16 research outputs found
Internal tide modeling : hydraulic & topographic controls
La marĂ©e interne gĂ©nĂ©rĂ©e sur une topographie est un Ă©lĂ©ment clĂ© des transferts Ă©nergĂ©tiques des Ă©chelles de forçage de l'ocĂ©an vers les Ă©chelles de mĂ©lange turbulent. Elle contribuerait Ă prĂšs de la moitiĂ© du mĂ©lange turbulent nĂ©cessaire au maintien de la stratification ocĂ©anique. Une comprĂ©hension plus approfondie des processus mis en jeu est nĂ©cessaire pour dĂ©crire plus prĂ©cisĂ©ment son rĂŽle dans le maintien de la circulation ocĂ©anique. Cette thĂšse s'inscrit dans la continuitĂ© des travaux d'Y. Dossmann (2012) portant sur les ondes internes solitaires gĂ©nĂ©rĂ©es au dessus d'une dorsale ocĂ©anique. Ces travaux reposent sur une utilisation complĂ©mentaire d'expĂ©riences physiques menĂ©es dans le grand canal du CNRM-GAME et d'expĂ©riences numĂ©riques Ă l'Ă©chelle du laboratoire effectuĂ©es avec le modĂšle d'ocĂ©anographie cĂŽtiĂšre SNH. L'utilisation simultanĂ©e de ces deux outils a notamment permis d'Ă©valuer la validitĂ© des hypothĂšses sous-jacentes de ce modĂšle et le dĂ©veloppement de nouveaux schĂ©mas numĂ©riques. Dans cette thĂšse, des simulations numĂ©riques utilisant la version non-hydrostatique et non-Boussinesq du modĂšle SNH sont utilisĂ©es pour dĂ©crire les diffĂ©rents rĂ©gimes d'ondes internes dans des rĂ©gions " supercritiques ". Le terme supercritique dĂ©signe Ă la fois des courants de marĂ©e intenses dont la vitesse U est supĂ©rieure Ă la vitesse de propagation des ondes internes cn, et des topographies trĂšs abruptes dont l'angle de la pente est supĂ©rieur Ă l'angle du rayon d'onde interne dans la pycnocline . De telles conditions environnementales correspondent Ă des rĂ©gions de mĂ©lange intense, jusqu'Ă 10 000 fois supĂ©rieur au mĂ©lange turbulent observĂ© dans l'ocĂ©an ouvert. Les processus physiques ayant lieu dans ces rĂ©gions restent encore mal compris et mal reprĂ©sentĂ©s par les paramĂ©trisations d'ondes internes existantes. De plus, ces rĂ©gions sont Ă©galement des zones propices Ă la gĂ©nĂ©ration d'ondes internes non-linĂ©aires pouvant se propager pendant plusieurs jours et entraĂźnant ainsi des transferts d'Ă©nergie significatifs loin de leur zone de gĂ©nĂ©ration. La description des processus turbulents en jeu dans ces rĂ©gions " extrĂȘmes " constitue le cĆur de ma thĂšse. Dans une premiĂšre partie, des configurations acadĂ©miques Ă l'Ă©chelle du laboratoire sont mises en place pour Ă©tudier les processus en jeu dans diffĂ©rents rĂ©gimes " supercritiques " de gĂ©nĂ©ration d'onde internes. Des simulations numĂ©riques directes sont rĂ©alisĂ©es et permettent d'identifier un nombre limitĂ© de paramĂštres physiques adimensionnĂ©s contrĂŽlant la dynamique des ondes internes dans ces rĂ©gions. Une attention particuliĂšre est portĂ©e sur le rĂŽle jouĂ© par la topographie sur la gĂ©nĂ©ration des modes verticaux d'ondes internes et sur la formation de modes " hauts " d'ondes internes solitaires. Le second objectif de cette thĂšse est de faire le lien entre les prĂ©cĂ©dentes Ă©tudes acadĂ©miques Ă l'Ă©chelle du laboratoire et l'Ă©chelle ocĂ©anique. Pour cela, un principe de similitude permettant de conserver la dynamique des ondes internes tout en modifiant l'Ă©chelle de l'Ă©coulement est mise en place. Par le biais de ce principe de similitude, nous partons de cas idĂ©alisĂ©s Ă l'Ă©chelle du laboratoire, que nous transposons Ă l'Ă©chelle ocĂ©anique, pour nous rapprocher de cas ocĂ©aniques plus rĂ©alistes et de plus en plus complexes. Puis notre Ă©tude de rĂ©gime est Ă©tendue Ă deux rĂ©gions ocĂ©aniques " supercritiques " bien connues : le dĂ©troit de Gibraltar et le plateau situĂ© Ă l'entrĂ©e du golfe du Maine (nommĂ© " Georges Bank " en anglais). L'applicabilitĂ© de nos paramĂštres clĂ©s est Ă©tudiĂ©e dans le cas de ces deux environnements complexes par le biais de simulations haute rĂ©solution de grande Ă©chelle (LES).Internal tides are involved in the Meridional Overturning Circulation energy balance. The issue about the relative importance of the mechanical and thermodynamical energy sources induces a need for a quantitative evaluation of the energy transfers and for a clear understanding of the physical processes involved in these energy transfers. In supercritical regions such as the strait of Gibraltar or the Hawaiian Ridge, large topography variations and strong currents lead to more complex generation mechanisms of internal waves and environmental interactions. They can be subject locally to spectacular breaking, with turbulent structures observed hundreds of meters above the seafloor, and driving turbulence orders of magnitude higher than open-ocean levels. These regions are also effective at generating nonlinear internal waves (ISWs) which persist for days after their generation and are suspected to be responsible for important remote energy transfers. In these "extreme" regions, ISWs dynamics is also more difficult to model. These situations are highly non-hydrostatic and non-linear with strong instabilities, strong velocity and density gradients and steep slopes. Moreover, in these regions, actual internal wave's parameterizations are often inadequate. So there is a real need to understand and represent better the ISWs dynamic in these areas. This thesis follows the line of research of Dossmann (2012), on topographically induced internal solitary waves which used a complementary approach relying on numerical and experimental configurations at laboratory scale. In this context, we continue to explore internal tide regimes but in "supercritical" regions: internal tide generation area with supercritical topography and hydraulic control. Simulations are performed using the nonhydrostatic and non-Boussinesq version of the regional oceanic circulation model SNH. In a first part, taking an idealized modeling approach at laboratory scale, we examined a range of different internal waves regimes in "supercritical" regions. Relying on quasi-direct numerical simulations (quasi-DNS), a regime analysis has been proposed using and identifying key non-dimensional parameters for ISWs dynamics. This analysis has permitted to recover a topographic control on vertical mode generation characterized by the ratio of vertical mode wavelength to topography width, even above supercritical topography. The topographic selection criterion has proven to be a useful indicator of high mode solitary wave formation in non-linear regime. The purpose of the second part is to extend the previous studies at laboratory scales towards more realistic oceanic conditions. In this regard, the regime analysis is applied to a idealized large scale oceanic strait through a similitude principle. The idealized strait configuration succeeds in representing laboratory scale strait regime at largest and realistic scales. Then our analysis is applied to two well-known realistic cases: the Strait of Gibraltar and Georges Bank through large eddy simulations. These two oceanographic "supercritical" regions are particularly interesting for their specific topography and stratification conditions
Modélisation numérique de la marée interne : ContrÎles hydrauliques et topographiques
Internal tides are involved in the Meridional Overturning Circulation energy balance. The issue about the relative importance of the mechanical and thermodynamical energy sources induces a need for a quantitative evaluation of the energy transfers and for a clear understanding of the physical processes involved in these energy transfers. In supercritical regions such as the strait of Gibraltar or the Hawaiian Ridge, large topography variations and strong currents lead to more complex generation mechanisms of internal waves and environmental interactions. They can be subject locally to spectacular breaking, with turbulent structures observed hundreds of meters above the seafloor, and driving turbulence orders of magnitude higher than open-ocean levels. These regions are also effective at generating nonlinear internal waves (ISWs) which persist for days after their generation and are suspected to be responsible for important remote energy transfers. In these âextremeâ regions, ISWs dynamics is also more difficult to model. These situations are highly non-hydrostatic and non-linear with strong instabilities, strong velocity and density gradients and steep slopes. Moreover, in these regions, actual internal waveâs parameterizations are often inadequate. So there is a real need to understand and represent better the ISWs dynamic in these areas.This thesis follows the line of research of Dossmann (2012), on topographically induced internal solitary waves which used a complementary approach relying on numerical and experimental configurations at laboratory scale. In this context, we continue to explore internal tide regimes but in âsupercriticalâ regions: internal tide generation area with supercritical topography and hydraulic control. Simulations are performed using the nonhydrostatic and non-Boussinesq version of the regional oceanic circulation model SNH.In a first part, taking an idealized modeling approach at laboratory scale, we examined a range of different internal waves regimes in âsupercriticalâ regions. Relying on quasi-direct numerical simulations (quasi-DNS), a regime analysis has been proposed using and identifying key non-dimensional parameters for ISWs dynamics. This analysis has permitted to recover a topographic control on vertical mode generation characterized by the ratio of vertical mode wavelength to topography width, even above supercritical topography. The topographic selection criterion has proven to be a useful indicator of high mode solitary wave formation in non-linear regime.The purpose of the second part is to extend the previous studies at laboratory scales towards more realistic oceanic conditions. In this regard, the regime analysis is applied to a idealized large scale oceanic strait through a similitude principle. The idealized strait configuration succeeds in representing laboratory scale strait regime at largest and realistic scales. Then our analysis is applied to two well-known realistic cases: the Strait of Gibraltar and Georges Bank through large eddy simulations. These two oceanographic âsupercriticalâ regions are particularly interesting for their specific topography and stratification conditions.La marĂ©e interne gĂ©nĂ©rĂ©e sur une topographie est un Ă©lĂ©ment clĂ© des transferts Ă©nergĂ©tiques des Ă©chelles de forçage de lâocĂ©an vers les Ă©chelles de mĂ©lange turbulent. Elle contribuerait Ă prĂšs de la moitiĂ© du mĂ©lange turbulent nĂ©cessaire au maintien de la stratification ocĂ©anique. Une comprĂ©hension plus approfondie des processus mis en jeu est nĂ©cessaire pour dĂ©crire plus prĂ©cisĂ©ment son rĂŽle dans le maintien de la circulation ocĂ©anique. Cette thĂšse sâinscrit dans la continuitĂ© des travaux dâY. Dossmann (2012) portant sur les ondes internes solitaires gĂ©nĂ©rĂ©es au dessus dâune dorsale ocĂ©anique. Ces travaux reposent sur une utilisation complĂ©mentaire dâexpĂ©riences physiques menĂ©es dans le grand canal du CNRM-GAME et dâexpĂ©riences numĂ©riques Ă lâĂ©chelle du laboratoire effectuĂ©es avec le modĂšle dâocĂ©anographie cĂŽtiĂšre SNH. Lâutilisation simultanĂ©e de ces deux outils a notamment permis dâĂ©valuer la validitĂ© des hypothĂšses sous-jacentes de ce modĂšle et le dĂ©veloppement de nouveaux schĂ©mas numĂ©riques. Dans cette thĂšse, des simulations numĂ©riques utilisant la version non-hydrostatique et non-Boussinesq du modĂšle SNH sont utilisĂ©es pour dĂ©crire les diffĂ©rents rĂ©gimes dâondes internes dans des rĂ©gions « supercritiques ». Le terme supercritique dĂ©signe Ă la fois des courants de marĂ©e intenses dont la vitesse U est supĂ©rieure Ă la vitesse de propagation des ondes internes cn, et des topographies trĂšs abruptes dont lâangle de la pente est supĂ©rieur Ă lâangle du rayon dâonde interne dans la pycnocline . De telles conditions environnementales correspondent Ă des rĂ©gions de mĂ©lange intense, jusquâĂ 10 000 fois supĂ©rieur au mĂ©lange turbulent observĂ© dans lâocĂ©an ouvert. Les processus physiques ayant lieu dans ces rĂ©gions restent encore mal compris et mal reprĂ©sentĂ©s par les paramĂ©trisations dâondes internes existantes. De plus, ces rĂ©gions sont Ă©galement des zones propices Ă la gĂ©nĂ©ration dâondes internes non-linĂ©aires pouvant se propager pendant plusieurs jours et entraĂźnant ainsi des transferts dâĂ©nergie significatifs loin de leur zone de gĂ©nĂ©ration. La description des processus turbulents en jeu dans ces rĂ©gions « extrĂȘmes » constitue le cĆur de ma thĂšse.Dans une premiĂšre partie, des configurations acadĂ©miques Ă lâĂ©chelle du laboratoire sont mises en place pour Ă©tudier les processus en jeu dans diffĂ©rents rĂ©gimes « supercritiques » de gĂ©nĂ©ration dâonde internes. Des simulations numĂ©riques directes sont rĂ©alisĂ©es et permettent dâidentifier un nombre limitĂ© de paramĂštres physiques adimensionnĂ©s contrĂŽlant la dynamique des ondes internes dans ces rĂ©gions. Une attention particuliĂšre est portĂ©e sur le rĂŽle jouĂ© par la topographie sur la gĂ©nĂ©ration des modes verticaux dâondes internes et sur la formation de modes « hauts » dâondes internes solitaires.Le second objectif de cette thĂšse est de faire le lien entre les prĂ©cĂ©dentes Ă©tudes acadĂ©miques Ă lâĂ©chelle du laboratoire et lâĂ©chelle ocĂ©anique. Pour cela, un principe de similitude permettant de conserver la dynamique des ondes internes tout en modifiant lâĂ©chelle de lâĂ©coulement est mise en place. Par le biais de ce principe de similitude, nous partons de cas idĂ©alisĂ©s Ă lâĂ©chelle du laboratoire, que nous transposons Ă lâĂ©chelle ocĂ©anique, pour nous rapprocher de cas ocĂ©aniques plus rĂ©alistes et de plus en plus complexes. Puis notre Ă©tude de rĂ©gime est Ă©tendue Ă deux rĂ©gions ocĂ©aniques « supercritiques » bien connues : le dĂ©troit de Gibraltar et le plateau situĂ© Ă lâentrĂ©e du golfe du Maine (nommĂ© « Georges Bank » en anglais). LâapplicabilitĂ© de nos paramĂštres clĂ©s est Ă©tudiĂ©e dans le cas de ces deux environnements complexes par le biais de simulations haute rĂ©solution de grande Ă©chelle (LES)
Computation of Density Perturbation and Energy Flux of Internal Waves from Experimental Data
We hereby present two different spectral methods for calculating the density anomaly and the vertical energy flux from synthetic Schlieren data, for a periodic field of linear internal waves (IW) in a density-stratified fluid with a uniform buoyancy frequency. The two approaches operate under different assumptions. The first method (hereafter Mxzt) relies on the assumption of a perfectly periodic IW field in the three dimensions (x, z, t), whereas the second method (hereafter MxtUp) assumes that the IW field is periodic in x and t and composed solely of wave components with downward phase velocity. The two methods have been applied to synthetic Schlieren data collected in the CNRM large stratified water flume. Both methods succeed in reconstructing the density anomaly field. We identify and quantify the source of errors of both methods. A new method mixing the two approaches and combining their respective advantages is then proposed for the upward energy flux. The work presented in this article opens new perspectives for density and energy flux estimates from laboratory experiments data
Régimes d'ondes internes topographiques: génération de solitons et contrÎle hydraulique
L'étude des cascades d'échelles aboutissant au mélange turbulent de l'océan constitue le fil rouge de nos travaux. En associant étroitement simulation numérique et simulation physique, nous avons en particulier exploré le rÎle joué par les marées internes générées au dessus d'un talus, d'une dorsale océanique et plus récemment au passage d'un détroit. Nous avons pour cela été amenés à implémenter voire à développer de nouveaux outils expérimentaux (stéréo-corrélation...) et numériques (modÚle océanique à toit libre non-hydrostatique et non-Boussinesq...). Nous présenterons plus spécifiquement deux régimes d'ondes internes topographiques: la génération d'ondes solitaires au dessus d'une dorsale océanique et le contrÎle hydraulique des ondes au passage d'un détroit
Nonlinear processes generated by supercritical tidal flow in shallow straits
International audienceNumerical experiments have been carried out using a nonhydrostatic and non-Boussinesq regional oceanic circulation model to investigate the nonlinear processes generated by supercritical tidal flow in shallow straits. Our approach relies on idealized direct numerical simulations inspired by oceanic observations. By analyzing a large set of simulations, a regime diagram is proposed for the nonlinear processes generated in the lee of these straits. The results show that the topography shape of the strait plays a crucial role in the formation of internal solitary waves (ISWs) and in the occurrence of local breaking events. Both of these nonlinear processes are important turbulence producing phenomena. The topographic control, observed in mode 1 ISW formation in previous studies [Y. Dossmann, F. Auclair, and A. Paci, "Topographically induced internal solitary waves in a pycnocline: Primary generation and topographic control," Phys. Fluids 25, 066601 (2013) and Y. Dossmann et al., "Topographically induced internal solitary waves in a pycnocline: Ultrasonic probes and stereo-correlation measurements," Phys. Fluids 26, 056601 (2014)], is clearly reproducible for mode-2 ISW above shallow straits. Strong plunging breaking events are observed above "narrow" straits (straits with a width less than mode 1 wavelength) when the fluid velocity exceeds the local mode 1 wave speed. These results are a step towards future works on vertical mixing quantification and localization around complex strait areas
In situ observations of the smallâscale dynamics at Camarinal Sill â Strait of Gibraltar
Recently, PROTEVS GIB20 experiment was performed in the Strait of Gibraltar. Part of this experiment was dedicated to observe the high frequency dynamics near Camarinal Sill, considered as a mixing hotspot in the region. Mooring lines equipped with current profilers and temperature/salinity probes provided data which evidence two dynamical regimes depending on the tidal current intensity; in neap tide floods, local internal hydraulic control is never observed over CS while in spring tide, local internal hydraulic control depicts a tideâdependent and spatially variable pattern. In spring tide floods, measurements revealed the development of a hydraulic jump over the sill and its advection on the lee side. Cross sill sections with CTD casts and acoustic images confirmed this dynamics and depicted a well developed hydraulic jump on the eastern flank of the sill during spring tide ebbs. The northâsouth and temporal variability of the internal hydraulics was analyzed from several zonal sections over Camarinal Sill, the mean topographic feature of the Strait of Gibraltar. We highlighted a complex series of local hydraulic jumps constrained by topography and a significant northâsouth variability. The spatiotemporal variability of local hydraulics questions the two dimensional representation of the exchange flow in the Strait of Gibraltar. During neap tide flood, the dynamics of the Mediterranean outflow was investigated from a fixed station. We imaged the development of instabilities at the interface between Atlantic and Mediterranean waters jointly with the generation of much thicker billows deeper. Finally, we discuss our findings in relation to other straits dynamics
Observation of non linear internal waves on the Landes shelf (Bay of Biscay)
The Bay of Biscay (Bob) is a hotspot of internal tide and non-linear internal wave generation. Yet, the internal waveson the inner shelf have never been observed nor described. An oceanographic campaign has been conducted overthe Landes shelf (Southern BoB) during the summer of 2017 to better characterize internal wave dynamics. Usingvelocity current profiles, thermistor chains and MVP (Moving Vessel Profiler) measurements in 65m water depth, wehere describe a part of the first field observations of non-linear internal waves (NLIWs) on the southern shelf of theBay of Biscay. Frequent NLIWs, characterized by isotherm excursions up to 20m and vertical currents of 0.2m/s, wereobserved. NLIWs of depression propagate close to the surface and polarity reversal was evidence for some of thoseNLIWs. No polarity reversals were observed when background stratification was maximum close to the seafloor.Trains of NLIWs of elevation, associated with a maximum density gradient close to the bottom, were observed at thebeginning of the campaign. The latter observation seems to be linked to a deep cold water advection. Velocity measurements show two flow regions within the wave frame of reference. The backscatter signal is used to proxysuspended materials and showed different responses depending on the flow region. In conclusion, we observechanges in the NLIWs response with the background stratification. Clear elevation and depression NLIWs exist whenthere was strong near-bed stratification. Twenty days later, maximum stratification was in the middle of the watercolumn and packets of mixed polarity waves were observed. Increased backscatter signal within the core of someNLIWs suggests that they could potentially impact the dynamics of near-bed material. However, the processesinducing increased backscatter have not been identified, and dedicated measurements will be conducted in the futureto better understand the underlying mechanisms
Evidence of Reflected Internal Solitary Waves in the Strait of Gibraltar
Large-amplitude internal solitary waves (ISWs) propagating eastward toward the Alboran Sea have long been known in the Strait of Gibraltar. New in-situ data and satellite images evidence northwestward propagating ISWs. These waves are probably the reflection of the well-known eastward propagating wave along the Moroccan shelf. A simple 2D-vertical section, run with the compressible non-hydrostatic Coastal and Regional Ocean COmmunity model, illustrates that the Moroccan slope is conducive to reflection of incident solitary waves of amplitude observed in the Strait of Gibraltar. A clear signature of these waves in the Tarifa high-frequency sea level oscillations is depicted that paves the way to studies of the seasonality of ISWs in the Strait of Gibraltar with the long-time series of sea level at Tarifa. The polarity of the reflected waves, observed with Acoustic Doppler Current Profilers/Conductivity-Temperature-Depth array moorings, presents a slight variability possibly due to the internal tide oscillations. The reflected ISWs arrives at the mooring location in phase with the peak of the internal tide. For the strongest tide, the pynocline approaches a critical point where the polarity of the ISWs might reverse.
Key Points
Northwestward propagating internal solitary waves (ISWs) suspected to be reflections are evidenced in the Strait of Gibraltar
A simple numerical approach illustrates that incident ISWs of amplitude observed in the area can reflect on the Moroccan slope
High-frequency sea level oscillations at Tarifa reveal a signature consistent with the observed ISWs
Plain Language Summary
The Strait of Gibraltar is known as a hotspot for internal solitary waves (ISWs) generation, which propagate eastward toward the Alboran Sea. A new in-situ data set combined with satellite images reveal northwestward propagating ISWs in the Strait of Gibraltar. These waves are likely generated by the reflection of the eastward propagating waves on the Moroccan shelf as shown by simple numerical simulations. The reflected wave train, whose leading wave can be either an elevation or a depression wave, arrives at the observation site in phase with the internal tide trough (maximum sinking of the isopycnals). In strong spring tides, the interface depth approaches the critical point, which allows the possibility of change of wave-polarity, that is, depression versus elevation, thus explaining the double nature of the observed leading waves. Signatures of the reflected waves have been also identified in the high-frequency sea level oscillations at Tarifa. This study paves the way for long-term observations of ISWs in the Strait of Gibraltar
Evidence of Reflected Internal Solitary Waves in the Strait of Gibraltar
International audienceLarge-amplitude internal solitary waves (ISWs) propagating eastward toward the Alboran Sea have long been known in the Strait of Gibraltar. New in-situ data and satellite images evidence northwestward propagating ISWs. These waves are probably the reflection of the well-known eastward propagating wave along the Moroccan shelf. A simple 2D-vertical section, run with the compressible non-hydrostatic Coastal and Regional Ocean COmmunity model, illustrates that the Moroccan slope is conducive to reflection of incident solitary waves of amplitude observed in the Strait of Gibraltar. A clear signature of these waves in the Tarifa high-frequency sea level oscillations is depicted that paves the way to studies of the seasonality of ISWs in the Strait of Gibraltar with the long-time series of sea level at Tarifa. The polarity of the reflected waves, observed with Acoustic Doppler Current Profilers/Conductivity-Temperature-Depth array moorings, presents a slight variability possibly due to the internal tide oscillations. The reflected ISWs arrives at the mooring location in phase with the peak of the internal tide. For the strongest tide, the pynocline approaches a critical point where the polarity of the ISWs might reverse. Plain Language Summary The Strait of Gibraltar is known as a hotspot for internal solitary waves (ISWs) generation, which propagate eastward toward the Alboran Sea. A new in-situ data set combined with satellite images reveal northwestward propagating ISWs in the Strait of Gibraltar. These waves are likely generated by the reflection of the eastward propagating waves on the Moroccan shelf as shown by simple numerical simulations. The reflected wave train, whose leading wave can be either an elevation or a depression wave, arrives at the observation site in phase with the internal tide trough (maximum sinking of the isopycnals). In strong spring tides, the interface depth approaches the critical point, which allows the possibility of change of wave-polarity, that is, depression versus elevation, thus explaining the double nature of the observed leading waves. Signatures of the reflected waves have been also identified in the high-frequency sea level oscillations at Tarifa. This study paves the way for long-term observations of ISWs in the Strait of Gibraltar
Turbulence Over Camarinal Sill and Its Impact on Water MixingâStrait of Gibraltar
traits and narrows are the place of intensified turbulent mixing. Deep understanding of the turbulent dynamics at these locations is of crucial importance as it conditions the properties of the water masses flowing in the open ocean. A new extensive field experiment, PROTEVS GIB20, with high frequency measurements has been conducted in the Strait of Gibraltar. It allows us to infer dissipation rates of the turbulent kinetic energy, Ï”, from two consistent methods. The range of Ï” is depicted for the different processes which developed in the vicinity of Camarinal Sill, the main topographic feature of the Strait of Gibraltar. It evidences that the bottom boundary layer, hydraulic jumps and large overturns are the main loci of intensified turbulence reaching 10â3W.kgâ1. The variability of the turbulence is mainly controlled by semi-diurnal, diurnal and fortnightly tidal oscillations. Spatially, the western flank of Camarinal Sill is evidenced as the hotspot for turbulent mixing. We confirm a weak variation of the spatially averaged vertically integrated turbulent dissipation rate. This result needs to be qualified in view of the differentiated impact of the various processes on adjacent water masses. The dynamics of the spring tide directly mix Atlantic and Mediterranean waters, resulting in a large spreading of the T-S diagram.
Key Points
Dissipation rates of TKE are inferred near the Camarinal Sill with maximum values of 10â3Â W/kg in the hydraulic jump
The spatio-temporal distribution of dissipation rates near Camarinal Sill shows maxima on the western flank in tidal outflows
Mixing rates are similar during spring and neap tides except for the Atlantic waters more impacted by spring tide hydraulic jumps
Plain Language Summary
A recent field experiment, PROTEVS GIB20, was performed in the Strait of Gibraltar in October 2020. This experiment was partly designed to observe the turbulent dynamics of the flow in this region. The Strait of Gibraltar presents an important topographic sill, Camarinal Sill, where the current reaches its maximum value enhancing the turbulent processes. Quantifying the turbulence intensity and the associated mixing that develops there is of crucial importance to understand the impact of small-scale process on larger/regional scales which is now recognized but remains incompletely understood. Such measurements are necessary to assess the performances of the parametrization use in climate model which to dot represent such small-scale dynamics. Direct measurements of dissipation rate of turbulent kinetic energy are rare near Camarinal Sill, due to the rough environment and strong maritime traffic. From different methods, we infer time series and spatial averages of the turbulence dissipation rate. We describe the variability of the turbulence, the physical processes involved and the impact on water mass mixing