Patrick Marchesiello - Academia.edu (original) (raw)

Papers by Patrick Marchesiello

Research Square (Research Square), Apr 2, 2024

Coastal risk assessment requires accurate coastal bathymetry, which is largely lacking in many re... more Coastal risk assessment requires accurate coastal bathymetry, which is largely lacking in many regions of the world's coastal oceans. This is particularly true in Guinea-Bissau, where the tidal range is the highest of any country in West Africa, affecting several socioeconomic sectors such as agriculture and urban development. Here we present a selection of coastal bathymetry datasets available for generally poorly sampled coastal regions. These include the global GEBCO product, digital nautical charts (CMAP), and depth retrieval from satellite wave detection (S2Shores). While GEBCO provides the right regional envelop of bathymetry, the coastal zone is plagued with random bumpiness at scales below 10 km. S2Shores can be used to correct these errors and instead reveal the presence of small scale channels, reminiscent of very high-resolution satellite color images. These structures are also confirmed by nautical charts, which show the channels to be deeper than that indicated by the satellite retrieval. In simulations with the hydro-sedimentary coastal ocean model CROCO, the tidal energy flux is better channeled towards the coast in the CMAP and S2shores cases, while GEBCO produces larger form drag and sediment bed response, acting as a bathymetric correction to minimize friction. This result highlights the potential of combining different approaches to estimate up-to-date coastal bathymetry in the most challenging areas.

The nearshore zone, including the surf zone and the inner shelf (up to about 20 m depth), is a pa... more The nearshore zone, including the surf zone and the inner shelf (up to about 20 m depth), is a particularly chaotic zone where processes with very different scales coexist and interact permanently. This very active region, which marks the transition from the continent to the sea, is of crucial importance in many aspects (beach erosion/accretion, dispersion of pollutants or larvae, users’ safety, etc.). Several studies have already looked at the transport of passive tracers in the nearshore zone, but they generally use depth-averaged models. As a result, little is known about the non-hydrostatic 3D processes governing the nearshore zone, while depth-averaged models struggle to accurately represent measured data. For example, they tend to underestimate transport in the surf zone and overestimate it on the inner shelf. Recently, 3D wave-resolving models with a free surface have been made available to researchers. One of these models, CROCO (Coastal and Regional Ocean Community model) is used here and allows the simulation of nearshore processes with a reduced number of unknown parameters. The model is applied to a large-scale dye release experiment in California (Imperial Beach, 2009), for which simulations with the depth-averaged funwaveC model have already been performed. The IB09 survey is ideal to study processes as the beach is almost alongshore uniform. Several diagnostics are performed, such as tracer mass balance or quantification of the exchange between the surf zone and the inner shelf. The diagnostics highlight the vertical shear and 3D instabilities at work in the nearshore zone and infer the ability of 3D wave-resolving models to adequately reproduce the available observations, particularly with respect to surf-shelf exchange. This work may in the future suggest feedback for better parameterization in coarser, simpler models for a more accurate depiction of coastal pollution fate and sediment transport.

The Gulf Stream (GS) is one of the strongest ocean currents on the planet. Eddy-rich resolution m... more The Gulf Stream (GS) is one of the strongest ocean currents on the planet. Eddy-rich resolution models are needed to properly represent the dynamics of the GS; however, kinetic energy (KE) can be in excess in these models if not dissipated efficiently. The question of how and how much energy is dissipated and in particular how it flows through ocean scales thus remains an important and largely unanswered question. Using a high-resolution (∼2 km) ocean model [Coastal and Regional Ocean Community (CROCO)], we characterize the spatial and temporal distribution of turbulent cascades in the GS based on a coarse-grained method. We show that the balanced flow is associated with an inverse cascade while the forward cascade is explained by ageostrophic advection associated with frontogenesis. Downscale fluxes are dominant at scales smaller than about 20 km near the surface and most intense at the GS North Wall. There is also strong seasonal variability in KE flux, with the forward cascade intensifying in winter and the inverse cascade later in spring. The forward cascade, which represents an interior route to dissipation, is compared with both numerical and boundary dissipation processes. The contribution of interior dissipation is an order of magnitude smaller than that of the other energy sinks. We thus evaluate the sensitivity of horizontal momentum advection schemes on energy dissipation and show that the decrease in numerical dissipation in a high-order scheme leads to an increase in dissipation at the boundaries, not in the downscale flux.

The wind-driven circulation in the Senegalese upwelling system is investigated using ROMS. The mo... more The wind-driven circulation in the Senegalese upwelling system is investigated using ROMS. The model is initialized and forced at its lateral boundaries with the ECCO model decadal outputs (horizontal currents, temperature, salinity, sea surface height), at the surface with QuikSCAf daily winds over 2000-2004 and WRF wind for one experiment. The model is forced with climatological heat and salinity fluxes from COADS. The Cap Vert peninsula subdivides the Senegalese ecosystem into two distinct regions. When there is upwelling, the structure of the surface water on each side of the Cape differs. To its north the structure of the upwelling is classical with low SST at the 'coast and increases offshore whereas at the south the upwelling core is in the middle of the continental shelf with lowest SST, increasing in both offshore and onshore. The ROMS model outputs are in good agreement with MODIS sea surface temperature observations and reproduce the seasonal cycle rather well. Using ...

The South Atlantic, 1996

The South Atlantic ocean is widely open to the Indian Ocean and the North Atlantic Ocean, and has... more The South Atlantic ocean is widely open to the Indian Ocean and the North Atlantic Ocean, and has a large inflow from the Pacific Ocean through the Drake passage. Strategies of modelling the ocean circulation in this area require to consider inter-ocean exchanges. The paper discusses various numerical approaches of the problem. One consists in modelling the world ocean and to study the South Atlantic as a sub-domain. Exchanges between the various oceans are thus determined by the model, and the circulation obtained for the South Atlantic depends upon the overall model behaviour. It is a consistent way to diagnose the circulation of the water masses in the ocean with coarse resolution models. This approach is also possible at eddy resolving resolution since the global ocean modelling effort undertaken by Semtner and Chervin (1988, 1992), and the simulation of the southern ocean circulation realised by the Fine Resolution Antarctic Model experiment (FRAM, Webb et al. 1991). However, there still are large differences between the various hydrographie or model estimates of the fluxes at the limits of the South Atlantic.

Visites, et invitations de chercheurs 9. Diffusion des résultats 9.1. Animation de la Communauté ... more Visites, et invitations de chercheurs 9. Diffusion des résultats 9.1. Animation de la Communauté scientifique 9.2. Enseignement 9.3. Participation à des colloques, séminaires, invitations 10. Bibliographie 1. Composition de l'équipe Le projet IDOPT est un projet commun au CNRS (département SPM), à l'université Joseph Fourier (Grenoble 1), à l'INPG et à l'INRIA Rhône-Alpes. Ce projet est localisé au laboratoire LMC de l'IMAG. Note : Certains chercheurs ou enseignants-chercheurs (Anestis Antoniadis, Isabelle Charpentier, Patrick Witomski, Dinh Tuan Pham) n'effectuent qu'une partie de leur recherche au sein du projet IDOPT.

Le transport en zone littorale, qui comprend la zone de déferlement et le plateau interne, est en... more Le transport en zone littorale, qui comprend la zone de déferlement et le plateau interne, est encore relativement mal compris. Pourtant, le devenir des traceurs (sédiments, polluants, larves, etc.) est d'une grande importance pour l'écosystème côtier et sa vulnérabilité. Plusieurs études se sont déjà intéressées au transport de traceurs passifs en zone littorale en utilisant des modèles à profondeur moyennée et à vagues résolues pour reproduire les observations. Des diagnostics pertinents ont été proposés, tels que le bilan de masse des traceurs ou la quantification des échanges entre la zone de déferlement et le plateau interne. Cependant, les modèles à profondeur moyennée ont tendance à sousestimer le transport dans la zone de déferlement et à le surestimer sur le plateau interne, ce qui rend difficile la représentation précise des données mesurées. L'objectif principal de cet article est donc de revisiter ces études en utilisant un modèle 3D non-hydrostatique à vagues résolues (CROCO). Les données utilisées pour la comparaison sont issues de l'expérience IB09 où un colorant a été libéré en zone de déferlement et hautement mesuré. Ces données ont déjà été comparées à un modèle à vagues résolues moyenné en profondeur par HALLY-ROSENDAHL & FEDDERSEN (2016). Les diagnostics de ces simulations chercheront à mettre en évidence les instabilités 3D à l'oeuvre dans la zone littorale et à déduire la capacité d'un modèle 3D à vagues résolues à améliorer la représentation des observations disponibles.

Ocean Modelling, Aug 1, 2023

Ocean Modelling, Dec 1, 2021

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.

Ocean Modelling, Nov 1, 2022

The advantage of a smooth representation of bathymetry in terrain-following σ-coordinate ocean mo... more The advantage of a smooth representation of bathymetry in terrain-following σ-coordinate ocean models is compromised by the need to avoid numerical errors on steep slopes associated with pressure gradient discretization or spurious diapycnal diffusion. Geopotential z-coordinate models avoid these errors, but greatly underrepresent the interaction of flow with a topographic slope, especially when the bathymetry is underresolved. Hybrid coordinate models are also deficient because it is difficult to find a satisfactory compromise between z and σ coordinates. With volume penalization, we do not seek a compromise, but rather a correction to the usual coordinate systems that realistically recovers continuous and steep bathymetry. The Brinkman volume penalization method studied here is a modified version of the one introduced in Debreu et al. (2020) that simplifies the numerical implementation of the penalization, increases robustness and improves its computational performance for realistic long-term simulations, while preserving accuracy. We apply this penalization method to the Gulf Stream separation problem that has puzzled modelers for decades. The method improves the representation of the flow-topography interaction and achieves realistic separation of the Gulf Stream at resolutions as coarse as 1/8 •. In addition, it provides a tool to separate the effect of eddy activity and topographic slope when changing grid resolution. This has never before been possible because at coarse resolution none of the usual coordinate systems can properly represent a steep continental slope. Our results show that realistic bathymetry is more important than eddy activity in ensuring realistic Gulf Stream separation, even though many recent studies tend to focus on the eddy activity. A steep slope can exert a stabilizing influence that promotes a strong mean slope current with strong inertia that helps it separate from the coast at the topographic curvature of Cape Hatteras. We anticipate that a successful topographic slope correction will be very valuable to climate models, as their current resolution is far from sufficient to represent western boundary currents (WBCs) using traditional coordinate systems. Our results suggest that a climate model with a 1/4 • resolution using volume penalization-and perhaps also some 1 parameterization of the eddy-mean flow interaction to energize the WBCs-would represent ocean circulation much more realistically.

Ocean Modelling, 2020

Accurate and stable implementation of bathymetry boundary conditions remains a challenging proble... more Accurate and stable implementation of bathymetry boundary conditions remains a challenging problem. The dynamics of ocean flow often depend sensitively on satisfying bathymetry boundary conditions and correctly representing their complex geometry. Generalized (e.g. σ) terrain-following coordinates are often used in ocean models, but they require smoothing the bathymetry to reduce pressure gradient errors (Mellor et al., 1994). Geopotential z-coordinates are a common alternative that avoid pressure gradient and numerical diapycnal diffusion errors, but they generate spurious flow due to their "staircase" geometry. We introduce a new Brinkman volume penalization to approximate the no-slip boundary condition and complex geometry of bathymetry in ocean models. This approach corrects the staircase effect of z-coordinates, does not introduce any new stability constraints on the geometry of the bathymetry and is easy to implement in an existing ocean model. The porosity parameter allows modelling subgrid scale details of the geometry. We illustrate the penalization and confirm its accuracy by applying it to

Journal of Marine Science and Engineering, Jul 19, 2019

Although coastal morphology results essentially from underwater sediment transports, the evolutio... more Although coastal morphology results essentially from underwater sediment transports, the evolution of underwater beach profiles along the diverse coastlines of the world is still poorly documented. Bathymetry inversion from shore-based video cameras set forth a more systematic evaluation and is becoming more commonly used. However, there are limitations to this profiling method that are insufficiently assessed, undermining confidence in operational applications. In this paper, we investigate the daily evolution of a low tide terrace (LTT) in Nha Trang beach, Vietnam, under strong seasonal forcing: from weak wind waves during summer monsoon to moderate waves during winter monsoon. A new error estimation for depth inversion is presented based on tidal evaluation. The method compares video-based estimate and direct measurement of tidal amplitudes to provide a quality criterion. It reveals three types of errors, the main one being a deep water error associated with physical limits-loss of celerity-bathymetry relationship in deep water. This error is dependent on wave period and thus has a strong seasonal pattern in Vietnam. It is generally detrimental to depth inversion where wind waves are dominant (in summer here). On the contrary, the second error type is larger for larger waves and is located at breakpoint, altering wave detection. The last error type is due to nonlinear effects and wave setup in shallow water. After removing the faulty data, we finally present the first reliable three-year time-series of a beach profile in Nha Trang, Vietnam. A main result is the overall stability demonstrated for the LTT beach, with rapid exchange of sediment between the terrace and the upper beach during typhoons, monsoon events or seasonal cycles. These tropical environments may provide faster beach recovery compared with mid-latitude configurations.

Geophysical Research Letters, Mar 14, 2019

Western boundary currents (WBCs) are critical to Earth's climate. In the last decade, mesoscale a... more Western boundary currents (WBCs) are critical to Earth's climate. In the last decade, mesoscale air-sea interactions emerged as an important factor of WBC dynamics. Recently, coupled models including the feedback of surface oceanic currents to the atmosphere confirmed the existence of a physical process called eddy killing, which may correct long-lasting biases in the representation of WBCs by providing an unambiguous energy sink mechanism. Using ocean-atmosphere coupled simulations of the Gulf Stream and the Agulhas Current, we show that eddy killing reduces the eddy-mean flow interaction (both forward and inverse cascades) and leads to more realistic solutions. Model and data fluxes are in good agreement when the same coarse grid is used for their computation, although in this case they are underestimated. We conclude that the uncoupled approach is no longer suitable for continued ocean model improvement and discuss new formulations that should better account for air-sea interactions. Plain Language Summary Western boundary currents (WBCs), such as the Gulf Stream and the Agulhas Current play a crucial role in global ocean circulation and in determining and stabilizing the Earth's climate. In the last decade, mesoscale air-sea interactions emerged as important in WBC dynamics. Recently, coupled models including the feedback of surface oceanic currents to the atmosphere revealed a process called eddy killing, which potentially corrects long-lasting biases in the representation of WBCs. In this study, using ocean-atmosphere coupled simulations of the Gulf Stream and Agulhas Current, we show that eddy killing reduces the interactions between eddies and mean flow. The influence of the eddies on the mean flow can be measured by the cascade of energy, and, in particular, the inverse cascade of energy. The reduction of inverse energy flux by eddy killing leads to realistic solutions and, in particular, to the observed stabilization of WBCs. Model and data fluxes are in good agreement when the same coarse grid is used for their computation, although in this case they are underestimated. We conclude that uncoupled models are no longer suitable for continuing our model improvement of ocean dynamics and discuss new formulations that should better account for air-sea interactions.

Journal of Physical Oceanography, Nov 1, 2012

The present study investigates the integrated ocean response to tropical cyclones (TCs) in the So... more The present study investigates the integrated ocean response to tropical cyclones (TCs) in the South Pacific convergence zone through a complete ocean heat budget. The TC impact analysis is based on the comparison between two long-term (1979-2003) oceanic simulations forced by a mesoscale atmospheric model solution in which extreme winds associated with cyclones are either maintained or filtered. The simulations provide a statistically robust experiment that fills a gap in the current modeling literature between coarse-resolution and short-term studies. The authors' results show a significant thermal response of the ocean to at least 500-m depth, driven by competing mixing and upwelling mechanisms. As suggested in previous studies, vertical mixing largely explains surface cooling induced by TCs. However, TC-induced upwelling of deeper waters plays an unexpected role as it partly balances the warming of subsurface waters induced by vertical mixing. Below 100 m, vertical advection results in cooling that persists long after the storm passes and has a signature in the ocean climatology. The heat lost through TC-induced vertical advection is exported outside the cyclogenesis area with strong interannual variability. In addition, 60% of the heat input below the surface during the cyclone season is released back to the oceanic mixed layer through winter entrainment and then to the atmosphere. Therefore, seasonal modulation reduces the mean surface heat flux due to TCs to about 3 3 10 23 PW in this region exposed to 10%-15% of the world's cyclones. The resulting climatological anomaly is a warming of about 0.18C in the subsurface layer and cooling below the thermocline (less than 0.18C).

Journal Of Geophysical Research: Oceans, Feb 24, 2023

Bottom drag and top drag—associated with the ocean current feedback (CFB) to the atmosphere—are t... more Bottom drag and top drag—associated with the ocean current feedback (CFB) to the atmosphere—are the energy dissipation processes in the oceanic boundary layers. Both are instrumental in regulating western boundary currents, such as the Gulf Stream (GS). However, the sensitivity of model results to bottom drag parameterization has not yet been considered in simulations where top drag is present. So far, in the absence of top drag, the bottom drag coefficient turns out to be a very sensitive parameter of ocean models. Here, we re‐examine this sensitivity when both top and bottom drag are considered in eddy‐rich ocean simulations of the North Atlantic basin. The top drag is addressed with a parameterization that considers the wind adjustment to CFB. In agreement with previous studies, we confirm that top drag has a large control on GS dynamics by reducing mesoscale activity. Next, we show that the energy sink due to bottom drag increases when top drag is neglected, but that this increase only partially compensates for the absence of top drag. As a result, the GS representation is too sensitive to bottom drag. Finally, we propose a simple scale‐aware parameterization of the subgrid‐scale topographic effect on bottom drag. Our results suggest that for a realistic representation of GS dynamics and energy pathways, top and bottom drags must be considered together. Future studies should include processes missing from this study, such as submesoscale dynamics and the surface gravity waves at the ocean‐atmosphere interface.