Shelf Impact on Buoyant Coastal Current Instabilities

Pennel, Romain; Stegner, Alexandre; Béranger, Karine

doi:10.1175/jpo-d-11-016.1

Cited by 28 publications

(30 citation statements)

References 49 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Thus, the concept of a ''residual'' circulation, in which mean and eddy components exactly oppose each other to leading order (e.g., Marshall and Speer 2012) does not hold in this shelf-slope system. This is consistent with previous studies that point to suppression of baroclinic instability over steeply sloping topography (e.g., Pennel et al 2012;Isachsen 2011;Stewart and Thompson 2013). Both u and S vary substantially along the isopycnals crossing the continental slope (see Figs.…”

Section: Role Of Eddies In Cross-slope Exchangesupporting

confidence: 92%

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Stewart

Thompson

2016

Journal of Physical Oceanography

View full text Add to dashboard Cite

Along various stretches of the Antarctic margins, dense Antarctic Bottom Water (AABW) escapes its formation sites and descends the continental slope. This export necessarily raises the isopycnals associated with lighter density classes over the continental slope, resulting in density surfaces that connect the near-freezing waters of the continental shelf to the much warmer circumpolar deep water (CDW) at middepth offshore. In this article, an eddy-resolving process model is used to explore the possibility that AABW export enhances shoreward heat transport by creating a pathway for CDW to access the continental shelf without doing any work against buoyancy forces. In the absence of a net alongshore pressure gradient, the shoreward CDW transport is effected entirely by mesoscale and submesoscale eddy transfer. Eddies are generated partly by instabilities at the pycnocline, sourcing their energy from the alongshore wind stress, but primarily by instabilities at the CDW-AABW interface, sourcing their energy from buoyancy loss on the continental shelf. This combination of processes induces a vertical convergence of eddy kinetic energy and alongshore momentum into the middepth CDW layer, sustaining a local maximum in the eddy kinetic energy over the slope and balancing the Coriolis force associated with the shoreward CDW transport. The resulting slope turbulence self-organizes into a series of alternating along-slope jets with strongly asymmetrical contributions to the slope energy and momentum budgets. Cross-shore variations in the potential vorticity gradient cause the jets to drift continuously offshore, suggesting that fronts observed in regions of AABW down-slope flow may in fact be transient features.

show abstract

Section: Role Of Eddies In Cross-slope Exchangesupporting

confidence: 92%

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Stewart

Thompson

2016

Journal of Physical Oceanography

View full text Add to dashboard Cite

show abstract

“…We stress here that what we call the non-linear saturation parameter returns only the saturation of the cross-shore perturbations, and does not take into account the along-shore perturbations (which never reach finite-amplitude values). Similar analyses were performed by Pennel et al (2012) and Geheniau et al (2017) using the surface kinetic energy because only the surface velocity field, derived with a good accuracy from particle image velocimetry (PIV), is available in laboratory experiments. We now present the impact that a variable s and/or H have on the growth rate of the instabilities of a buoyant current.…”

Section: The Effect Of Variable Topographic Slope (S) and Depth (H )mentioning

confidence: 79%

“…In order to estimate the most unstable wavenumber, we perform, as in Pennel et al (2012), a Fourier decomposition of the cross-shore velocity at the surface (z = 0). We then select the unstable mode k m that both follows an exponential growth and reaches the highest amplitude.…”

Section: The Role Of the Topographic Parameter T Pmentioning

confidence: 99%

“…Moreover, previous studies have shown that a sloping topography has a strong impact on the non-linear saturation of unstable surface flows (Sutyrin, 2001). Experimental studies (using a two-layer stratification) of coastal fronts or coastal currents over linear shelf slopes (Pennel et al, 2012;Geheniau et al, 2017) have shown that there are no large meanders or any eddy detachments when the topographic parameter reaches values below T 0 −3. This is evidence that the non-linear saturation of the linear instability becomes important when the topographic slopes are comparable to the isopycnal slopes.…”

Section: Introductionmentioning

confidence: 97%

“…Hence, the impact of a sloping bathymetry on the development and the evolution of meanders and eddies has been the topic of several studies. The first studies were devoted to the linear stability of coastal flows, while the more recent numerical simulations focus on the non-linear formation of meso-and submesoscale eddies from shelf/slope density fronts or currents (Pennel et al, 2012;Stewart and Thompson, 2013;.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

2017

Self Cite

View full text Add to dashboard Cite

Abstract. This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the crossshore transport. We use the Regional Ocean Modeling System (ROMS) to run simulations in an idealized channel configuration, using a fixed coastal current structure and testing its unstable evolution for various depths and topographic slopes. The experiments are integrated beyond the linear stage of the instability, since our focus is on the non-linear end state, namely the formation of coastal eddies or meanders, to classify the dynamical regimes. We find three nonlinear end states, whose properties cannot be deduced solely from the linear instability analysis. They correspond to a quasi-stable coastal current, the propagation of coastal meanders, and the formation of coherent eddies. We show that the topographic parameter T p , defined as the ratio of the topographic Rossby wave speed over the current speed, plays a key role in controlling the amplitude of the unstable crossshore perturbations. This result emphasizes the limitations of linear stability analysis to predict the formation of coastal eddies, because it does not account for the non-linear saturation of the cross-shore perturbations, which is predominant for large negative T p values. We show that a second dimensionless parameter, the vertical aspect ratio γ , controls the transition from meanders to coherent eddies.We suggest the use of the parameter space (T p , γ ) to describe the emergence of coastal eddies or meanders from an unstable buoyant current. By knowing the values of T p and γ for an observed flow, which can be calculated from hydrological sections, we can identify which non-linear end state characterizes that flow -namely if it is quasi-stable, meanders, or forms eddies.

show abstract

Long‐lived mesoscale eddies in the eastern Mediterranean Sea: Analysis of 20 years of AVISO geostrophic velocities

et al. 2014

Self Cite

View full text Add to dashboard Cite

We analyzed 20 years of AVISO data set to detect and characterize long-lived eddies, which stay coherent more than 6 months, in the eastern Mediterranean Sea. In order to process the coarse gridded (1=8 ) AVISO geostrophic velocity fields, we optimized a geometrical eddy detection algorithm. Our main contribution was to implement a new procedure based on the computation of the Local and Normalized Angular Momentum (LNAM) to identify the positions of the eddy centers and to follow their Lagrangian trajectories. We verify on two mesoscale anticyclones, sampled during the EGYPT campaign in 2006, that our methodology provides a correct estimation of the eddy centers and their characteristic radius corresponding to the maximal tangential velocity. Our analysis reveals the dominance of anticyclones among the longlived eddies. This cyclone-anticyclone asymmetry appears to be much more pronounced in eastern Mediterranean Sea than in the global ocean. Then we focus our study on the formation areas of long-lived eddies. We confirm that the generations of the Ierapetra and the Pelops anticyclones are recurrent and correlated to the Etesian wind forcing. We also provide some evidence that the smaller cyclonic eddies formed at the southwest of Crete may also be induced by the same wind forcing. On the other hand, the generation of long-lived eddies along the Libyo-Egyptian coast are not correlated to the local wind-stress curl but surprisingly, their initial formation points follow the Herodotus Trough bathymetry. Moreover, we identify a new formation area, not discussed before, along the curved shelf off Benghazi.

show abstract

Shelf Impact on Buoyant Coastal Current Instabilities

Cited by 28 publications

References 49 publications

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

Long‐lived mesoscale eddies in the eastern Mediterranean Sea: Analysis of 20 years of AVISO geostrophic velocities

Contact Info

Product

Resources

About