Arms winding around a meddy seen in seismic reflection data close to the Morocco coastline

Ménesguen, Claire; Hua, Bach Lien; Carton, Xavier; Klingelhoëfer, Frauke; Schnurle, Philippe; Reichert, C.

doi:10.1029/2011gl050798

Cited by 36 publications

(42 citation statements)

References 18 publications

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“…The finite-amplitude growth of modes m = 2 (and also m = 1) leads to the formation of two slightly asymmetric satellites (figure 7b). Such satellites connected by spiral arms to the main eddy core have been observed in situ in the seismic data for another Meddy structure (Ménesguen et al 2012). A subsequent ejection of a long PV filament which destabilizes into rolls that coalesce into smaller size vortices is seen in figure 7(c), while the vortex core undergoes polygonal deformations due to azimuthal modes 3, 4, 5 of slower growth rate.…”

Section: Critical-level Baroclinic Instabilitymentioning

confidence: 63%

Layering and turbulence surrounding an anticyclonic oceanic vortex: in situ observations and quasi-geostrophic numerical simulations

et al. 2013

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Evidence of persistent layering, with a vertical stacking of sharp variations in temperature, has been presented recently at the vertical and lateral periphery of energetic oceanic vortices through seismic imaging of the water column. The stacking has vertical scales ranging from a few metres up to 100 m and a lateral spatial coherence of several tens of kilometres comparable with the vortex horizontal size. Inside this layering, in situ data display a $[{ k}_{h}^{- 5/ 3} { k}_{h}^{- 2} ] $ scaling law of horizontal scales for two different quantities, temperature and a proxy for its vertical derivative, but for two different ranges of wavelengths, between 5 and 50 km for temperature and between 500 m and 5 km for its vertical gradient. In this study, we explore the dynamics underlying the layering formation mechanism, through the slow dynamics captured by quasi-geostrophic equations. Three-dimensional high-resolution numerical simulations of the destabilization of a lens-shaped vortex confirm that the vertical stacking of sharp jumps in density at its periphery is the three-dimensional analogue of the preferential wind-up of potential vorticity near a critical radius, a phenomenon which has been documented for barotropic vortices. For a small-Burger (flat) lens vortex, baroclinic instability ensures a sustained growth rate of sharp jumps in temperature near the critical levels of the leading unstable modes. Such results can be obtained for a background stratification which is due to temperature only and does not require the existence of salt anomalies. Aloft and beneath the vortex core, numerical simulations well reproduce the $[{ k}_{h}^{- 5/ 3} { k}_{h}^{- 2} ] $ scaling law of horizontal scales for the vertical derivative of temperature that is observed in situ inside the layering, whatever the background stratification. Such a result stems from the tracer-like behaviour of the vortex stretching component and previous studies have shown that spectra of tracer fields can be steeper than $- 1$, namely in $- 5/ 3$ or $- 2$, if the advection field is very compact spatially, with a $- 5/ 3$ slope corresponding to a spiral advection of the tracer. Such a scaling law could thus be of geometric origin. As for the kinetic and potential energy, the ${ k}_{h}^{- 5/ 3} $ scaling law can be reproduced numerically and is enhanced when the background stratification profile is strongly variable, involving sharp jumps in potential vorticity such as those observed in situ. This raises the possibility of another plausible mechanism leading to a $- 5/ 3$ scaling law, namely surface-quasi-geostrophic (SQG)-like dynamics, although our set-up is more complex than the idealized SQG framework. Energy and enstrophy fluxes have been diagnosed in the numerical quasi-geostrophic simulations. The results emphasize a strong production of energy in the oceanic submesoscales range and a kinetic and potential energy flux from mesoscale to submesoscales range near the critical levels. Such horizontal submesoscale production, which is correlated to the accumulation of thin vertical scales inside the layering, thus has a significant slow dynamical component, well-captured by quasi-geostrophy.

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Section: Critical-level Baroclinic Instabilitymentioning

confidence: 63%

Layering and turbulence surrounding an anticyclonic oceanic vortex: in situ observations and quasi-geostrophic numerical simulations

et al. 2013

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“…From 35 to 40 km in the profile, 20 km far away from the lateral right edge of the meddy there is a vertical thermohaline anomaly that is similar to the spiral arm of a meddy described by Ménesguen et al . [].…”

Section: Resultsmentioning

confidence: 99%

Recovery of temperature, salinity, and potential density from ocean reflectivity

Biescas

Ruddick

Nedimović

et al. 2014

J. Geophys. Res. Oceans

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This work explores a method to recover temperature, salinity, and potential density of the ocean using acoustic reflectivity data and time and space coincident expendable bathythermographs (XBT). The acoustically derived (vertical frequency >10 Hz) and the XBT-derived (vertical frequency <10 Hz) impedances are summed in the time domain to form impedance profiles. Temperature (T) and salinity (S) are then calculated from impedance using the international thermodynamics equations of seawater (GSW TEOS-10) and an empirical T-S relation derived with neural networks; and finally potential density (q) is calculated from T and S. The main difference between this method and previous inversion works done from real multichannel seismic reflection (MCS) data recorded in the ocean, is that it inverts density and it does not consider this magnitude constant along the profile, either in vertical or lateral dimension. We successfully test this method on MCS data collected in the Gulf of Cadiz (NE Atlantic Ocean). T, S, and q are inverted with accuracies of dT sd 50:1 C, dS sd 50:09, and dq sd 50:02kg=m 3 . Inverted temperature anomalies reveal baroclinic thermohaline fronts with intrusions. The observations support a mix of thermohaline features created by both double-diffusive and isopycnal stirring mechanisms. Our results show that reflectivity is primarily caused by thermal gradients but acoustic reflectors are not isopycnal in all domains.

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“…Many pancake vortices are also observed in oceans. Famous examples are the Mediterranean eddies (Meddies) which are formed by salty water flowing from the Mediterranean sea into the mid-Atlantic ocean (Armi et al 1989;Hobbs 2007;Ménesguen et al 2012). Meddies typically have a horizontal extension of O(100) km and vertical thickness of O(1) km (Richardson, Bower & Zenk 2000).…”

Section: Introductionmentioning

confidence: 99%

Analogies and differences between the stability of an isolated pancake vortex and a columnar vortex in stratified fluid

Yim

Billant

2016

J. Fluid Mech.

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In order to understand the dynamics of pancake shaped vortices in stably stratified fluids, we perform a linear stability analysis of an axisymmetric vortex with Gaussian angular velocity in both the radial and axial directions with an aspect ratio of α. The results are compared to those for a columnar vortex (α = ∞) in order to identify the instabilities. Centrifugal instability occurs when R > c(m) where R = ReF 2h is the buoyancy Reynolds number, F h the Froude number, Re the Reynolds number and c(m) a constant which differs for the three unstable azimuthal wavenumbers m = 0, 1, 2. The maximum growth rate depends mostly on R and is almost independent of the aspect ratio α. For sufficiently large buoyancy Reynolds number, the axisymmetric mode is the most unstable centrifugal mode whereas for moderate R, the mode m = 1 is the most unstable. Shear instability for m = 2 develops only when F h 0.5α. By considering the characteristics of shear instability for a columnar vortex with the same parameters, this condition is shown to be such that the vortex is taller than the minimum wavelength of shear instability in the columnar case. For larger Froude number F h 1.5α, the isopycnals overturn and gravitational instability can operate. Just below this threshold, the azimuthal wavenumbers m = 1, 2, 3 are unstable to baroclinic instability. A simple model shows that baroclinic instability develops only above a critical vertical Froude number F h /α because of confinement effects.

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Arms winding around a meddy seen in seismic reflection data close to the Morocco coastline

Cited by 36 publications

References 18 publications

Layering and turbulence surrounding an anticyclonic oceanic vortex: in situ observations and quasi-geostrophic numerical simulations

Layering and turbulence surrounding an anticyclonic oceanic vortex: in situ observations and quasi-geostrophic numerical simulations

Recovery of temperature, salinity, and potential density from ocean reflectivity

Analogies and differences between the stability of an isolated pancake vortex and a columnar vortex in stratified fluid

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