thermohaline intrusions, internal wave breakdown is the only mechanism capable of producing mixing. Both the near-inertial feature and the mixing patch were observed for over three days and then faded out. It is not possible to determine whether they disappeared because the near-inertial feature was dissipated by the mixing or because the drogue drifted away; both are plausible.

Kinematical models of mixing use a standard internal wave spectrum to predict the frequency of occurrence and persistence of shear instabilities. Observed distributions of ε and χ patches thinner than 2 m are similar to the model predictions, although the dissipation rates are low. Most are just at or below the transition dissipation rate εtr. Laboratory experiments have established that if ε<εtr the turbulence is too weak to produce a net buoyancy flux.

Observed, but not predicted by the models, are patches 5 to 10 m thick, which have average dissipation rates well above εtr. They produce most of the temporal variability in the mixing rates and are mainly associated with near-inertial features. These thicker patches are not described by present kinematical modes, which neglect the mixing in near-inertial features and assume that overturns mix to completion. Because these mixing zones are more intense, thicker, wider, and more persistent than those produced by random superposition of internal waves they are potentially more important. If these measurements are characteristic of the main thermocline, the global distribution of near-inertial futures is a major factor controlling the distribution of mixing.

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A. C. Haza
,
E. D’Asaro
,
H. Chang
,
S. Chen
,
M. Curcic
,
C. Guigand
,
H. S. Huntley
,
G. Jacobs
,
G. Novelli
,
T. M. Özgökmen
,
A. C. Poje
,
E. Ryan
, and
A. Shcherbina

Abstract

The Lagrangian Submesoscale Experiment (LASER) was designed to study surface flows during winter conditions in the northern Gulf of Mexico. More than 1000 mostly biodegradable drifters were launched. The drifters consisted of a surface floater extending 5 cm below the surface, containing the satellite tracking system, and a drogue extending 60 cm below the surface, hanging beneath the floater on a flexible tether. On some floats, the drogue separated from the floater during storms. This paper describes methods to detect drogue loss based on two properties that distinguish drogued from undrogued drifters. First, undrogued drifters often flip over, pointing their satellite antenna downward and thus intermittently reducing the frequency of GPS fixes. Second, undrogued drifters respond to wind forcing more than drogued drifters. A multistage analysis is used: first, two properties are used to create a preliminary drifter classification; then, the motion of each unclassified drifter is compared to that of its classified neighbors in an iterative process for nearly all of the drifters. The algorithm classified drifters with a known drogue status with an accuracy of virtually 100%. Drogue loss times were estimated with a precision of less than 0.5 and 3 h for 60% and 85% of the drifters, respectively. An estimated 40% of the drifters lost their drogues in the first 7 weeks, with drogue loss coinciding with storm events, particularly those with steep waves. Once the drogued and undrogued drifters are classified, they can be used to quantify the differences in material dispersion at different depths.

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E.A. D'Asaro
,
P. G. Black
,
L. R. Centurioni
,
Y.-T. Chang
,
S. S. Chen
,
R. C. Foster
,
H. C. Graber
,
P. Harr
,
V. Hormann
,
R.-C. Lien
,
I.-I. Lin
,
T. B. Sanford
,
T.-Y. Tang
, and
C.-C. Wu

Tropical cyclones (TCs) change the ocean by mixing deeper water into the surface layers, by the direct air–sea exchange of moisture and heat from the sea surface, and by inducing currents, surface waves, and waves internal to the ocean. In turn, the changed ocean influences the intensity of the TC, primarily through the action of surface waves and of cooler surface temperatures that modify the air–sea fluxes. The Impact of Typhoons on the Ocean in the Pacific (ITOP) program made detailed measurements of three different TCs (i.e., typhoons) and their interaction with the ocean in the western Pacific. ITOP coordinated meteorological and oceanic observations from aircraft and satellites with deployments of autonomous oceanographic instruments from the aircraft and from ships. These platforms and instruments measured typhoon intensity and structure, the underlying ocean structure, and the long-term recovery of the ocean from the storms' effects with a particular emphasis on the cooling of the ocean beneath the storm and the resulting cold wake. Initial results show how different TCs create very different wakes, whose strength and properties depend most heavily on the nondimensional storm speed. The degree to which air–sea fluxes in the TC core were reduced by ocean cooling varied greatly. A warm layer formed over and capped the cold wakes within a few days, but a residual cold subsurface layer persisted for 10–30 days.

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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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