Evaluation objective de réseaux d'observation en domaine côtier par la modélisation d'ensemble

L'océan côtier est le siège de nombreux processus physiques d'échelles variées ; les attentes du public y sont nombreuses. Il convient d'améliorer notre connaissance de cette zone complexe, par la modélisation et la collecte de données. Les modèles numériques sont utiles à la conception de réseaux d'observation, car ils constituent une représentation de la réalité riche en informations sur la zone. Les méthodes d'ensemble permettent de représenter les statistiques d'erreurs du modèle, qu'un réseau performant sera capable de détecter ou de contraindre. Dans un premier temps, nous assimilons à l'aide d'un filtre de Kalman d'ensemble des données simulant les mesures d'un altimètre à fauchée dans un modèle barotrope en Mer du Nord. La mesure de la pente de surface de la mer permet de mieux représenter les courants et étend le domaine d'influence des observations. Le roulis du satellite induit une corrélation dans les erreurs d'observation le long de la trace, qui s'il est pris en compte dans le schéma d'assimilation ne remet pas en cause l'apport de l'instrument. Dans un second temps, nous développons une technique d'analyse de performances de réseaux d'observation, qui permet de définir un critère quantitatif de discrimination de divers réseaux ainsi qu'une analyse qualitative permettant de comprendre l'impact du réseau considéré sur toutes les dimensions du modèle. A l'aide d'un modèle 3D du Golfe de Gascogne, cette technique nous permet de démontrer les capacités d'un altimètre à fauchée à observer la dynamique méso échelle de l'océan côtier, ainsi que l'apport de mesures de courant dans la conception d'une campagne de mesures, et enfin l'apport de données marégraphiques.The coastal ocean is characterised by a rich spectrum of interacting scales and processes, and is subject to heavy societal pressure and demand. Our understanding of this complex area needs to be improved, through modelling and data gathering. Realistic numerical models are potentially useful to design observational arrays. Ensemble methods provide a proxy for model errors, which may be detected or constrained by an efficient array. In a first study, an ensemble Kalman filter is used to assimilate simulated data of a wide swath altimeter in a barotropic model of the North Sea. Measurements of the cross-track slope of the sea surface lead to a better representation of the currents, and to a wider domain of influence of observations. The roll of the satellite induces along-track correlation of the observation errors; when it is taken into account in the data assimilation process, it does not jeopardize the improvement potentially brought by the instrument. In a second study, a new analysis technique for assessing the performance of observational networks is proposed. It implements a quantitative criterion to discriminate between various networks, and a qualitative one which allows understanding the impact of the considered network on all the model dimensions. Using a 3D model of the Bay if Biscay, this technique illustrates (1) the capacity of a wide swath altimeter to retrieve useful coastal ocean mesoscale dynamical signals, (2) the positive contributions of current measurements of currents in the design of a coastal cruise, and (3) the contribution of tide gauge data at detecting errors on the shelf

Influence of Caribbean eddies on the Loop current system evolution

The Loop Current (LC) system dynamics are an essential component of the processes influencing circulation and transport in the Gulf of Mexico (GoM). The LC evolution is influenced by various factors, including the rich eddy field of the region and the flow exchange through the Yucatan Strait with the neighboring Caribbean Sea. These factors contribute to the complexity of the LC and, as a result, to the limitations in the predictability of the system. The focus of this study is to further elucidate the evolution of the LC, by quantifying the influence of coherent eddy fluxes originating in the Caribbean Sea. This is achieved by employing the Lagrangian-Averaged Vorticity Deviation (LAVD) method, an objective metric to evaluate eddy coherence in the Caribbean Sea that allows, for the first time, to quantify at different depths the evolution of coherent Caribbean eddies through the Yucatan Channel towards the GoM. The physical connectivity between the Caribbean Sea and the GoM is addressed using Lagrangian techniques to analyze processes that take place south of the Yucatan Channel and help quantify their strong relationship with the GoM eddy field. Coherent anticyclonic vorticity fluxes, as well as the net coherent anticyclonic volume transport between the Caribbean Sea and the GoM are associated with Loop Current Eddy (LCE) detachments through direct connectivity between the coherent Caribbean anticyclones and the forming LCE. The findings have important implications for understanding and predicting the LC system and the physical connectivity processes between the GoM and the Caribbean Sea

Influence of the Caribbean Sea eddy field on Loop Current predictions

Previous studies have shown how the passage of eddies from the Caribbean Sea (CS) to the Gulf of Mexico (GoM) can impact the Loop Current (LC) system, in particular the detachments of LC Eddies (LCEs). Here we used numerical modeling to investigate the impact of the eddy field in the CS on LC predictions. We used a HYCOM ocean model configuration of the North Atlantic at 1/12° resolution to perform two data-assimilative experiments: one in which all available observations were assimilated (Ref), and one in which all available observations were assimilated except in the CS, where climatological altimetry values were assimilated instead of actual observations, leading to dampening the mesoscale activity there (NoCarib). These experiments took place in 2015, when the LC was very active with several LCE detachments, re-attachments, and separations. Each of these experiments was used to initialize 28 60-day forecast simulations every 10 days. In terms of model Sea Surface Height (SSH), the forecasts initialized with the Ref experiment had, on average, lower errors than the forecasts initialized with the NoCarib experiment in the southeastern part of the GoM, with a peak during the 31-40 day forecast period. More importantly, the errors in predicting the date of the next LCE detachment or separation were smaller in the forecasts initialized from the more realistic Ref experiment. Finally, the forecasts initialized by the NoCarib experiment showed a much higher level of false negatives predictions, meaning that no LCE detachment was predicted whereas a detachment actually happened. Overall, 68% of LCE detachments were predicted with an error smaller than 15 days in the forecasts initialized from the more realistic Ref experiment, but only 32% in the forecasts initialized from the NoCarib experiment, stressing the importance of the CS eddy field for predicting the LC evolution. These findings have implications on the GoM predictability, highlighting the need to either run data-assimilative models covering both the GoM and the CS, or pay particular attention to accurate boundary conditions for limited-area GoM models

Long term simulations of potential oil spills around Cuba

Simulations over eight years of continuous surface oil spills around Cuba are carried out to identify the most likely stranding (beaching) locations. The open source Lagrangian oil drift model OpenOil is applied with high resolution hydrodynamic forcing. The actual fraction of the released oil mass reaching different regions is calculated, revealing small differences between a light and a heavy crude oil type. Similar stranding rates for the two oil types are found. Another important conclusion is that, due to the high temporal variability in stranding rates, short term simulations of a few weeks are not suitable to assess environmental risk. The highest stranding rates are simulated in winter in Northern Cuba. It is also found that oil could reach Northern Cuba, Yucatan or Florida in about 3–5 days after a spill.publishedVersio

Highly variable upper and abyssal overturning cells in the South Atlantic

The Meridional Overturning Circulation (MOC) is a primary mechanism driving oceanic heat redistribution on Earth, thereby affecting Earth’s climate and weather. However, the full-depth structure and variability of the MOC are still poorly understood, particularly in the South Atlantic. This study presents unique multiyear records of the oceanic volume transport of both the upper (~3100 meters) overturning cells based on daily moored measurements in the South Atlantic at 34.5°S. The vertical structure of the time-mean flows is consistent with the limited historical observations. Both the upper and abyssal cells exhibit a high degree of variability relative to the temporal means at time scales, ranging from a few days to a few weeks. Observed variations in the abyssal flow appear to be largely independent of the flow in the overlying upper cell. No meaningful trends are detected in either cell.Fil: Kersalé, Marion. National Ocean And Atmospheric Administration; Estados Unidos. University of Miami; Estados UnidosFil: Meinen, Christopher S.. National Ocean And Atmospheric Administration; Estados UnidosFil: Perez, Renellys C.. National Ocean And Atmospheric Administration; Estados UnidosFil: Le Hénaff, Matthieu. National Ocean And Atmospheric Administration; Estados Unidos. University of Miami; Estados UnidosFil: Valla, Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval. Departamento Oceanografía; ArgentinaFil: Lamont, Tarron. University of Cape Town; SudáfricaFil: Sato, Olga T.. Universidade de Sao Paulo; BrasilFil: Dong, Shenfu. National Ocean And Atmospheric Administration; Estados UnidosFil: Terre, T.. University of Brest; Francia. Centre National de la Recherche Scientifique; FranciaFil: van Caspel, M.. Universidade de Sao Paulo; BrasilFil: Chidichimo, María Paz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval. Departamento Oceanografía; ArgentinaFil: van den Berg, Marcel Alexander. Department of Environmental Affairs; SudáfricaFil: Speich, Sabrina. University Of Cape Town; SudáfricaFil: Piola, Alberto Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ecole Normale Superieure. Laboratoire de Meteorologie Dynamique; Francia. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval. Departamento Oceanografía; Argentina. Instituto Franco-Argentino sobre Estudios del Clima y sus Impactos; Argentina. Universidad de Buenos Aires; ArgentinaFil: Campos, Edmo. Universidade de Sao Paulo; Brasil. American University Of Sharjah.; Emiratos Árabes UnidosFil: Ansorge, Isabelle. University of Cape Town; SudáfricaFil: Volkov, Denis L.. University of Miami; Estados Unidos. National Ocean And Atmospheric Administration; Estados UnidosFil: Lumpkin, Rick. National Ocean And Atmospheric Administration; Estados UnidosFil: Garzoli, S. L.. University of Miami; Estados Unidos. National Ocean And Atmospheric Administration; Estados Unido

Ocean observations in support of studies and forecasts of tropical and extratropical cyclones

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Domingues, R., Kuwano-Yoshida, A., Chardon-Maldonado, P., Todd, R. E., Halliwell, G., Kim, H., Lin, I., Sato, K., Narazaki, T., Shay, L. K., Miles, T., Glenn, S., Zhang, J. A., Jayne, S. R., Centurioni, L., Le Henaff, M., Foltz, G. R., Bringas, F., Ali, M. M., DiMarco, S. F., Hosoda, S., Fukuoka, T., LaCour, B., Mehra, A., Sanabia, E. R., Gyakum, J. R., Dong, J., Knaff, J. A., & Goni, G. Ocean observations in support of studies and forecasts of tropical and extratropical cyclones. Frontiers in Marine Science, 6, (2019): 446, doi:10.3389/fmars.2019.00446.Over the past decade, measurements from the climate-oriented ocean observing system have been key to advancing the understanding of extreme weather events that originate and intensify over the ocean, such as tropical cyclones (TCs) and extratropical bomb cyclones (ECs). In order to foster further advancements to predict and better understand these extreme weather events, a need for a dedicated observing system component specifically to support studies and forecasts of TCs and ECs has been identified, but such a system has not yet been implemented. New technologies, pilot networks, targeted deployments of instruments, and state-of-the art coupled numerical models have enabled advances in research and forecast capabilities and illustrate a potential framework for future development. Here, applications and key results made possible by the different ocean observing efforts in support of studies and forecasts of TCs and ECs, as well as recent advances in observing technologies and strategies are reviewed. Then a vision and specific recommendations for the next decade are discussed.This study was supported by the National Oceanic and Atmospheric Administration and JSPS KAKENHI (Grant Numbers: JP17K19093, JP16K12591, and JP16H01846)

Mississippi waters reaching South Florida reefs under no flood conditions: synthesis of observing and modeling system findings

In August 2014, in situ measurements revealed an intense salinity drop impacting South Florida coral reefs, between Pulley Ridge (Southwest Florida Shelf) and the Florida Keys. The low salinity waters had a surface signal of 32 (down from 35.2) and extended over a 15–20-m deep lens. Satellite observations showed that this abrupt drop in salinity was due to a southeastward export of Mississippi River waters from the Northern Gulf of Mexico (GoM), revealing strong interaction between coastal and oceanic flows. Unlike previous events of marked long-distance Mississippi water export, this episode is not associated with Mississippi flooding conditions, which makes it a unique study case. We have developed a high-resolution (~2 km) comprehensive hydrodynamic numerical model of the GoM to study the conditions that controlled the 2014 Mississippi River water export episode. It is based on the Hybrid Coordinate Ocean Model (HYCOM) and assimilates remotely sensed altimetry and sea surface temperature observations, to ensure that the simulated upper-ocean is realistic. This regional model has a detailed representation of coastal physics (especially river plume dynamics) and employs high-frequency river discharge and atmospheric forcing. The combined use of the simulation and observations reveals a unique pathway that brought Mississippi waters first eastward along the Northern GoM continental shelf, under prevailing winds and the presence of an anticyclonic Loop Current eddy, then southward along the edge of the West Florida Shelf, before reaching the deep GoM. Unlike usually observed, the offshore advection of Mississippi River waters thus took place far from the Delta area, which is another specificity of the 2014 episode. Finally, in the Florida Straits, Mississippi waters were advected from the deep ocean to the continental shelf under the influence of both deep sea (particularly a cyclonic Loop Current frontal eddy) and shelf flows (wind-induced Ekman transport). The simulation, in tandem with data, thus helped analyze processes that are likely to affect the connectivity between reefs in the southern Florida region (Florida Keys, Dry Tortugas, Pulley Ridge) and remote areas (Mississippi Delta), as well as the local connectivity between neighboring reefs

Contribution of a wide-swath altimeter in a shelf seas assimilation system: Impact of the satellite roll errors

12 pages, 8 figures, 1 tableThe authors investigate the potential qualitative improvement brought by wide-swath, interferometry-based ocean altimetry measurements with respect to classical nadir altimeters in a coastal/shelf data assimilation system. In addition, particular attention is paid to roll errors, which could significantly reduce the expected benefits of wide-swath altimetry. A barotropic, nonlinear free-surface model is set up over the European shelf as part of an ensemble Kalman filter. Experiments assimilating simulated data are performed over the North Sea to test the ability of altimeter configurations to reduce model errors due to the action of meteorological forcing in the presence of bathymetric uncertainties. A simplified wide-swath observation scheme is used, composed of nadir altimeter height plus a nadir-centered cross-track sea level slope measurement. The simplified wide-swath measurements are found to be able to constrain events unsampled by a single nadir altimeter owing to a wider domain of influence in the cross-track direction and the ability to detect cross-track gradients.Since the satellite-borne interferometer is highly sensitive to the platform behavior, especially satellite roll, experiments taking roll errors into account are then carried out. Whereas observational errors are considered independent in most data assimilation studies, the roll of the platform correlates those errors along the path of the satellite. Despite the large amplitude of the roll errors, the contribution of the wide-swath altimeter in coastal zones remains valuable as long as the roll frequency is known (within Gaussian error) and the assimilation scheme is designed to take observational error correlations into accountSupport of Matthieu Le Hénaff by Délégation Générale pour l’Armement is acknowledgedPeer reviewe

Characterizing the Navidad current interannual variability using coastal altimetry

In this study, the Navidad current, which flows along the northern coast of Spain in winter, is observed and characterized using coastal altimetry data over the period 1992–2002. This coastal current, marked by a strong interannual variability, is associated with eastward transport of warm waters along the shelf slope. Specific data editing and processing strategies have been applied to the along-track altimeter data, which allows us to retrieve altimetric sea level anomalies closer to the coast, with a better spatial coverage and improved quality when compared with standard altimetric products. The current variability observed upstream by in situ time series after November 1996 is well reproduced by the satellite across-track surface geostrophic current anomalies up until September 1999; this agreement degrades later in time. The combined use of satellite-derived current anomalies and sea-surface temperature anomalies allows us to develop indices of Navidad occurrences, in the first long-term, systematic survey of that current based on a multi-sensor approach. The satellite analyses confirm the previously identified Navidad occurrences in winter of 1995–1996, 1997–1998, and 2000–2001. Furthermore, a weak Navidad event was identified in winter 1996–1997. These four winters are associated with a negative North Atlantic Oscillation index in the previous fall, but the intensity of the Navidad is not correlated to the amplitude of that index.Peer reviewe

Assessment of observational networks with the Representer Matrix Spectra method—application to a 3D coastal model of the Bay of Biscay

International audienceThe development of coastal ocean modeling in the recent years has allowed an improved representation of the associated complex physics. Such models have become more realistic, to the point that they can now be used to design observation networks in coastal areas, with the idea that a "good" network is a network that controls model state error. To test this ability without performing data assimilation, we set up a technique called Representer Matrix Spectra (RMS) technique that combines the model state and observation error covariance matrices into a single scaled representer matrix. Examination of the spectrum and the eigenvectors of that matrix informs us on which model state error modes a network can detect and constrain amidst the observation error background. We applied our technique to a 3D coastal model in the Bay of Biscay, with a focus on mesoscale activity, and tested the performance of various altimetry networks and an in situ array deployment strategy. It appears that a single nadir altimeter is not efficient enough at capturing coastal mesoscale physics, while a wide swath altimeter would do a much better job. Testing various local in situ array configurations confirms that adding a current meter to a vertical temperature measurement array improves the detection of secondary variability modes, while shifting the array higher on the shelf break would obviously enhance the model constraint along the coast. The RMS technique is easily set up and used as a "black box," but the utility of its results is maximized by previous knowledge of model state error physics. The technique provides both quantitative (eigenvalues) and qualitative (eigenvectors) tools to study and compare various network options. The qualitative approach is essential to discard possibly inconsistent modes