Etude numérique de la mer d'Iroise : dynamique, variabilité du front d'Ouessant et évaluation des échanges cross-frontaux

This thesis falls in a realistic ocean modelling framework, focused on the study of the Iroise Sea. The first two parts deals with the numerical set up and the validation compared with observations. The emphasis is then on the analysis of the main physical structures and process at work in Iroise Sea: first the Ushant tidal front and mixed layer connexion process (surface and bottom) associated, then we analysed the mean eulerian residual circulation, for which we tried to isolate the relative contributions of the different physical process acting on it. Then, we mapped the different dynamical regime at work: we observe cyclostrophic regime near islands and in shallow water area and a strong geostrophic at the tidal front, especially in summer. Nevertheless, in the main part of the study domain, the different dynamical terms are acting of an equivalent. To a thermal point of view, the analysis of heat budget shows that the air-sea flux dominate largely the advective flux, although these ones induce a high frequency modulation in the net heat gain. Then, a diagnostic parameter has been established and the role of different external forcings (air/sea heat fluxes, wind stress, spring/neap tidal cycles) upon the Ushant front variability has been highlighted through sensitivity experiments. Finally, the study of the cross-frontal mass transport, based on a lagrangian particles tracking technique, has been established. The methodology is reliable and the first results reveal that these exports take place through specific latitude "channel".Cette thèse s'insère dans un cadre de modélisation réaliste pour l'étude de la mer d'Iroise. Les deux premières parties concernent la mise en œuvre numérique et la validation par rapport aux observations. L'accent est ensuite mis sur l'analyse des principales structures et processus physiques à l'œuvre en mer l'Iroise : d'abord le front de marée d'Ouessant et les processus associés de connexion de couches de mélange de surface et de fond puis la circulation résiduelle eulérienne moyenne pour laquelle nous avons cherché à quantifier la part relative des différents processus la régissant. Nous avons ensuite cartographié les différents régimes dynamiques présents : on observe ainsi des régimes cyclostrophiques autour des îles et en zone peu profonde et un régime géostrophique apparaît nettement au niveau du front, en particulier en été. Néanmoins, dans la plus grande partie du domaine d'étude, les différents termes dynamiques interviennent de manière équivalente. Du point de vue thermique, l'analyse du bilan de chaleur indique que les flux air-mer dominent nettement les flux advectifs bien qu'ils induisent une modulation haute fréquence marquée du gain net de chaleur. Ensuite, un paramètre diagnostic du front a ensuite été établi et le rôle de différents forçages externes (flux de chaleur air/mer, tension de vent, cycle de marée mortes-eaux/vives-eaux) sur la variabilité du front d'Ouessant est mis en relief grâce à des expériences de sensibilité. Enfin, l'étude des échanges de masse cross-frontaux, basée sur une technique de suivi de particules lagrangiennes, a été mise en place. La méthodologie apparaît robuste et les premiers résultats indiquent que ces exports s'effectuent selon des 'canaux' privilégiés de latitude

Étude numérique de la mer d'Iroise (dynamique, variabilité du front d'Ouessant et évaluation des échanges cross-frontaux)

Cette thèse s insère dans un cadre de modélisation réaliste pour l étude de la mer d Iroise. Les deux premières parties concernent la mise en oeuvre numérique et la validation par rapport aux observations. L accent est ensuite mis sur l analyse des principales structures et processus physiques à l oeuvre en mer d Iroise d abord le front de marée d Ouessant et les processus de stratification/déstratification associés, puis la circulation résiduelle eulérienne moyenne afin d isoler et d évaluer ses différentes composantes. La cartographie des différents régimes dynamiques présents a permis l observation de régimes cyclostrophiques autour des îles et en zone peu profonde et fait clairement apparaître un régime géostrophique au niveau du front, en particulier en été. Néanmoins, dans la plus grande partie du domaine d étude, les différents termes dynamiques interviennent de manière équivalente. Du point de vue thermique, l analyse du bilan de chaleur indique que les flux air-mer dominent nettement les flux advectifs bien qu ils induisent une modulation haute fréquence marquée du gain net de chaleur. Ensuite, un paramètre diagnostic du front a ensuite été établi et le rôle de différents forçages externes (flux de chaleur air/mer, tension de vent, cycle de marée mortes-eaux/vives-eaux) sur la variabilité du front d Ouessant est mis en relief grâce à des expériences de sensibilité. Enfin, l étude des échanges de masse cross-frontaux, basée sur une technique de suivi de particules lagrangiennes, a été mise en place. La méthodologie apparaît robuste été les premiers résultats indiquent que ces exports s effectuent selon des canaux privilégiés de latitude.This thesis work falls within a realistic ocean modelling framework focused on the study of the Iroise Sea. The first two parts deal with the numerical implementation and validation through comparison with observations. Then, emphasis is put on the analysis of the main physical structures and processes at work in the Iroise Sea At first, the ushant tidal front along with the associated stratification/destratification processes, then the mean eulerian residual circulation in order to isolate and assess its different components. Then, the mapping of the different dynamical regimes at work allowed us to observe cyclostrophic regimes around the islands and in shallow water area and made clearly appear a strong geostrophic regime at the tidal front, especially in summer. Nevertheless, in the main part of the domain under study, the contributions by the different dynamical terms are quite alike. From a thermal point of view, the analysis of heat budget showed a high domination of the advective fluxes by the air-sea ones despite the induction by the latter of a marked high-frequency modulation of the net heat gain. Then, a diagnostic parameter was established, and the role of different external forcings (air/sea heat fluxes, wind stress, springjneap tidal cycles) upon the ushant front variability was highlighted through sensitivity experiments. Finally, a study of the cross-frontal mass transport by a lagrangian particles tracking technique was launched. The methodology proved to be reliable, and according to the first results, these exports are developing through specific latitude channels .BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Two-way nesting in split-explicit ocean models : algorithms, implementation and validation

A full two-way nesting approach for split-explicit, free surface ocean models is presented. It is novel in three main respects: the treatment of grid refinement at the fast mode (barotropic) level; the use of scale selective update schemes; the conservation of both volume and tracer contents via refluxing. An idealized application to vortex propagation on a beta plane shows agreement between nested and high resolution solutions. A realistic application to the California Current System then confirm these results in a complex configuration. The selected algorithm is now part of ROMS_AGRIF. It is fully consistent with ROMS parallel capabilities on both shared and distributed memory architectures. The nesting implementation authorizes several nesting levels and several grids at any particular level. This operational capability, combined with the inner qualities of our two-way nesting algorithm and generally high-order accuracy of ROMS numerics, allow for realistic simulation of coastal and ocean dynamics at multiple, interacting scales

The influence of the land-sea breeze on coastal upwelling systems: locally forced vs internal wave vertical mixing and implications for thermal fronts

International audienceLand-sea breeze forcing near a land boundary drives both a locally forced response and an associated offshore propagating internal wave response, the effects of which can be difficult to separate. These processes enhance vertical mixing near the critical latitude for diurnal-inertial resonance (30° N/S), and are a feature of all four major eastern boundary upwelling systems. Here, we employ 1D- and 2D-vertical model configurations forced by a land-sea breeze to quantify the relative contributions of the locally forced and internal wave responses to surface currents and vertical mixing, and test sensitivity to latitude and bottom slope. We further include a sub-inertial alongshore wind to consider the role of the land-sea breeze in the context of upwelling systems. At the critical latitude, the internal waves generated via thermocline pumping near the land boundary are evanescent (in agreement with theory) and largely absent ∼50 km offshore. The internal waves are shown to contribute to vertical mixing, which can be ∼20% greater than that due to the forced response alone, further deepening the surface Ekman boundary layer. This deepening reduces the sub-inertial offshore advection of surface waters, thereby retaining the upwelling front closer to the land boundary and driving a net warming of the nearshore surface waters. Cross-shore horizontal oscillations of the upwelling front generated by the land-sea breeze drive strong diurnal variability in sea surface temperature, in agreement with observations from a cross-shore mooring array in the southern Benguela (∼32.3° S)

Lagrangian pathways in the southern Benguela upwelling system

The effect of ocean currents on fish eggs and larvae during their journey from spawning to nursery grounds in the Southern Benguela upwelling system is poorly understood. The survival and successful transport of fish eggs and larvae results from complex biological and physical processes. This study focuses on the advective processes, more specifically on how the dynamics and characteristics of the ocean currents shape the Lagrangian pathways in the Southern Benguela. A mesoscale eddy resolving interannual (1989–2010) simulation of the region, with a horizontal resolution of 7.5 km, is used to study processes impacting the connectivity between the western edge of the Agulhas Bank and the west coast upwelling region. A set of Lagrangian experiments are conducted with particles being released within the top 100 m of the water column along an across-shore transect off Cape Point (34∘ S). Transport success is given by the ratio of the number of particles that reach St Helena Bay (32∘ S) over the total number of particles released. The model results show a strong seasonal cycle in transport success which is governed by the complex three-dimensional structure of the along-shore jets, their variability, together with the wind-induced Ekman drift. Transport success is most efficient in spring when the Benguela Jet consists of one coherent intensified single-core branch that flows over the 300 m isobath, and when wind-induced Ekman transport favours the retention of particles within the jet. At this time of the year, the pathway leading to successful transport is located inshore, with 90% of particles released inshore the 300 m isobath being successfully transported to St Helena Bay in <15 days. This pathway is also characterized by low eddy kinetic energy values. During the upwelling season (December–March), transport success becomes less efficient, and less sensitive to the initial across-shore position of the particles. The inshore route no longer dominates, as the majority of particles follow offshore pathways. The Benguela Jet shifts offshore and splits into several branches due to the shoaling of the poleward undercurrent. The entrainment of particles within the offshore branch of the jet is favored by the dominating offshore wind-induced Ekman transport. Particles trapped in the offshore branch get exposed to higher mesoscale variability. Their northward progression is slower, which leads to journeys generally exceeding 20 days. This study shows that successful transport from the Agulhas Bank to the west coast upwelling region cannot be attributed to only a simple wind induced modulation of the jet. It explores how the seasonal modulation of the Benguela Jet, poleward undercurrent and offshore Ekman transport combine together with the turbulent off-shelf eddy field to set-up the characteristics of transport success

Assessing the impact of downscaled winds on a regional ocean model simulation of the Humboldt system

International audienceSimulating the oceanic circulation in Eastern Boundary Upwelling Systems (EBUS) is a challenging issue due to the paucity of wind stress products of a sufficiently high spatial resolution to simulate the observed upwelling dynamics. In this study, we present the results of regional simulations of the Humboldt current system (Peru and Chile coasts) to assess the value of a statistical downscaling model of surface forcing. Twin experiments that differ only from the momentum flux forcing are carried out over the 1992-2000 period that encompasses the major 1997/98 El Niño/La Niña event. It is shown that the mean biases of the oceanic circulation can be drastically reduced simply substituting the mean wind field of NCEP reanalysis by a higher resolution mean product (QuikSCAT). The statistical downscaling model improves further the simulations allowing more realistic intraseasonal and interannual coastal undercurrent variability, which is notoriously strong off Central Peru and Central Chile. Despite some limitations, our results suggest that the statistical approach may be useful to regional oceanic studies of present and future climates

The land-sea breeze influences the oceanography of the southern Benguela upwelling system at multiple time-scales

The physical and biogeochemical functioning of eastern boundary upwelling systems is generally understood within the context of the upwelling - relaxation cycle, driven by sub-diurnal wind variability (i.e. with a time-scale of greater than a day). Here, we employ a realistically configured and validated 3D model of the southern Benguela upwelling system to quantify the impact of super-diurnal winds associated with the land-sea breeze (LSB). The ocean response to the LSB is found to be particularly enhanced within St Helena Bay (SHB), a hotspot for productivity which is also prone to Harmful Algal Bloom (HAB) development. We attribute the enhanced response to a combination of near-critical latitude for diurnal-inertial resonance (~32.5°S), the local enhancement of the LSB, and the local development of a shallow stratified surface layer through bay retention. Pronounced advection of the surface layer by diurnal-inertial oscillations contributes to large differences in day- and night-time sea surface temperatures (SST’s) (more than 2°C on average in SHB). Event-scale diapycnal mixing is particularly enhanced within SHB, as highlighted by a numerical experiment initialised with a subsurface passive tracer. These super-diurnal processes are shown to influence sub-diurnal dynamics within SHB through their modulation of the vertical water column structure. A deeper thermocline retains the upwelling front closer to land during active upwelling, while geostrophically-driven alongshore flow is impacted through the modulation of cross-shore pressure gradients. The results suggest that the LSB is likely to play an important role in the productivity and therefore HAB development within SHB, and highlight potential challenges for observational systems and models aiming to improve our understanding of the physical and biological functioning of the system

Inter‐annual variability of the along‐shore Lagrangian transport success in the southern Benguela Current upwelling system

A 3-km resolution regional ocean model is used to investigate the role of wind-driven coastal circulation and mesoscale variability on the inter-annual variability of transport success in the southern Benguela between Cape Point (34°S) and St Helena Bay (32°S) from 1992 to 2011. Lagrangian particles are released within the top 100 m of the water column along an across-shore transect off Cape Point. Transport success is given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released. The analysis of transport success anomalies and their relationship with the local circulation and wind forcing reveal that there is no single driver of the inter-annual variability. The transport success variability of particles released on the shelf (depths 500 m), across-shore transports induced by mesoscale eddies are the main contributors to transport success variability. Rare passage of Agulhas rings near the shelf edge can induce strong offshore advection of particles into the open ocean. In contrast, shelf-edge cyclonic eddies favour the onshore transport of particles originating from the outer shelf-edge and thus contribute to increase transport success. Plain Language Summary This study investigates the inter-annual variation of Lagrangian transport in the southern Benguela Current upwelling system using a high-resolution regional ocean model and particle tracking experiments. Transport of fish eggs and larvae by upper ocean currents is crucial for the marine ecosystem in this highly productive region since the spawning and nursery areas used by anchovies are separated by large distances ( ∼400 km). The alongshore connectivity between the Cape Peninsula and St Helena Bay from 1992 to 2011 is analysed and linked to the regional ocean circulation and wind-forcing on an inter-annual time scale. We find that transport success is influenced by several drivers including the Benguela Jet, Ekman transport, the coastal inner-shelf poleward counter-current, and occasional interactions with eddies such as Agulhas rings and shelf-edge cyclonic eddies. These findings provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs in the southern Benguela, underlining that no single driver can account solely for extreme positive or negative events in transport success

Enhanced Vertical Mixing in Coastal Upwelling Systems Driven by Diurnal‐Inertial Resonance: Numerical Experiments

International audienceThe land-sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30 degrees N/S. 1-D vertical numerical experiments were undertaken to study the key drivers of enhanced diapycnal mixing in coastal upwelling systems driven by diurnal-inertial resonance near the critical latitude. The effect of the land boundary was implicitly included in the model through the "Craig approximation" for first-order cross-shore surface elevation gradient response. The model indicates that for shallow water depths (<similar to 100 m), bottom shear stresses must be accounted for in the formulation of the "Craig approximation," as they serve to enhance the cross-shore surface elevation gradient response, while reducing shear and mixing at the thermocline. The model was able to predict the observed temperature and current features during an upwelling/mixing event in 60 m water depth in St Helena Bay (similar to 32.5 degrees S, southern Benguela), indicating that the locally forced response to the land-sea breeze is a key driver of diapycnal mixing over the event. Alignment of the subinertial Ekman transport with the surface inertial oscillation produces shear spikes at the diurnal-inertial frequency; however their impact on mixing is secondary when compared with the diurnal-inertial resonance phenomenon. The amplitude of the diurnal anticyclonic rotary component of the wind stress represents a good diagnostic for the prediction of diapycnal mixing due to diurnal-inertial resonance. The local enhancement of this quantity over St Helena Bay provides strong evidence for the importance of the land-sea breeze in contributing to primary production in this region through nutrient enrichment of the surface layer