Geostrophic and ageostrophic circulation of a shallow anticyclonic eddy off Cape Bojador

Ruiz, Simón; Pelegrí, Josep Lluís; Emelianov, Mikhail; Pascual, Ananda; Mason, Evan

doi:10.1002/2013jc009169

Cited by 14 publications

(14 citation statements)

References 45 publications

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“…For a long time, the Canary Islands were considered as the dominating generation area in the CANUS . More recently, eddy formation was also linked to the destabilization of the fronts [Meunier et al, 2012] or coastal currents [Meunier et al, 2010;Ben ıtez-Barrios et al, 2011;Ruiz et al, 2014]. Despite their different origins, the vertical structure of the eddies in these studies were quite similar, showing a core centered around 200 m depth, as in our study.…”

Section: 1002/2015jc010950supporting

confidence: 77%

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

2015

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In the four major Eastern Boundary Upwelling Systems (EBUS), mesoscale eddies are known to modulate the biological productivity and transport near‐coastal seawater properties toward the offshore ocean, however little is known about their main characteristics and vertical structure. This study combines 10 years of satellite‐altimetry data and Argo float profiles of temperature and salinity, and our main goals are (i) to describe the main surface characteristics of long‐lived eddies formed in each EBUS and their evolution, and (ii) to depict the main vertical structure of the eddy‐types that coexist in these regions. A clustering analysis of the Argo profiles surfacing within the long‐lived eddies of each EBUS allows us to determine the proportion of surface and subsurface‐intensified eddies in each region, and to describe their vertical structure in terms of temperature, salinity and dynamic height anomalies. In the Peru‐Chile Upwelling System, 55% of the sampled anticyclonic eddies (AEs) have subsurface‐intensified maximum temperature and salinity anomalies below the seasonal pycnocline, whereas 88% of the cyclonic eddies (CEs) are surface‐intensified. In the California Upwelling System, only 30% of the AEs are subsurface‐intensified and all of the CEs show maximum anomalies above the pycnocline. In the Canary Upwelling System, ∼40% of the AEs and ∼60% of the CEs are subsurface‐intensified with maximum anomalies extending down to 800 m depth. Finally, the Benguela Upwelling System tends to generate ∼40–50% of weak surface‐intensified eddies and ∼50–60% of much stronger subsurface‐intensified eddies with a clear geographical distribution. The mechanisms involved in the observed eddy vertical shapes are discussed.

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Section: 1002/2015jc010950supporting

confidence: 77%

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

2015

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“…The origin of the recirculation cell seems to be related to the pool of cold waters left by the gigantic filament south of Cape Ghir that develops in the summer, when the trade winds are stronger, as indicated by the minimum in the sea level observed in the composite at approximately 10°30″W, 31°30″N, a hypothesis also suggested by previous authors [ Pelegrí et al ., ; Mason et al ., ]. Just south of Cape Bojador, the fall composite shows a strong anticyclonic eddy that is marginally observed with the hydrographic data set and that has been previously reported [ Ruiz et al ., ].…”

Section: Resultsmentioning

confidence: 99%

On the seasonal variability of the Canary Current and the Atlantic Meridional Overturning Circulation

et al. 2017

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The Atlantic Meridional Overturning Circulation (AMOC) is continually monitored along 26°N by the RAPID‐MOCHA array. Measurements from this array show a 6.7 Sv seasonal cycle for the AMOC, with a 5.9 Sv contribution from the upper mid‐ocean. Recent studies argue that the dynamics of the eastern Atlantic is the main driver for this seasonal cycle; specifically, Rossby waves excited south of the Canary Islands. Using inverse modeling, hydrographic, mooring, and altimetry data, we describe the seasonal cycle of the ocean mass transport around the Canary Islands and at the eastern boundary, under the influence of the African slope, where eastern component of the RAPID‐MOCHA array is situated. We find a seasonal cycle of −4.1 ± 0.5 Sv for the oceanic region of the Canary Current, and +3.7 ± 0.4 Sv at the eastern boundary. This seasonal cycle along the eastern boundary is in agreement with the seasonal cycle of the AMOC that requires the lowest contribution to the transport in the upper mid‐ocean to occur in fall. However, we demonstrate that the linear Rossby wave model used previously to explain the seasonal cycle of the AMOC is not robust, since it is extremely sensitive to the choice of the zonal range of the wind stress curl and produces the same results with a Rossby wave speed of zero. We demonstrate that the seasonal cycle of the eastern boundary is due to the recirculation of the Canary Current and to the seasonal cycle of the poleward flow that characterizes the eastern boundaries of the oceans.

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“…Except for experiments based on neutrally buoyant RAFOS floats (e.g., Bower & Rossby, ; Levine et al, ; Rossby, ; Song et al, ), observations cannot provide direct measurements of the vertical velocities, which are thus commonly inferred from 2‐D or 3‐D reconstructions of the density field and horizontal velocities through the Omega equation (Giordani et al, ; Hoskins, ; Hoskins & Draghici, ; Hoskins et al, ), a well‐known diagnostic tool that has been applied to the data collected during several field campaigns (Allen et al, ; Barceló‐Llull et al, ; Buongiorno Nardelli et al, ; Gomis et al, ; Naveira Garabato et al, ; Pollard & Regier, ; Rodríguez et al, ; Rudnick, ; Ruiz et al, ; Tintoré et al, ). Oceanographic surveys, however, suffer from the difficulty to guarantee both high spatial resolution and synopticity.…”

Section: Introductionmentioning

confidence: 99%

“…This is due to the fact that observations only provide limited views of the three-dimensional (3-D) ocean state, primarily in terms of resolution and coverage, but also reflects our incomplete understanding of boundary layer dynamics and imperfect or missing parameterization of processes occurring at the unresolved scales, such as turbulent mixing and diffusion or submesoscale (Bachman et al, 2016(Bachman et al, , 2017Canuto, 2015;Fox-Kemper and Ferrari, 2008;Hamlington et al, 2014;Li et al, 2017). 1977;Hoskins et al, 1978Hoskins et al, , 2003, a well-known diagnostic tool that has been applied to the data collected during several field campaigns Barcel o-Llull et al, 2017;Buongiorno Nardelli et al, 2001;Gomis et al, 2001;Naveira Garabato et al, 2001;Pollard & Regier, 1992;Rodr ıguez et al, 2001;Rudnick, 1996;Ruiz et al, 2014;Tintor e et al, 1991). Oceanographic surveys, however, suffer from the difficulty to guarantee both high spatial resolution and synopticity.…”

Section: Introductionmentioning

confidence: 99%

Three‐Dimensional Ageostrophic Motion and Water Mass Subduction in the Southern Ocean

2018

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Vertical velocities at the ocean mesoscale are several orders of magnitude smaller than corresponding horizontal flows, making their direct monitoring a still unsolved challenge. Vertical motion is generally retrieved indirectly by applying diagnostic equations to observation‐based fields. The most common approach relies on the solution of an adiabatic version of the Omega equation, neglecting the ageostrophic secondary circulation driven by frictional effects and turbulent mixing in the boundary layers. Here we apply a diabatic semigeostrophic diagnostic model to two different 3‐D reconstructions covering the Southern Ocean during the period 2010–2012. We incorporate the effect of vertical mixing through a modified K‐profile parameterization and using ERA‐interim data, and perform an indirect validation of the ageostrophic circulation with independent drifter observations. Even if horizontal gradients and associated vertical flow are likely underestimated at 1/4° × 1/4° resolution, the exercise provides an unprecedented relative quantification of the contribution of vertical mixing and adiabatic internal dynamics on the vertical exchanges along the Antarctic Circumpolar Current. Kinematic estimates of subduction rates show the destruction of poleward flowing waters lighter than 26.6 kg/m3 (14 ÷ 15 Sv) and two main positive bands associated with the Antarctic Intermediate Water (7 ÷ 11 Sv) and Sub‐Antarctic Mode Waters (4 ÷ 7 Sv) formation, while Circumpolar Deep Water upwelling attains around 3 ÷ 6 Sv. Diabatic and adiabatic terms force distinct spatial responses and vertical velocity magnitudes along the water column and the restratifying effect of adiabatic internal dynamics due to mesoscale eddies is shown to at least partly compensate the contribution of wind‐driven vertical exchanges to net subduction.

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Geostrophic and ageostrophic circulation of a shallow anticyclonic eddy off Cape Bojador

Cited by 14 publications

References 45 publications

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

On the seasonal variability of the Canary Current and the Atlantic Meridional Overturning Circulation

Three‐Dimensional Ageostrophic Motion and Water Mass Subduction in the Southern Ocean

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