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In this thesis, we show the effects of the surface current on the wave field with a particular interest in the significant wave height (Hs) variability and how Hs spatial gradients are related to the underlying current gradients. With a theoretical and idealized framework, we show that the current-induced refraction leads to a spatial redistribution of the wave action resulting in an inhomogeneity in the Hs field. Then, this inhomogeneity is described in an idealized numerical framework in an isolated mesoscale eddy. The nonlocal effects of the current in the Hs field have been highlighted in this study. Also, the variability of the Hs field is much more important in an eddy field dynamical at the meso- and the submesoscale range than in an eddy with a Gaussian shape. As eddies are not isolated in the ocean, we studied Hs gradients in a realistic framework in the Agulhas current. The sharp Hs gradients monitored by the new filtered altimeter data, are, on average, reproduced by the numerical wave models if forcing currents are sufficiently resolved. It proved the importance of small scale currents in the wave action spatial redistribution. The proportionality between the spatial gradients of the surface Kinetic Energy and Hs has been verified both by the numerical outputs and altimeter data at the mesoscale range. The directional data from SWIM sensors have shown first results motivating for future studies of current effects on the waves., Dans cette thèse, nous présentons les effets des courants sur les vagues avec un intérêt particulier pour la variabilité de leurs hauteurs significatives (Hs) et comment leurs gradients spatiaux sont liés aux gradients spatiaux des courants sous-jacents. Dans un premier temps, nous montrons, dans un cadre théorique et simplifié que la réfraction des vagues induite par les gradients de courants se traduit par une redistribution spatiale de l’action des vagues, conduisant à une inhomogenéité dans le champ des Hs. Ensuite, cette inhomogéneité est étudiée dans un cadre numérique idéalisé dans un tourbillon océanique isolé de mésoéchelle. Cette étude permet de souligner l’effet non local des courants sur les Hs. Aussi, dans un tourbillon dynamique à la fois à la méso- et à la sousmésoechelle, la variabilité du champ des Hs est bien plus importante que pour un tourbillon gaussien.Comme les tourbillons ne sont pas isolés en réalité, nous étudions les gradients spatiaux des Hs dans un cadre réaliste dans le courant des Aiguilles. Les forts gradients observés par les nouvelles données altimétriques filtrées sont, en moyenne, reproductibles par les modèles de vagues si les courants de forçages sont suffisamment résolus démontrant l’importance des petites échelles dans la redistribution de l’action des vagues par les courants. La proportionnalité entre les gradients d’énergie cinétique de surface et les gradients de Hs est vérifiée à la fois par les données altimétriques et le modèle sur toute la gamme de la mésoéchelle. Les mesures directionnelles du capteur SWIM ont montré de premiers résultats motivant quant à l’observation des effets des courants sur le champ de vagues et de ses paramètres.

This paper examines the potential of an optical flow video-based technique to estimate wave-filtered surface currents in the nearshore where wave-breaking induced foam is present. This approach uses the drifting foam, left after the passage of breaking waves, as a quasi-passive tracer and tracks it to estimate the surface water flow. The optical signature associated with sea-swell waves is first removed from the image sequence to avoid capturing propagating waves instead of the desired foam motion. Waves are removed by applying a temporal Fourier low-pass filter to each pixel of the image. The low-pass filtered images are then fed into an optical flow algorithm to estimate the foam displacement and to produce mean velocity fields (i.e., wave-filtered surface currents). We use one week of consecutive 1-Hz sampled frames collected during daylight hours from a single fixed camera located at La Petite Chambre d’Amour beach (Anglet, SW France) under high-energy conditions with significant wave height ranging from 0.8 to 3.3 m. Optical flow-computed velocities are compared against time-averaged in situ measurements retrieved from one current profiler installed on a submerged reef. The computed circulation patterns are also compared against surf-zone drifter trajectories under different field conditions. Optical flow time-averaged velocities show a good agreement with current profiler measurements: coefficient of determination (r2)= 0.5–0.8; root mean square error (RMSE) = 0.12–0.24 m/s; mean error (bias) =−0.09 to −0.17 m/s; regression slope =1±0.15; coherence2 = 0.4–0.6. Despite an underestimation of offshore-directed velocities under persistent wave breaking across the reef, the optical flow was able to correctly reproduce the mean flow patterns depicted by drifter trajectories. Such patterns include rip-cell circulation, dominant onshore-directed surface flow and energetic longshore current. Our study suggests that open-source optical flow algorithms are a promising technique for coastal imaging applications, particularly under high-energy wave conditions when in situ instrument deployment can be challenging.

Aim For 80 years, popular opinion has held that most of Madagascar's terrestrial vertebrates arrived from Africa by transoceanic dispersal (i.e. rafting or swimming). We reviewed this proposition, focussing on three ad hoc hypotheses proposed to render this unlikely scenario more feasible: (a) Could hibernation have helped mammals to reach Madagascar? (b) Could the aquatic abilities of hippopotamuses have enabled them to swim the Mozambique Channel? (c) How valid is the Ali‐Huber model predicting that eastward Palaeogene surface currents allowed rafts to reach Madagascar in 3–4 weeks? Finally, we explored the alternative hypothesis of geodispersal via short‐lived land bridges between Africa and Madagascar. Location East Africa, Madagascar, Mozambique Channel. Taxa Fish, amphibians, reptiles, birds, mammals. Methods We established colonization timeframes using molecular divergence dates estimated for Malagasy vertebrate lineages. We reviewed the likelihood of the “torpid waif” and “swimming hippopotamus” hypotheses, and re‐investigated Ali and Huber's model of Eocene jet‐like currents by tracking particle trajectories in currents simulated using the Institut Pierre‐Simon Laplace Earth System Model. Finally, we summarized recent geological findings from the Mozambique Channel, and used them to compile palaeosedimentological maps using PLACA4D. Results Madagascar's vertebrate fauna has complex origins. Hibernation is probably an adaptation to Madagascar's hypervariable climate, rather than a facilitator of mammal dispersal. Hippopotamus physiology precludes the ability to cross an oceanic channel deeper than 4 m and hundreds of km wide. The Ali‐Huber model of Palaeogene currents considerably underestimated the time required to cross the Mozambique Channel under simulated palaeogeographic conditions. New geological data indicate the existence of three short‐lived land bridges between Africa and Madagascar at 66–60 Ma, 36–30 Ma and 12–05 Ma. Main conclusion The three Cenozoic land bridges afford a more grounded hypothesis for the dispersal of Madagascar's extant biota than transoceanic rafting or swimming, although vicariance, island hopping and limited rafting also played a role.

The current feedback to the atmosphere (CFB) contributes to the oceanic circulation by damping eddies. In an ocean‐atmosphere coupled model, CFB can be correctly accounted for by using the wind relative to the oceanic current. However, its implementation in a forced oceanic model is less straightforward as CFB also enhances the 10‐m wind. Wind products based on observations have seen real currents that will not necessarily correspond to model currents, whereas meteorological reanalyses often neglect surface currents or use surface currents that, again, will differ from the surface currents of the forced oceanic simulation. In this study, we use a set of quasi‐global oceanic simulations, coupled or not with the atmosphere, to (i) quantify the error associated with the different existing strategies of forcing an oceanic model, (ii) test different parameterizations of the CFB, and (iii) propose the best strategy to account for CFB in forced oceanic simulation. We show that scatterometer wind or stress are not suitable to properly represent the CFB in forced oceanic simulation. We furthermore demonstrate that a parameterization of CFB based on a wind‐predicted coupling coefficient between the surface current and the stress allows us to reproduce the main characteristics of a coupled simulation. Such a parameterization can be used with any forcing set, including future coupled reanalyses, assuming that the associated oceanic surface currents are known. A further assessment of the thermal feedback of the surface wind in response to oceanic surface temperature gradients shows a weak forcing effect on oceanic currents. We appreciate the support from the National Science Foundation (OCE‐1419450), from the California Natural Resources Agency (C0100400), and from the National Oceanic and Atmospheric Agency (NA15NOS4780186). This work was performed using HPC resources from GENCI‐[CINES/IDRIS/TGCC] (Grants 2017‐A0010106895, 2018‐A0030106895, and 2019‐A0050106895) and the Engineering Discovery Environment (XSEDE). T. A. and L. R. would like to thank HPC‐Europa3 program, application HPC17IUTPN and HPC17MM0RX, respectively, and Barcelona Supercomputing Center (BSC) for hosting them. Data can be downloaded online (from http://research.atmos.ucla.edu/lrenault/HowTo/). The authors thank B. Barnier, E. Chassignet, G. Danabasoglu, F. Lemarié, K. Mogensen, J. Small, A.M. Treguier, and H. Tsujino for useful discussions. The authors want to thank three anonymous reviewers and J. Small for their careful and constructive comments.

Place: Oxford Publisher: Pergamon-Elsevier Science Ltd WOS:000582696800013; International audience; In the Tropical Atlantic Ocean, we assessed the accuracy of a Lagrangian model (Ichthyop) forced with velocity fields from a hydrodynamical model (CROCO) and two different remote sensing products (GlobCurrent and OSCAR) using trajectories of oceanographic drifters. Additionally, we evaluated the possibility to expand the drifters data using trajectories of GPS-buoy equipped drifting Fish Aggregating Devices (FADs). The observed and simulated trajectories were compared in terms of spatial distribution, velocity distribution and a nondimensional skill score. For the drifters and FADs, the GlobCurrent and OSCAR products lead to similar performances as the CROCO model-ouputs in the broad studied domain. In the Gulf of Guinea, however, the CROCO model performed significantly better than the other two because the parent solution of CROCO benefited from its communication with a child grid of finer resolution in this region. On average, the simulations lead to an underestimation of the drifter and FAD velocities, likely because the spatial resolutions of the forcing products were insufficient and the time frequency at which they were produced were too low to resolve the relevant oceanic processes properly. We found a low skill for all models to simulate FAD trajectories, possibly because of the devices vertical structure that prevent FADs from drifting like water parcels. Our results therefore suggest that in the Tropical Atlantic the FAD dataset may not be appropriate to use for corroborating Lagrangian simulations.

The first eXpendable BathyThermographs (XBTs) were deployed in the 1960s in the North Atlantic Ocean. In 1967 XBTs were deployed in operational mode to provide a continuous record of temperature profile data along repeated transects, now known as the Global XBT Network. The current network is designed to monitor ocean circulation and boundary current variability, basin-wide and trans-basin ocean heat transport, and global and regional heat content. The ability of the XBT Network to systematically map the upper ocean thermal field in multiple basins with repeated trans-basin sections at eddy-resolving scales remains unmatched today and cannot be reproduced at present by any other observing platform. Some repeated XBT transects have now been continuously occupied for more than 30 years, providing an unprecedented long-term climate record of temperature, and geostrophic velocity profiles that are used to understand variability in ocean heat content (OHC), sea level change, and meridional ocean heat transport. Here, we present key scientific advances in understanding the changing ocean and climate system supported by XBT observations. Improvement in XBT data quality and its impact on computations, particularly of OHC, are presented. Technology development for probes, launchers, and transmission techniques are also discussed. Finally, we offer new perspectives for the future of the Global XBT Network GG, FB, SD, UR, MB, RD, and DV were supported by a grant from the NOAA/Ocean Observing and Monitoring Division (OOMD) and by NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML). The participation of JS and NZ in this study was supported by NOAA's Global Ocean Monitoring and Observing Program through Award NA15OAR4320071 and NSF Award 1542902. CD was funded by the Australian Research Council (FT130101532 and DP160103130); the Scientific Committee on Oceanic Research (SCOR) Working Group 148, funded by national SCOR committees and a grant to SCOR from the U.S. National Science Foundation (Grant OCE-1546580); and the Intergovernmental Oceanographic Commission of UNESCO/International Oceanographic Data and Information Exchange (IOC/IODE) IQuOD Steering Group. LC was supported by 2016YFC1401800 SI

Understanding fishing vessel dynamics at a fine spatial scale is of great interest for defining appropriate spatial management plans. Different models have been developed to detect fishing activity from Vessel Monitoring System (VMS) data. While mathematical and statistical methods differ, all rely on the idea that vessel speed over ground provides information on fishing vessel activity. However, trawling with constant speed relative to the water mass may as well prove a winning strategy for both technical (to ensure sufficient trawl opening) and biological (to limit escapement of fish) reasons. Therefore, considering speed through water instead of speed over ground might provide insights into fishing activities. We developed a method combining surface currents derived from ocean circulation models with VMS data to estimate vessel speed through water. We then used vessel speed through water as input to two previously published segmentation methods to infer fishing activity during a fishing trip. We illustrate the approach by analysing trajectories of trawlers operating in the Eastern English Channel. All vessels were equipped with VMS and part of the Obsmer national discard sampling programme. Overall, results showed that surface currents influenced fishing behaviour, and trawling preferentially occured parallel to surface currents. Speed over ground associated with trawling behaviour was much more variable than speed through water, suggesting that trawling occured at constant engine regimes. However, for both segmentation methods using speed through water instead of speed over ground did not improve our capacity to identify trawling sequences. In both cases the amount of time spent trawling during a trip was overestimated.

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The meanders of a baroclinic coastal current in the Northwestern Mediterranean Sea have already been reported in the literature. These meanders can be surrounded by vortices. Such vortices have been observed in the western part of the Gulf of Lions but the location and the mechanism of their formation are poorly documented. In this paper, we use the current measurements of a one-year experiment, which was conducted in the eastern part of the Gulf of Lions to detect and characterize the vortex activity. A vortex detection algorithm based on few velocity data was developed. Current measurements were available at the sea surface (HF radars) and in the water column from 50 to 140 m depth (four current meter moorings). SST images and hydrologic data were also used. Results focus on observations that are coherent 50 m and at the surface. Vortices are anticyclonic, of submesoscale size and present maximal velocities of 30-50 cm/s. The drift speed of the vortices is comparable to but less than the velocity of the Northern Current. These observations enable to estimate the minimum vortex occurrence in this area. The presence of vortex structures is strongly correlated with a specific sequence of wind patterns. (C) 2010 Elsevier Ltd. All rights reserved.

International audience; A decomposition in sub-arrays is used to reduce the computation time in the modelling of a finite phased array of microstrip antennas. The surface currents are considered of equal amplitude on the patches which have the same neighbourhood configuration. This simplifies the resolution of the integral equation by reducing the number of unknown coefficients. This method can be applied in the case of large phased arrays or arrays with parasitic elements. In the case of a four-element linear array the theory agrees well with the experiment and this method is less time-consuming than the "element by element" approach.< >