Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field

Shcherbina, Andrey Y.; D’Asaro, Eric A.; Lee, Craig M.; Klymak, Jody M.; Molemaker, M. Jeroen; McWilliams, James C.

doi:10.1002/grl.50919

Cited by 196 publications

(243 citation statements)

References 24 publications

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“…Here, we refer to the along-track direction as x, regardless of orientation, and the cross-track direction as y. This unidirectional velocity field is not a serious constraint on the analysis if the glider orientation during a given transect is perpendicular to the front orientation (Shcherbina et al 2013). Our choice of sampling pattern results in a glider orientation that is arbitrary with respect to front orientation within the OSMOSIS domain.…”

Section: B Potential Vorticity Calculationmentioning

confidence: 99%

“…From this characterization, it becomes clear that submesoscale processes are intensified in regions of strong lateral buoyancy gradients, strong vorticity, and weak vertical stratification. These conditions are often met in or near western boundary currents of the Pacific (D'Asaro et al 2011) and Atlantic (Thomas et al 2013;Shcherbina et al 2013), in coastal oceans, and in the Southern Ocean.…”

Section: Introductionmentioning

confidence: 99%

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Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Thompson

Lazar

Buckingham

et al. 2016

Journal of Physical Oceanography

175

227

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The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km 2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (;20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September-November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December-March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the openocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

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Section: B Potential Vorticity Calculationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Thompson

Lazar

Buckingham

et al. 2016

Journal of Physical Oceanography

175

227

View full text Add to dashboard Cite

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“…Theory , modeling (e.g. Capet et al, 2008e), and observations (Shcherbina et al, 2013) indicate that these mixed layer instabilities can substantially enhance submesoscale energy in the presence of a deep winter mixed layer. Such mixed layer flows have energy spectra close to k h −2 .…”

Section: Mixed-layer Flowsmentioning

confidence: 99%

“…The data were collected as part of two separate observational programs: the Oleander project along a transect between Elisabeth, New Jersey, and Hamilton, Bermuda, occupied weekly in [2005][2006][2007][2008][2009][2010][2011][2012][2013], and the Lateral Mixing Experiment (LatMix) along several straight transects off Cape Hatteras in June 2011 (summer) and just south of the Gulf Stream extension in March 2012 (winter) (Shcherbina et al, 2013) (Fig. 5.1).…”

Section: Datamentioning

confidence: 99%

Submesoscale turbulence in the upper ocean

Callies¹

2016

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Submesoscale flows, current systems 1-100 km in horizontal extent, are increasingly coming into focus as an important component of upper-ocean dynamics. A range of processes have been proposed to energize submesoscale flows, but which process dominates in reality must be determined observationally. We diagnose from observed flow statistics that in the thermocline the dynamics in the submesoscale range transition from geostrophic turbulence at large scales to inertia-gravity waves at small scales, with the transition scale depending dramatically on geographic location. A similar transition is shown to occur in the atmosphere, suggesting intriguing similarities between atmospheric and oceanic dynamics. We furthermore diagnose from upper-ocean observations a seasonal cycle in submesoscale turbulence: fronts and currents are more energetic in the deep wintertime mixed layer than in the summertime seasonal thermocline. This seasonal cycle hints at the importance of baroclinic mixed layer instabilities in energizing submesoscale turbulence in winter. To better understand this energization, three aspects of the dynamics of baroclinic mixed layer instabilities are investigated. First, we formulate a quasigeostrophic model that describes the linear and nonlinear evolution of these instabilities. The simple model reproduces the observed wintertime distribution of energy across scales and depth, suggesting it captures the essence of how the submesoscale range is energized in winter. Second, we investigate how baroclinic instabilities are affected by convection, which is generated by atmospheric forcing and dominates the mixed layer dynamics at small scales. It is found that baroclinic instabilities are remarkably resilient to the presence of convection and develop even when rapid overturns keep the mixed layer unstratified. Third, we discuss the restratification induced by baroclinic mixed layer instabilities. We show that the rate of restratification depends on characteristics of the baroclinic eddies themselves, a dependence not captured by a previously proposed parameterization. These insights sharpen our understanding of submesoscale dynamics and can help focus future inquiry into whether and how submesoscale flows influence the ocean's role in climate.Thesis supervisor: Raffaele Ferrari Title: Breene M. Kerr Professor of Oceanography

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“…Stewart et al [18,24] find that submesoscale eddies are generated over the continental shelf and shelf break, but are suppressed over the continental slope due to the strong potential vorticity gradient [17]. To date, much of the work on submesoscale dynamics has been limited to idealized processes models or observational studies in strong western boundary currents [25][26][27] and the open ocean [28]. Recently, however, studies on the influence of topographic slopes on submesoscale dynamics are beginning to emerge.…”

Section: Introductionmentioning

confidence: 99%

Submesoscale Turbulence over a Topographic Slope

Lazar

Zhang

Thompson

2018

Fluids

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Abstract:Regions of the ocean near continental slopes are linked to significant vertical velocities caused by advection over a sloping bottom, frictional processes and diffusion. Oceanic motions at submesoscales are also characterized by enhanced vertical velocities, as compared to mesoscale motions, due to greater contributions from ageostrophic flows. These enhanced vertical velocities can make an important contribution to turbulent fluxes. Sloping topography may also induce large-scale potential vorticity gradients by modifying the slope of interior isopycnal surfaces. Potential vorticity gradients, in turn, may feed back on mesoscale stirring and the generation of submesoscale features. In this study, we explore the impact of sloping topography on the characteristics of submesoscale motions. We conduct high-resolution (1 km × 1 km) simulations of a wind-driven frontal current over an idealized continental shelf and slope. We explore changes in the magnitude, skewness and spectra of surface vorticity and vertical velocity across different configurations of the topographic slope and wind-forcing orientations. All of these properties are strongly modulated by the background topography. Furthermore, submesoscale characteristics exhibit spatial variability across the continental shelf and slope. We find that changes in the statistical properties of submesoscale motions are linked to mesoscale stirring responding to differences in the interior potential vorticity distributions, which are set by frictional processes at the ocean surface and over the sloping bottom. Improved parameterizations of submesoscale motions over topography may be needed to simulate the spatial variability of these features in coarser-resolution models, and are likely to be important to represent vertical nutrient fluxes in coastal waters.

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Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field

Cited by 196 publications

References 24 publications

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Submesoscale turbulence in the upper ocean

Submesoscale Turbulence over a Topographic Slope

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