This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (We... more This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low-and high-water seasons. Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-monthly scales, the lagoon is tidally choked: ocean tides can reach 90 cm, whereas in the lagoon semi-diurnal and diurnal tides hardly reach few centimeters. Choking conditions vary with river inflow and ocean tide amplitude, correctly represented by a simple tidal choking model. Diurnal modulation and asymmetry of the tide are stronger (weaker) during high (low) water period. We also observed WL variations of ±5-10 cm at a fortnightly frequency, stronger during wet (high-water) season. Superimposed on the seasonal, fortnightly and tidal WL variations, we further observed short-term high-frequency seiche events. Mostly observed during dry (low-water) conditions, they are characterized by typical standing-wave oscillations of 5-10 cm amplitudes and 3 h periods. They are forced by the passage of fast-moving squall-lines that induce strong wind variations, heavy rainfalls and rapid drop-off of the air temperature. Results obtained in this study provide useful metrics for the validation of flood forecasting models to be implemented in Benin, and elsewhere on the West African coastline.
Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo f... more Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo floats, we analyze the isopycnal θ/S anomalies (θ'/S') within newborn eddies in the tropical Atlantic Ocean (TAO). Our results show that on density‐coordinates, both anticyclonic eddies (AEs) and cyclonic eddies (CEs) can exhibit positive, negative, or nonsignificant θ'/S'. Almost half of the sampled eddies do not have significant θ'/S' at their generation site. The other half exhibits significant positive or negative θ', up to ±0.5°C typically. More than 70% of them have a subsurface signature. Refined analyses of the vertical structure of new‐born eddies in three selected subregions show the dominance of cold (warm) subsurface AEs (CEs) likely due to isopycnal advection of large‐scale potential vorticity (PV) and θ. PV is a key parameter to analyze eddy generation and dynamics. Isopycnal advection, friction or diapycnal mixing can be involved in the generatio...
Waves propagate in a free-surface ocean due to compressibility and gravity (and surface tension a... more Waves propagate in a free-surface ocean due to compressibility and gravity (and surface tension at much smaller scale). Analytical solutions have long been derived independently for acoustic and gravity waves, i.e., acoustic waves or internal-gravity rays in an unbounded ocean, surface-gravity waves in a free-surface-ocean, and acoustic or internal modes in a bounded ocean. In the present study, surface tension and earth-rotation are neglected and a simple, unified model based on inner and boundary dispersion relations is derived for waves propagating in a compressible, stratified, freesurface ocean. Branches of acoustic gravity wave solutions are identified and visually analysed in phasespace. Taylor developments are then carried out with respect to small parameters describing stratification and compressibility and are compared with numerical approximations of the intersection of inner and boundary dispersion surfaces. Finally, the model recovers the known approximations for swell, longsurface waves, internal-gravity rays, internal modes, acoustic waves or acoustic modes, and also provides modification of these solutions due to stratification and compressibility. Two peculiar regions of the acoustic-gravity wave phase-space are more specifically highlighted and studied in details: one for long waves shedding new light on the distinction between surface waves and low-order internal modes, the other for marginally stable surface waves of intermediate length-scale.
Nonlinear Processes in Geophysics Discussions, 2019
We apply a coupled modelling system composed of a state-of-the-art hydrodynamical model and a low... more We apply a coupled modelling system composed of a state-of-the-art hydrodynamical model and a low complexity biogeochemical model to an idealized Iberian Peninsula upwelling system to identify the main drivers of dissolved oxygen variability and to study its response to changes in the duration of the upwelling season and in phytoplankton growth regime. We find that the export of oxygenated waters by upwelling front turbulence is a major sink for nearshore dissolved oxygen. In our simulations of summer upwelling, when phytoplankton population is generally dominated by diatoms whose growth is largely enhanced by nutrient input, net primary production and air-sea exchange compensate dissolved oxygen depletion by offshore export over the shelf. A shorter upwelling duration causes relaxation of upwelling winds and a decrease in offshore export, resulting in a slight increase of net dissolved oxygen enrichment in the coastal region as compared to longer upwelling durations. When phytoplankton is dominated by groups less sensitive to nutrient inputs, growth rates decrease and the coastal region becomes net heterotrophic. Together with the physical sink, this lowers the net oxygenation rate of coastal waters, that remains positive only because of air-sea exchanges. These findings help disentangling the physical and biogeochemical controls of dissolved oxygen in upwelling systems and, together with projections of increased duration of upwelling seasons and phytoplankton community changes, suggest that the Iberian coastal upwelling region may become more vulnerable to hypoxia and deoxygenation.
In this study, the authors first show that it is difficult to reconstruct the vertical structure ... more In this study, the authors first show that it is difficult to reconstruct the vertical structure of vortices using only surface observations. In particular, they show that the recent surface quasigeostrophy (SQG) and interior and surface quasigeostrophy (ISQG) methods systematically lead to surface-intensified vortices, and those subsurface-intensified vortices are thus not correctly modeled. The authors then investigate the possibility of distinguishing between surface- and subsurface-intensified eddies from surface data only, using the sea surface height and the sea surface temperature available from satellite observations. A simple index, based on the ratio of the sea surface temperature anomaly and the sea level anomaly, is proposed. While the index is expected to give perfect results for isolated vortices, the authors show that in a complex environment, errors can be expected, in particular when strong currents exist in the vicinity of the vortex. The validity of the index is t...
Titre: Influence des processus physiques à mésoéchelle sur l'écosystème planctonique: applic... more Titre: Influence des processus physiques à mésoéchelle sur l'écosystème planctonique: application aux zones d'Upwelling de Bord Est.
• Global deoxygenation is a crucial issue with severe consequences to coastal ecosystems (Levin[2... more • Global deoxygenation is a crucial issue with severe consequences to coastal ecosystems (Levin[2018],Breitburgh et al[2018]) • Eastern boundary upwelling systems are key players in global biogeochemical cycles due to their intense biological productivity and their connection with the oligotrophic open ocean. • Climate change will bring about changes in physical and biogeochemical driving factors in EBUS.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastwa... more The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastward and westward jets. The origin and transport properties of these flow features remain in many ways an open question, all the more crucial since their usual underestimation in ocean global circulation models has been identified as a potential bias for the misrepresentation of the oxygen minimum zones. In this study, we analyze the water mass properties associated with these systems of jets using velocity and hydrographic sections. Data acquired during a dedicated cruise carried out in the western part of the basin and supplemented by cross-equatorial sections from historical cruises in the central and eastern parts are analyzed. While it is confirmed that the near-equatorial jets carry oxygen anomalies, contributing to the ventilation of the eastern tropical Pacific, the data also revealed unexpected features. Tracer distributions (oxygen, salinity, and potential vorticity) show the pre...
Taking advantage of alternative expressions for potential vorticity (PV) in divergence forms, we ... more Taking advantage of alternative expressions for potential vorticity (PV) in divergence forms, we derive balances between volume integral of PV and boundary conditions, that are then applied to practical computations of PV: • we propose a new method for diagnosing the Ertel potential vorticity from model output, that preserves the balances; • we show how the expression of PV can be derived in general coordinate systems. This is here emphasised with isopycnic coordinates by generalising the PV expression to the general Navier-Stokes equations; • we propose a generalised derivation for the Haynes-McIntyre impermeability theorem, which highlights the role of the bottom boundary condition choice (e.g. no-slip vs free-slip) and mixing near the bottom
ABSTRACT In this paper, the authors study the influence of the wind on the dynamics of the contin... more ABSTRACT In this paper, the authors study the influence of the wind on the dynamics of the continental shelf and margin, in particular the formation of a secondary upwelling (or downwelling) front along the shelf break.Observations during the MOUTON2007 campaign at sea along the Portuguese coast in summer 2007 reveal the presence of several upwelling fronts, one being located near the shelf break. All upwellings are characterized by deep cold waters close to or reaching the surface and with high chlorophyll concentrations. Simplified numerical models are built in order to study a possible physical mechanism behind this observation. First, a simple shallow water model with three distinct layers is used to study the formation of secondary upwelling fronts. We show that the physical mechanism behind this process is associated with onshore transport of high potential vorticity anomalies of the shelf for upwelling favorable conditions. Sensitivity studies to bottom friction, shelf width, continental slope steepness, shelf “length” are analysed in terms of potential vorticity dynamics. In particular bottom friction is analyzed in detail and we find that, even though bottom friction limits the barotropic velocity field, it enhances the cross-shore circulation, so that no steady state is possible when stratification is taken into account. Bottom friction accelerates the onshore advection of high potential vorticity, but also drastically reduces its amplitude because of diabatic effects. The net effect of bottom friction is to reduce the secondary upwelling development. Based on similar mechanisms, previous results are then extended to downwelling favorable conditions. Finally a more realistic configuration, with bottom topography, wind forcing and stratification set up from observations, is then developed and the results confronted to the observations. Simulations overestimate the velocity amplitude but exhibit good agreement in terms of density ranges brought over the shelf and general isopycnal patterns.The application and extension of the results to more general oceanic regions is discussed and we conclude on the influence of such process on the dynamics of wind driven circulation over a shelf.
In this paper, the authors analyse the stability of particular numerical schemes used in oceanic ... more In this paper, the authors analyse the stability of particular numerical schemes used in oceanic general circulation models to deal with the barotropic momentum advection term. It is shown that, when this term is integrated using time splitting, its stability properties can be drastically reduced in configurations where there exists shallow areas, where velocities become comparable to the propagation speed of external gravity waves. A simple alternative scheme with improved stability is proposed and discussed.
... the first one, and 2 m for the other two); and U s , T s and q s are the wind speed, temperat... more ... the first one, and 2 m for the other two); and U s , T s and q s are the wind speed, temperature ... 3.1.1. Description of the observation area. ... 1) are known for their intense tidal currents that generate intense mixing, thought to be mainly associated with the bottom boundary layer (see ...
This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount... more This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole inf...
Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of t... more Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of the circulation in the Bay of Biscay has been computed. The buoys deployed during several cruises are fitted with drogues at depth between~15 m and~80 m to follow the current in surface layers. After a pre-processing of the dataset, 281 trajectories corresponding to a cumulated time series about 71 years long have been obtained to describe the residual seasonal circulation. Main features have been identified: an general northwesterly circulation over the deep ocean, a cyclonic along-slope circulation with a poleward slope current along the Aquitaine, Armorican and Celtic shelves (3.5 to 7 cm s −1), a marked winter eastward flow along the North Spanish slope (larger than 5 cm s −1 in average), and weak currents on the continental shelf from April to September (lower than 2.5 cm s −1 in summer). Further circulation patterns have been identified and quantified as the westward current in spring and summer along the North Spanish slope and shelf with speeds reaching 13.5 cm s −1. A cyclonic circulation cell has also been confirmed on the Armorican shelf from October to March. Intense poleward currents (about 10-15 cm s −1) have been observed from October to March on the shelf. The density of trajectories from 2004 to 2009 also allowed exploring the interannual variability of the circulation during two main half-year periods from October to March and April to September. Through this analysis, we described the variability of the Iberian Poleward Current intrusions in the Bay of Biscay during recent years as well as the interannual fluctuations in the development of a poleward autumn current on the Armorican shelf.
ABSTRACT This paper deals with the extension of mixed water plumes in a coastal environment. This... more ABSTRACT This paper deals with the extension of mixed water plumes in a coastal environment. This problem is connected with the dynamics of tidal fronts and is studied using simplified academic configurations where tidal mixing is represented as fixed areas where stratified waters are continuously homogenized in the vertical. The sensitivity of dispersive processes to seasonal and local parameters is analyzed.Localized mixing produces baroclinically unstable structures which are shown to generate vortices that transport mixed waters trapped in their core away from the mixing area. New stratified waters then enter the mixing area so that the homogenization process can be pursued. The production rate of homogenized waters is thus closely related to the ability of vortices, or other dispersive effects, to propagate away from the mixing area and disperse homogenized waters. A way to quantify dispersion is then proposed, based on this principle.Several mechanisms leading to vortex propagation and dispersion of mixed waters are identified. We use a four layer configuration to study their sensitivity to different parameters such as topography, the presence of a coast near the mixed water plume, the vertical mixing rate, bottom friction, stratification or the existence of a background current. Potential vorticity anomaly is used as a tracer of the mixed waters and to calculate production rates as a function of the latter parameters, and to analyze the dispersive mechanisms. It is shown that:•Baroclinic instability represents the most efficient mechanism for dispersing the homogenized waters. The emerging vortices are indeed mainly constituted of baroclinic dipoles (or hetons) with self propagating capacities.•On the f -plane, a bottom slope perturbs baroclinic instability and reduces the production rate. The homogenized waters are also funneled in a plume along lines of constant depth.•When mixing is produced against a wall, smaller vortices emerge, which drastically reduces the baroclinic instability efficiency. Mirror effects and a thin coastal (Kelvin) current developing along shore also favor dispersion but are of weaker influence. They also funnel dispersion along the coast (an effect which is strengthened by a bottom slope) so that the mixed waters form a plume extending Northward when the wall is located on the Eastern boundary. However these effects also counteract baroclinic instability so that the production rate and dispersion efficiency globally decrease when homogenization occurs along a coast.•An enhancement of mixing has a limited effect on the production rate and dispersion of homogenized waters because the production of mixed waters is strongly constrained by dispersive processes rather than the ability of mixing to renew the homogenized plume.•A bottom friction establishes a reduced gravity dynamics and inhibits the barotropic mode. This induces a reduction of the size of vortices emerging from baroclinic instability, which strongly diminishes the production rate and dispersion efficiency. It also modifies the vertical structure of the coastal (Kelvin) current which can significantly alter the dispersion pattern, but the effect associated with this process remains difficult to predict.•A decrease of stratification has two counteracting effects: it reduces the size of the emerging vortices but also reinforces the coupling between layers. We have found that the latter effect is not strong enough to overcome the reduction of the size of the emerging vortices, so that baroclinic instability and dispersion of homogenized waters are inhibited when stratification is reduced.•Finally, when a background barotropic current is added and if mixing occurs away from boundaries, dispersion increases linearly with the velocity field above a background velocity threshold. However a tidal front developing along a coast exhibits strong asymmetries depending on the direction of the current: production and dispersion increases linearly (above a threshold) with a Southward current, whereas it is shown that a Northward current inhibits baroclinic instabilities so that the average production rate remains almost constant. In that case, dispersion exhibits specific patterns with periodic release of homogenized water.The application of these results to the real ocean is finally discussed.
This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (We... more This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low-and high-water seasons. Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-monthly scales, the lagoon is tidally choked: ocean tides can reach 90 cm, whereas in the lagoon semi-diurnal and diurnal tides hardly reach few centimeters. Choking conditions vary with river inflow and ocean tide amplitude, correctly represented by a simple tidal choking model. Diurnal modulation and asymmetry of the tide are stronger (weaker) during high (low) water period. We also observed WL variations of ±5-10 cm at a fortnightly frequency, stronger during wet (high-water) season. Superimposed on the seasonal, fortnightly and tidal WL variations, we further observed short-term high-frequency seiche events. Mostly observed during dry (low-water) conditions, they are characterized by typical standing-wave oscillations of 5-10 cm amplitudes and 3 h periods. They are forced by the passage of fast-moving squall-lines that induce strong wind variations, heavy rainfalls and rapid drop-off of the air temperature. Results obtained in this study provide useful metrics for the validation of flood forecasting models to be implemented in Benin, and elsewhere on the West African coastline.
Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo f... more Based on 18 years of satellite altimetry data and temperature/salinity (θ/S) profiles from Argo floats, we analyze the isopycnal θ/S anomalies (θ'/S') within newborn eddies in the tropical Atlantic Ocean (TAO). Our results show that on density‐coordinates, both anticyclonic eddies (AEs) and cyclonic eddies (CEs) can exhibit positive, negative, or nonsignificant θ'/S'. Almost half of the sampled eddies do not have significant θ'/S' at their generation site. The other half exhibits significant positive or negative θ', up to ±0.5°C typically. More than 70% of them have a subsurface signature. Refined analyses of the vertical structure of new‐born eddies in three selected subregions show the dominance of cold (warm) subsurface AEs (CEs) likely due to isopycnal advection of large‐scale potential vorticity (PV) and θ. PV is a key parameter to analyze eddy generation and dynamics. Isopycnal advection, friction or diapycnal mixing can be involved in the generatio...
Waves propagate in a free-surface ocean due to compressibility and gravity (and surface tension a... more Waves propagate in a free-surface ocean due to compressibility and gravity (and surface tension at much smaller scale). Analytical solutions have long been derived independently for acoustic and gravity waves, i.e., acoustic waves or internal-gravity rays in an unbounded ocean, surface-gravity waves in a free-surface-ocean, and acoustic or internal modes in a bounded ocean. In the present study, surface tension and earth-rotation are neglected and a simple, unified model based on inner and boundary dispersion relations is derived for waves propagating in a compressible, stratified, freesurface ocean. Branches of acoustic gravity wave solutions are identified and visually analysed in phasespace. Taylor developments are then carried out with respect to small parameters describing stratification and compressibility and are compared with numerical approximations of the intersection of inner and boundary dispersion surfaces. Finally, the model recovers the known approximations for swell, longsurface waves, internal-gravity rays, internal modes, acoustic waves or acoustic modes, and also provides modification of these solutions due to stratification and compressibility. Two peculiar regions of the acoustic-gravity wave phase-space are more specifically highlighted and studied in details: one for long waves shedding new light on the distinction between surface waves and low-order internal modes, the other for marginally stable surface waves of intermediate length-scale.
Nonlinear Processes in Geophysics Discussions, 2019
We apply a coupled modelling system composed of a state-of-the-art hydrodynamical model and a low... more We apply a coupled modelling system composed of a state-of-the-art hydrodynamical model and a low complexity biogeochemical model to an idealized Iberian Peninsula upwelling system to identify the main drivers of dissolved oxygen variability and to study its response to changes in the duration of the upwelling season and in phytoplankton growth regime. We find that the export of oxygenated waters by upwelling front turbulence is a major sink for nearshore dissolved oxygen. In our simulations of summer upwelling, when phytoplankton population is generally dominated by diatoms whose growth is largely enhanced by nutrient input, net primary production and air-sea exchange compensate dissolved oxygen depletion by offshore export over the shelf. A shorter upwelling duration causes relaxation of upwelling winds and a decrease in offshore export, resulting in a slight increase of net dissolved oxygen enrichment in the coastal region as compared to longer upwelling durations. When phytoplankton is dominated by groups less sensitive to nutrient inputs, growth rates decrease and the coastal region becomes net heterotrophic. Together with the physical sink, this lowers the net oxygenation rate of coastal waters, that remains positive only because of air-sea exchanges. These findings help disentangling the physical and biogeochemical controls of dissolved oxygen in upwelling systems and, together with projections of increased duration of upwelling seasons and phytoplankton community changes, suggest that the Iberian coastal upwelling region may become more vulnerable to hypoxia and deoxygenation.
In this study, the authors first show that it is difficult to reconstruct the vertical structure ... more In this study, the authors first show that it is difficult to reconstruct the vertical structure of vortices using only surface observations. In particular, they show that the recent surface quasigeostrophy (SQG) and interior and surface quasigeostrophy (ISQG) methods systematically lead to surface-intensified vortices, and those subsurface-intensified vortices are thus not correctly modeled. The authors then investigate the possibility of distinguishing between surface- and subsurface-intensified eddies from surface data only, using the sea surface height and the sea surface temperature available from satellite observations. A simple index, based on the ratio of the sea surface temperature anomaly and the sea level anomaly, is proposed. While the index is expected to give perfect results for isolated vortices, the authors show that in a complex environment, errors can be expected, in particular when strong currents exist in the vicinity of the vortex. The validity of the index is t...
Titre: Influence des processus physiques à mésoéchelle sur l'écosystème planctonique: applic... more Titre: Influence des processus physiques à mésoéchelle sur l'écosystème planctonique: application aux zones d'Upwelling de Bord Est.
• Global deoxygenation is a crucial issue with severe consequences to coastal ecosystems (Levin[2... more • Global deoxygenation is a crucial issue with severe consequences to coastal ecosystems (Levin[2018],Breitburgh et al[2018]) • Eastern boundary upwelling systems are key players in global biogeochemical cycles due to their intense biological productivity and their connection with the oligotrophic open ocean. • Climate change will bring about changes in physical and biogeochemical driving factors in EBUS.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastwa... more The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastward and westward jets. The origin and transport properties of these flow features remain in many ways an open question, all the more crucial since their usual underestimation in ocean global circulation models has been identified as a potential bias for the misrepresentation of the oxygen minimum zones. In this study, we analyze the water mass properties associated with these systems of jets using velocity and hydrographic sections. Data acquired during a dedicated cruise carried out in the western part of the basin and supplemented by cross-equatorial sections from historical cruises in the central and eastern parts are analyzed. While it is confirmed that the near-equatorial jets carry oxygen anomalies, contributing to the ventilation of the eastern tropical Pacific, the data also revealed unexpected features. Tracer distributions (oxygen, salinity, and potential vorticity) show the pre...
Taking advantage of alternative expressions for potential vorticity (PV) in divergence forms, we ... more Taking advantage of alternative expressions for potential vorticity (PV) in divergence forms, we derive balances between volume integral of PV and boundary conditions, that are then applied to practical computations of PV: • we propose a new method for diagnosing the Ertel potential vorticity from model output, that preserves the balances; • we show how the expression of PV can be derived in general coordinate systems. This is here emphasised with isopycnic coordinates by generalising the PV expression to the general Navier-Stokes equations; • we propose a generalised derivation for the Haynes-McIntyre impermeability theorem, which highlights the role of the bottom boundary condition choice (e.g. no-slip vs free-slip) and mixing near the bottom
ABSTRACT In this paper, the authors study the influence of the wind on the dynamics of the contin... more ABSTRACT In this paper, the authors study the influence of the wind on the dynamics of the continental shelf and margin, in particular the formation of a secondary upwelling (or downwelling) front along the shelf break.Observations during the MOUTON2007 campaign at sea along the Portuguese coast in summer 2007 reveal the presence of several upwelling fronts, one being located near the shelf break. All upwellings are characterized by deep cold waters close to or reaching the surface and with high chlorophyll concentrations. Simplified numerical models are built in order to study a possible physical mechanism behind this observation. First, a simple shallow water model with three distinct layers is used to study the formation of secondary upwelling fronts. We show that the physical mechanism behind this process is associated with onshore transport of high potential vorticity anomalies of the shelf for upwelling favorable conditions. Sensitivity studies to bottom friction, shelf width, continental slope steepness, shelf “length” are analysed in terms of potential vorticity dynamics. In particular bottom friction is analyzed in detail and we find that, even though bottom friction limits the barotropic velocity field, it enhances the cross-shore circulation, so that no steady state is possible when stratification is taken into account. Bottom friction accelerates the onshore advection of high potential vorticity, but also drastically reduces its amplitude because of diabatic effects. The net effect of bottom friction is to reduce the secondary upwelling development. Based on similar mechanisms, previous results are then extended to downwelling favorable conditions. Finally a more realistic configuration, with bottom topography, wind forcing and stratification set up from observations, is then developed and the results confronted to the observations. Simulations overestimate the velocity amplitude but exhibit good agreement in terms of density ranges brought over the shelf and general isopycnal patterns.The application and extension of the results to more general oceanic regions is discussed and we conclude on the influence of such process on the dynamics of wind driven circulation over a shelf.
In this paper, the authors analyse the stability of particular numerical schemes used in oceanic ... more In this paper, the authors analyse the stability of particular numerical schemes used in oceanic general circulation models to deal with the barotropic momentum advection term. It is shown that, when this term is integrated using time splitting, its stability properties can be drastically reduced in configurations where there exists shallow areas, where velocities become comparable to the propagation speed of external gravity waves. A simple alternative scheme with improved stability is proposed and discussed.
... the first one, and 2 m for the other two); and U s , T s and q s are the wind speed, temperat... more ... the first one, and 2 m for the other two); and U s , T s and q s are the wind speed, temperature ... 3.1.1. Description of the observation area. ... 1) are known for their intense tidal currents that generate intense mixing, thought to be mainly associated with the bottom boundary layer (see ...
This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount... more This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole inf...
Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of t... more Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of the circulation in the Bay of Biscay has been computed. The buoys deployed during several cruises are fitted with drogues at depth between~15 m and~80 m to follow the current in surface layers. After a pre-processing of the dataset, 281 trajectories corresponding to a cumulated time series about 71 years long have been obtained to describe the residual seasonal circulation. Main features have been identified: an general northwesterly circulation over the deep ocean, a cyclonic along-slope circulation with a poleward slope current along the Aquitaine, Armorican and Celtic shelves (3.5 to 7 cm s −1), a marked winter eastward flow along the North Spanish slope (larger than 5 cm s −1 in average), and weak currents on the continental shelf from April to September (lower than 2.5 cm s −1 in summer). Further circulation patterns have been identified and quantified as the westward current in spring and summer along the North Spanish slope and shelf with speeds reaching 13.5 cm s −1. A cyclonic circulation cell has also been confirmed on the Armorican shelf from October to March. Intense poleward currents (about 10-15 cm s −1) have been observed from October to March on the shelf. The density of trajectories from 2004 to 2009 also allowed exploring the interannual variability of the circulation during two main half-year periods from October to March and April to September. Through this analysis, we described the variability of the Iberian Poleward Current intrusions in the Bay of Biscay during recent years as well as the interannual fluctuations in the development of a poleward autumn current on the Armorican shelf.
ABSTRACT This paper deals with the extension of mixed water plumes in a coastal environment. This... more ABSTRACT This paper deals with the extension of mixed water plumes in a coastal environment. This problem is connected with the dynamics of tidal fronts and is studied using simplified academic configurations where tidal mixing is represented as fixed areas where stratified waters are continuously homogenized in the vertical. The sensitivity of dispersive processes to seasonal and local parameters is analyzed.Localized mixing produces baroclinically unstable structures which are shown to generate vortices that transport mixed waters trapped in their core away from the mixing area. New stratified waters then enter the mixing area so that the homogenization process can be pursued. The production rate of homogenized waters is thus closely related to the ability of vortices, or other dispersive effects, to propagate away from the mixing area and disperse homogenized waters. A way to quantify dispersion is then proposed, based on this principle.Several mechanisms leading to vortex propagation and dispersion of mixed waters are identified. We use a four layer configuration to study their sensitivity to different parameters such as topography, the presence of a coast near the mixed water plume, the vertical mixing rate, bottom friction, stratification or the existence of a background current. Potential vorticity anomaly is used as a tracer of the mixed waters and to calculate production rates as a function of the latter parameters, and to analyze the dispersive mechanisms. It is shown that:•Baroclinic instability represents the most efficient mechanism for dispersing the homogenized waters. The emerging vortices are indeed mainly constituted of baroclinic dipoles (or hetons) with self propagating capacities.•On the f -plane, a bottom slope perturbs baroclinic instability and reduces the production rate. The homogenized waters are also funneled in a plume along lines of constant depth.•When mixing is produced against a wall, smaller vortices emerge, which drastically reduces the baroclinic instability efficiency. Mirror effects and a thin coastal (Kelvin) current developing along shore also favor dispersion but are of weaker influence. They also funnel dispersion along the coast (an effect which is strengthened by a bottom slope) so that the mixed waters form a plume extending Northward when the wall is located on the Eastern boundary. However these effects also counteract baroclinic instability so that the production rate and dispersion efficiency globally decrease when homogenization occurs along a coast.•An enhancement of mixing has a limited effect on the production rate and dispersion of homogenized waters because the production of mixed waters is strongly constrained by dispersive processes rather than the ability of mixing to renew the homogenized plume.•A bottom friction establishes a reduced gravity dynamics and inhibits the barotropic mode. This induces a reduction of the size of vortices emerging from baroclinic instability, which strongly diminishes the production rate and dispersion efficiency. It also modifies the vertical structure of the coastal (Kelvin) current which can significantly alter the dispersion pattern, but the effect associated with this process remains difficult to predict.•A decrease of stratification has two counteracting effects: it reduces the size of the emerging vortices but also reinforces the coupling between layers. We have found that the latter effect is not strong enough to overcome the reduction of the size of the emerging vortices, so that baroclinic instability and dispersion of homogenized waters are inhibited when stratification is reduced.•Finally, when a background barotropic current is added and if mixing occurs away from boundaries, dispersion increases linearly with the velocity field above a background velocity threshold. However a tidal front developing along a coast exhibits strong asymmetries depending on the direction of the current: production and dispersion increases linearly (above a threshold) with a Southward current, whereas it is shown that a Northward current inhibits baroclinic instabilities so that the average production rate remains almost constant. In that case, dispersion exhibits specific patterns with periodic release of homogenized water.The application of these results to the real ocean is finally discussed.
Ce cahier n°3 présente le compte rendu d’un atelier qui s’est tenu le 8 septembre 2016 à l’École ... more Ce cahier n°3 présente le compte rendu d’un atelier qui s’est tenu le 8 septembre 2016 à l’École des hautes études en sciences sociales Paris. Cet atelier rassemblait des chercheurs en sciences sociales,en sciences de gestion, des chercheurs en sciences de l’environnement et du climat ainsi que des ingénieurs du CNES.
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Papers by Yves Morel