Seminaires Septembre 2017-Aout 2018

Amaury Dehecq 

NASA Jet Propulsion Laboratory, California Institute of Technology, USA

Titre: Des images satellites espions au service du suivi multi-décennal des changements de surface : glaciers, glissement de terrain et hydrologie.

Résumé: La zone critique, enveloppe superficielle de la Terre où se concentre l'essentiel des activités humaines, a évolué de façon rapide au cours des 50 dernières années, en lien avec la pression anthropique et le changement climatique. Le suivi multi-décennal de cette zone critique est souvent limité aux échelles locales. L'explosion récente de la quantité de données de télédétection spatiale permet le passage à l'échelle régionale ou globale, mais avec une profondeur temporelle courte (~20 ans) face à la variabilité climatique. Néanmoins, il existe un nombre colossal d'images photographiques, acquises à des résolutions (sub)métriques depuis des plateformes aéroportées ou satellites "espion" déclassifiées, à une époque où les capteurs numériques avaient une résolution insuffisante. Ces images ont un potentiel important pour le suivi multi-décennal des changements de surface dans des régions où potentiellement aucune autre observation n'existe. Toutefois, ces données représentent plusieurs défis de taille : images scannées nécessitant un pré-traitement, distorsion des images, position de l'appareil peu ou pas connue... La majorité des études utilisant ces jeux de données requièrent un travail manuel fastidieux, limitant l'analyse à grande échelle. L'automatisation de ces traitements rendu possible aujourd'hui grâce aux améliorations technologiques permet de tirer pleinement profit des millions d'images existantes.

Dans cet exposé, je présenterai les jeux de données existants, ainsi que la méthodologie développée afin de traiter ces images de façon automatique. L'application aux images des satellites espions américains Hexagon (1972-1986) permet de générer des Modèles Numériques de Terrain (MNT) à 24 m de résolution et des ortho-images à 6 m de résolution. Je présenterai ensuite plusieurs applications pour le suivi des glaciers de montagne, des glissements de terrain et de l'hydrologie. Les glaciers ont contribué à environ un tiers de l'élévation du niveau des mers depuis 1971 (GIEC 2013), mais ces estimations, basées sur l'extrapolation d'une centaine d'observations, ont une incertitude de l'ordre du signal à observer (Gardner et al., 2013). La comparaison des MNT "espions" avec des MNTs récents (WorldView, SPOT) a permis le suivi sur 40 ans des changements de volume des glaciers de l'Alaska et de l'Arctique canadien, deux principaux contributeurs à l'apport de masse dans les océans sur la période 2003-2009. Nos résultats montrent une tendance similaire sur 40 ans, avec une contribution de l'Alaska au niveau des mers supérieure à 3 mm sur 1977-2015. Les mouvements de terrain sont une menace majeure dans tous les milieux montagneux. Sur la période 2003-2011, ils ont causé la mort de plus de 80 000 personnes (Petley, 2012). Au Pérou, le risque est accru en raison du risque sismique et des extrêmes climatiques. L'utilisation de MNT "espion" a permis de documenter le développement de larges glissements de terrains dans la vallée de Vitor, Pérou, suite au développement de l'agriculture intensive en 1953. Nous avons mis en évidence l'effet à long terme (38 ans) de l'irrigation sur l'érosion des déserts arides.

Jérôme Aucan

 LEGOS/IRD, Nouméa, NEW CALEDONIA

Title: SMART Subsea Cables for Observing the Global Ocean: Science and Implementation Plan

Abstract: The deep ocean is key to understanding environmental and societal threats, from climate and ocean warming to rising sea level, ocean acidification, and tsunamis. The deep ocean, however, is difficult and costly to monitor, and we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cable systems. This is the mission of the SMART cables initiative (Science Monitoring And Reliable Telecommunications), a Joint Task Force (JTF) sponsored by three UN agencies. SMART sensors would “piggyback” on the power and communications infrastructure of more than a million kilometers of undersea fiber optic cable and tens of thousands of repeaters, creating the potential for global coverage at modest incremental cost. Initial sensors would measure temperature, pressure, and acceleration. The resulting data would address two critical scientific and societal issues: a) the long-term need for sustained climate-quality data from the under-sampled deep oceans, and b) the near term need for improvements to global tsunami warning networks. This presentation explores the ocean observing improvements available from SMART cables. Simulations show deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating sensors into fiber optic cables are discussed, addressing sea and land-based elements plus delivery of real-time open data products to stakeholders. SMART cables have been endorsed by major ocean science organizations. JTF is working with cable suppliers and sponsors, development banks and end-users to incorporate SMART capabilities into future cable projects. By providing a global ocean network of long-lived sensors, SMART cables would be a first-order addition to the global ocean observing system.

Laurent Lebreton

The Ocean Cleanup, New Zealand

Titre: Etude de la pollution plastique en mer : modélisation, mesure in-situ et télédétection.

Résumé: Le problème des déchets plastiques dans les océans et plus généralement dans l’environnement soulève actuellement un nombre grandissant d’inquiétudes pour la santé publique et celle des écosystèmes ainsi que pour les préjudices sur l’économie. Notre addiction au plastique ainsi que notre inefficacité à soutenir le poids de déchets engendrés, sont souvent comparés à une tragédie des communs. A la différence du changement climatique, une autre tragédie des communs, il n’y a pas de déni sur l’accumulation grandissante de déchets plastiques sur les plages et les océans du monde entier. Les origines du plastique dans les océans peuvent être retracées aux objets du quotidien, aux activités nautiques, de pèche, d’aquaculture, d’agriculture, de construction ou de transport. Une partie conséquente de la recherche actuelle sur la pollution plastique s’intéresse aux questions de bilan massique dans l’environnement, impliquant des réflexions sur les sources, le transport, la dégradation et l’accumulation des déchets plastiques. Cette présentation couvrira une série d’exemples de projets sur la modélisation, la mesure in-situ et la télédétection des débris plastiques en mer, outils indispensables pour la formulation d’un bilan massique a échelle globale.

A propos: Laurent Lebreton est diplômé de l’école Centrale de Nantes, réside et travaille en Nouvelle Zélande en tant que consultant indépendant. Il a commencé ses travaux sur la pollution plastique en 2010 et a depuis publié une série d’articles scientifiques sur le sujet en s’intéressant particulièrement a la modélisation des sources et du transport des déchets plastiques dans les océans. Laurent travaille actuellement pour The Ocean Cleanup, une organisation non-gouvernementale Néerlandaise qui développe des technologies pour récupérer les débris marins. Laurent est en charge de l’équipe de recherche sur l’étude de la pollution plastique.

Title: High resolution coastal modelling prospects for South Africa

Abstract: South Africa is in the privileged position to be situated between the Atlantic and Indian Oceans. Part of the Exclusive Economic Zone (EEZ) of South Africa are vast areas in the Southern Ocean surrounding the Gough and Marion islands. The South African Weather Service (SAWS), together with other parastatal institutions, has various mandates when it comes to research, governance and management of our oceans. SAWS has recently embarked on a quest to provide high resolution coastal ocean forecasts for the southern and South African coastline. This quest is an element of the aptly named the Marine Master Plan, and consists of various phases of research and forecasting tool development. Most of the development is planned in-house, but with close collaboration with national and international partners. This talk will provide the audience with an overview of planned developments and research endeavours. The tool developments include an operational tool for storm surge forecasting, high resolution fully spectral wave model outputs for southern Africa, a wave energy exposure atlas and high resolution circulation modelling for key focus areas. Amongst others, applications of these models will include sewage plume behaviour research and eventual forecast (associated with desalination plant construction sites), assistance in search and rescue operations and eventually regional scale offshore to nearshore ship routing tools. Research topics include the calibration of the models to be developed and investigation of physical phenomena unique to the southern African coastline. Multi, inter and transdisciplinary research is also being conducted and an overview of these project will be discussed. 

Philippe Kubik, Céline Angelelis, Delphine Fontannaz

CNES

Titre: Séminaire Pléiades

Résumé: Lors d’une réunion que nous avions eue fin 2017 à propos des données Pléiades (https://pleiades.cnes.fr/) et de leur usage dans nos travaux sur les questions côtières et le littorales, certaines personnes ont exprimé un besoin d’informations sur cette mission. En réponse à cette demande, vous êtesi invité à réserver la matinée du jeudi 21/06 à 10h00 à midi en salle Coriolis pour assister à un séminaire sur cette mission spatiale et ses données.

Durant de ce séminaire, 3 présentations d’environ 20 minutes chacune seront faites par des collègues du CNES très impliqués sur le sujet Pléiades qu’ils aborderont selon 3 volets :

1. Les caractéristiques du système imageur et ses performances, par Philippe Kubik (responsable du service Qualité Images, Direction des Systèmes Orbitaux)

2. Un inventaire des usages actuels de ces images et leur positionnement dans les bases de données thématiques, par Céline Angelelis (responsable du service Analyse et Algorithmes, Direction des Systèmes Orbitaux)

3. L’organisation et les règles de mise à disposition des images, par Delphine Fontannaz du service Missions Terre et Atmosphère (Direction du Numérique et de l’Exploitation des Opérations).

Merci donc d’inscrire ce séminaire au solstice de votre agenda !

Pierre-Amaël Auger

Instituto Milenio de Oceanografia, Valparaiso, Chile

Title:  Regional to mesoscale organization of the primary productivity and the oxygen minimum zone in the Peru-Chile upwelling system

Abstract: Primary productivity and oxygen availability are key drivers for fish communities and the biological carbon pump in the ocean. First results of current projects which explore the organization, from regional to mesoscale, of primary productivity and the oxygen minimum zone (OMZ) in the Peru-Chile upwelling system (PCUS) are presented. ROMS-PISCES physical-biogeochemical modeling (now CROCO platform) is used to explore the mechanisms behind the observed ocean variability.
At regional scale, the model brings a new hypothesis to explain the meridional variability of the coastal upwelling productivity and its offshore extension in the coastal transition zone (CTZ). Iron limitation of the phytoplankton growth is indeed suggested to play a key role in the coastal upwelling of the PCUS. In the model, the iron limitation of coastal productivity (0-70 km from the coast) mostly relies on the iron inputs from the bottom sediments. These inputs likely depend on the contact time of upwelled waters with the sediments, and then on the continental shelf width. The offshore phytoplankton biomass (70-200 km from the coast) is mosty driven by the injection of living organisms from the coast, which itself relies on coastal productivity and cross-shore transport processes such as mesoscale eddies and filaments. The coastal productivity and offshore extension in the CTZ then depends on the continental shelf width and cross-shore exchange processes. The interaction between iron limitation and the level of eddy/filament activity may actually drive the regional distribution of plankton biomass in the PCUS. In recent years, quasi-zonal mesoscale jet-like features or striations have been ubiquitously detected in the time-mean circulation of the world ocean. Off central Chile (southern PCUS), striations were attributed to the organization of the mesoscale eddy field in preferred eddy tracks. Yet, their impact on biogeochemistry has not been assessed yet. In the CTZ, the model reproduces the surface chlorophyll striations detected in satellite records of ocean color. The interaction between preferred eddy tracks and sharp background gradients of biogeochemical properties seems to structure at mesoscale the mean horizontal distribution of the plankton biomass in the CTZ and the OMZ. The respective contributions of eddy trapping and eddy advection (stirring) to these striations are examined.

Yann Planton

LOCEAN

Titre: Relationship between warm water volume and El Niño–Southern Oscillation: physical interpretation and asymmetry

Résumé: El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability, with large environmental impacts at the global scale. With anomalous warming of up to 4°C in the eastern equatorial Pacific, extreme El Niño, such as in late 1982, 1997 and 2015, involve a complete re-organization of tropical convection with outsized societal impacts relative to moderate El Niño. Despite an improved understanding of ENSO dynamics over the past decades, predicting ENSO’s amplitude remains a challenge, especially at long lead-times. The recharge oscillator theory highlighted the role of the oceanic heat content averaged over the equatorial Pacific as a robust precursor of ENSO several seasons ahead, providing some hope to improve long-lead time ENSO forecasts. The water warmer volume (WWV), proxy of the equatorial Pacific heat content, is the most widely used precursor of ENSO. The standard interpretation of this lead relation in the context of the recharge oscillator theory is that anomalous easterlies during a La Niña, favour a slow recharge of the equatorial Pacific that will later favour a transition to El Niño. Using idealised experiments with a linear continuously stratified (LCS) ocean model, we demonstrate that WWV in spring mostly reflects the fast Kelvin wave response to wind anomalies during the early months of the year. It is hence not an appropriate index of the slow recharge invoked in the recharge oscillator. The evolution of the heat content over the western equatorial Pacific (WWVW) is dominated by the forced Rossby wave response to wind anomalies over the previous 10 months. WWVW hence involves a longer-term recharge than WWV, thus we recommend using WWVW rather than WWV as an index for the slow recharge before the spring predictability barrier. The recharge oscillator theory is however essentially linear and does not account for the potential asymmetries existing between El Niño and La Niña events. Yet, observations suggest that the ocean preconditioning is a more efficient predictor for La Niña than for El Niño events. Because of large uncertainties related to the short observational record, we analyse this asymmetry in a set of pre-industrial control experiments from eleven coupled models, selected for their capability to simulate observed ENSO variability. As suggested by observations, the discharge one year before ENSO peak in these models is a significantly better precursor of La Niña occurrence and amplitude than the recharge for El Niño. This asymmetry likely arises from (1) the asymmetry of the ocean preconditioning that promotes a larger influence on La Niña (larger discharge) than on El Niño (weaker recharge) and from (2) a nonlinear Bjerknes feedback that promotes the growth of El Niño rather than La Niña.

Ted Scambos

National Snow and Ice Data Center

University of Colorado at Boulder, Colorado, USA

Title: Très Grande Froideur: The lowest temperatures on Earth

Abstract: The lowest measured air temperature on Earth, in situ, is -89.2°C on 23 July, 1983 at Vostok Station in Antarctica. However, satellite data collected during the Antarctic polar night reveals a broad region of the uppermost East Antarctic Plateau that regularly reaches surface (‘skin’) temperatures of -90°C and below. The most frequent occurrence of -90°C measurements, and the locations of the lowest temperatures, are in shallow (~2m closure) topographic lows on the south side of the Dome A to Dome Fuji ice divide, at 3800 - 4050 m elevation. Roughly 100 sites in this broad area reach the lowest temperatures, -98.0 to -98.6°C. Comparison with automated weather station data in the same region indicate that the actual surface temperatures are -96 ± 2°C, and typical near-surface air temperature gradients suggest that the 2m air temperatures in the sites are likely in the range of -94°C ± 3°C. Ultra-cold conditions occur more frequently during periods when the Antarctic polar vortex is stronger. A consideration of the winter surface energy balance components suggests that the limit on the lowest temperatures is set by re-radiation from the atmosphere, controlled primarily by stratospheric clouds and water vapor in the air column. 

Note: Ted will be a guest scientist at LEGOS from 25/04/2018 to 25/05/2018.

Brian Arbic

University of Michigan, Ann Arbor, USA

Title:Global modeling of internal tides and the internal gravity wave continuum spectrum

Abstract: In these two talks I will discuss modeling of the internal gravity wave spectrum in two state-of-the-art global ocean models (HYCOM simulations run by the US Navy, and MITgcm simulations run on NASA computers).  There are many motivations for studying internal gravity waves.  Internal wave breaking underlies most of the mixing in the interior of the ocean, and internal-wave driven mixing has significant impacts on ocean biology, ocean circulation, and oceanic temperatures and salinities.  Internal waves also have a significant signal in datasets constructed from satellite remote sensing of the ocean, including present-generation nadir altimeters and the planned wide-swath SWOT mission.  Finally, internal waves have a large impact on acoustics and other operational oceanography considerations.  It has been recognized just in the last few years that global models that are forced by both atmospheric fields and the astronomical tidal potential are able to resolve some of the internal gravity wave continuum spectrum--the so-called Garrett-Munk spectrum.  In the first talk I will discuss the background, motivation, and technical aspects of such models in detail, and in the second talk I will discuss cutting edge research results including the comparisons of such models with observations, and open questions about such models, in particular, their potential use in understanding the development of the Garrett-Munk spectrum.

Ce séminaire fait suite à celui du mercredi 11 Avril - This seminar follows the one given on Wednesday 11 April.

John L. Wilkin

Rutgers, The State University of New Jersey,
New Brunswick, New Jersey, USA

Title: An Operational Data Assimilative Ocean Forecast System for Northeast U.S. Coastal Waters

Abstrac: Coastal ocean models that downscale basin and global scale models are widely used to study regional circulation and locally important ecosystem and biogeochemical processes. When operated as now-cast/forecast systems, these models offer predictions that assist decision-making for maritime applications. Rutgers University operates such a system for shelf waters of the northeast U.S. coast where Regional Associations of U.S. IOOS (Integrated Ocean Observing System) operate a network of more than 50 surface current measuring CODAR HF-radars, autonomous underwater glider vehicles (AUGV), telemetering moorings, and drifting buoys. Other U.S. national and global efforts deliver further sustained observations from moorings, ships, profiling floats, animal tags, and a constellation of satellites. The Rutgers University re-analysis and forecast system uses the Regional Ocean Modeling System (ROMS; myroms.org) with 4-dimensional Variational (4DVAR) data assimilation to adjust initial conditions, boundary conditions, and surface forcing in each analysis cycle. Data routinely assimilated include CODAR velocities, satellite altimeter sea surface height with coastal corrections, L2 satellite temperature (microwave and infrared), in situ CTD data from AUGV and ships (NMFS Ecosystem Monitoring voyages), and all in situ data reported to the WMO GTS network. The operational system makes extensive use of open source open access web services for the ingest of real-time data, output to users, and monitoring of data streams and model execution. By tracing the provenance of all data entering the 4DVAR analysis and using observation impact analysis tools that are part of the ROMS 4DVAR suite, we are exploring the relative impact of the various platforms and systems that comprise the integrated observatory on user-relevant metrics of the analyzed ocean state.

Marek Stastna

Department of Applied Mathematics

University of Waterloo, Canada

Title: A dynamics-based viewpoint on solitary-like internal waves in the presence of rotation.

Abstract: In this talk I will present the theory of large amplitude internal solitary waves using the Dubreil-Jacotin-Long equation.  No prior knowledge beyond basic fluid mechanics will be assumed. After providing some context for observations in the field, I will provide examples of solutions of large amplitude internal solitary waves and illustrate a particular behaviour in the large available potential energy limit, namely that of flat crested waves. I will use this limit as a motivation to explore a novel portion of parameter space for flow over topography, namely that of very large amplitude, trapped waves.  I will then discuss the manner in which the balance between nonlinearity and dispersion that allows internal solitary waves to remain solitary is altered in the presence of rotation.  I will illustrate how the dynamics changes using three case studies, classical stratified adjustment, supercritical flow over topography and the dynamics of mode-2 Kelvin waves.  In each of these three cases, dispersion allows for complex behaviour, including long lasting inertial oscillations, but only in the last of the three is there appreciable overturning and small scale mixing.  I will conclude with some comments on extensions to field scale and possible experimental confirmation of the findings.

Brian Arbic

University of Michigan, Ann Arbor, USA

Title:Fundamentals of global ocean internal tide and gravity wave modeling

Abstract:In these two talks I will discuss modeling of the internal gravity wave spectrum in two state-of-the-art global ocean models (HYCOM simulations run by the US Navy, and MITgcm simulations run on NASA computers).  There are many motivations for studying internal gravity waves.  Internal wave breaking underlies most of the mixing in the interior of the ocean, and internal-wave driven mixing has significant impacts on ocean biology, ocean circulation, and oceanic temperatures and salinities.  Internal waves also have a significant signal in datasets constructed from satellite remote sensing of the ocean, including present-generation nadir altimeters and the planned wide-swath SWOT mission.  Finally, internal waves have a large impact on acoustics and other operational oceanography considerations.  It has been recognized just in the last few years that global models that are forced by both atmospheric fields and the astronomical tidal potential are able to resolve some of the internal gravity wave continuum spectrum--the so-called Garrett-Munk spectrum.  In the first talk I will discuss the background, motivation, and technical aspects of such models in detail, and in the second talk I will discuss cutting edge research results including the comparisons of such models with observations, and open questions about such models, in particular, their potential use in understanding the development of the Garrett-Munk spectrum.

Ce séminaire sera suivi par une autre présentation donnée le 14 mai - This seminar will be followed by another talk given on May, the 14th.

Patrick Marchesiello

LEGOS

Title: Coastal erosion of the Mekong delta: from processes to countermeasures

Abstract: The Lower Mekong Delta Coastal Zone (LMDCZ) is emblematic of the coastal erosion problem in tropical delta regions. It has been generated by very large sedimentary fluxes reaching the ocean, but modulated also by waves and currents, which, in combination, redistribute the river intake to the southwest. This process has formed the southern tip of Vietnam for the last 3500 years. But now, in addition to natural forces, the LMDCZ is affected by local human activity, including reduction of river fluxes due to damming and sand mining, and reduction of protective coastal mangroves in favor of agriculture and aquaculture. Relative sea level rise is also a major challenge, although global warming is a minor factor in the current situation when compared with land subsidence due to groundwater extraction. Here, we present the findings of a European-Vietnamese project with the dual objectives of: 1) better assessing the hierarchy of causality for observed erosion patterns; and 2) proposing engineering solutions at two specific sites. A discussion on various aspects of coastal science is proposed based on questions that emerged from the project, e.g., what are the paradigms needed for understanding coastal erosion and protection measures in delta coastal zones? How do we handle the gap between science and civil engineering as researchers, particularly at IRD?

L'exposé sera donné en français

William Exbrayat

Observatoire Midi-Pyrénées

Titre: Les chercheurs sont tous des numéros ou comment utiliser un identifiant chercheur pour gérer ses publications et faire sa bibliométrie ?

Résumé : Qu’est ce qui se cache derrière les codes numériques suivants : C-3260-2008 et 0000-0002-2830-209X ? La même personne. Un chercheur. Pas n’importe qui. Williams Moerner, prix Nobel de Chimie en 2015. Il possède, comme des millions de chercheurs, un identifiant chercheur qui l’identifie de façon univoque et qui lui permet de relier ses publications, ses données, générer un CV et calculer automatiquement son H-index. 

Le problème, c’est qu’il y a pléthore de systèmes qui génèrent des identifiants chercheurs. C’est la jungle de l’identifiant numérique. 

 Aussi, nous allons nous concentrer sur deux identifiants auteurs MAJEURS (ReserachID et Orcid) qui peuvent être reliés et qui répondent aux besoins de base du chercheur en matière de gestion des publications scientifiques et des calculs bibliométriques comme par exemple le H-Index. 

 L’identifiant chercheur ou quand il est bon d’être un code numérique !

Rui Ponte

Atmospheric and Environmental Research - AER

Title: A few tales about ocean heat content variability in relation to forcing and the large-scale circulation 

Abstract: Variability in ocean heat content is a key metric of regional and global climate, but quantitative knowledge about the mechanisms responsible for such variability remains somewhat deficient. We will explore the forcing and dynamics behind some examples of both global and regional heat content variability, using a global ocean state estimate recently produced by the project for Estimating the Circulation and Climate of the Ocean (ECCO).

Isabelle Biegala

Institut Méditerranée d'Océanologie - MIO, Marseille

Titre: Fonctionnement de l’écosystème à la station SPOT

Résumé : Le programme d’observation SPOT (South Pacific Ocean Time Series) est unique dans l’hémisphère sud, car il regroupe des observations hauturières multiparamétriques (physiques, chimiques, biologiques), à une station fixe (20°S, 168°E), le long de la colonne d’eau (-5 à -4000m), dans le but de comprendre le fonctionnement de l’écosystème et d’évaluer sa capacité d’adaptation aux changements globaux en cours.

La station SPOT est située entre le Vanuatu et les îles Loyautés de la Nouvelle-Calédonie, sur le courant Sud Equatorial (South Equatorial Current), issu de la grande gyre oligotrophe du Pacifique Sud. Les paramètres étudiés s’étendent de la physique au zooplancton en passant par la caractérisation moléculaire, pigmentaire et satellite du pico et du phytoplancton. Ces paramètres comprennent aussi des mesures de biodiversité de groupes fonctionnels et d’activités métaboliques telles que les fixations de carbone et d’azote, l’excrétion d’ammonium et la biosynthèse et le transfert trophique de métabolites actifs.

Initiées par l’IRD (UMRs LEGOS et MIO, Grand Observatoire de l’environnement et de la biodiversité du Pacifique Sud GOPS, Observatoire OSU-Pytheas) en 2012, les premières campagnes SPOT ont permis de caractériser l’importance de la communauté des cyanobactéries fixatrices d’azote dans la productivité de l’écosystème planctonique de surface et profond. Ces travaux ont aussi permis de caractériser les fluctuations saisonnières dans cette zone particulière du Pacifique Sud-Ouest.

Grâce aux travaux sur les cyanobactéries, de nombreux outils moléculaires ont pu être développés en vue d’une surveillance de groupes pico-planctoniques clefs. Cette surveillance viendra compléter celle du satellite, où grâce au programme SPOT, 20 ans d’observations de la chl-a ont pu être reliés à la cyanobactérie Trichodesmium fixatrice de carbone et d’azote. La station SPOT est maintenant reconnue comme un point de référence pour la calibration des capteurs couleur de l’eau européen, japonais et américains.

En Janvier 2018, la 14^ème campagne océanographique SPOT s’associait aux chercheurs de l’Université de Nouvelle-Calédonie et faisait participer pour la troisième fois la société civile d’Ouvéa. Ouvéa est une ile Loyauté dont la principale ressource est la pêche, soupçonnée d’être directement influencée par l’écosystème planctonique issue de SPOT.

Mathieu Morvan

Laboratoire d'Océanographie Physique et Spatiale - LOPS, Brest

Title: Eddies in the Gulf of Aden and the Gulf of Oman, from the Mesoscale to Submesoscale Interactions

Abstract: The Gulf of Oman and the Gulf of Aden are intermediate basins which receive the outflows of salty water from the Persian Gulf and from the Red Sea, but also the waves and eddies from the Arabian Sea. These outflows settle at 250-300m and 600-1000m depths, respectively. The surface eddies in these two gulfs are dynamically deep reaching, and they strongly perturb these deep outflows. In the absence of these eddy fields, these two outflows should follow the coasts of Oman or of Somalia. But these eddies divert their paths away from the coast, advect them along curved trajectories around the eddy rims, elongate these outflows as salty filaments into these two gulfs and finally, can break these filaments into small eddies. First, we will show that during the spring 2011 and 2014, the PhysIndien 2011 and PhysIndien 2014 surveys revealed the presence of submesoscale eddies at the depth of the Persian Gulf Water in the Sea of Oman. In the same periods of time, the altimetry suggests that the surface dynamics was dominated by a row of mesoscale eddies alternatively signed. Thanks to high resolution realistic numerical simulation performed with the HYCOM model over the Gulf of Aden and the Gulf of Oman, we will show the formation of submesoscale eddies and filaments at the surface and at depth and compare such submesoscale dynamics between both regions. Then, we will investigate the lifecycle of small scale eddies and filaments through idealized numerical simulation performed with CROCO at high resolution. Finally, since strong interactions between submesoscale and mesoscale eddies were observed, we will propose to use the Contour-Advective Semi-Lagrangian (non-hydrostatic) algorithm to investigate the full properties of the interaction between a large vortex and a smaller but more intense one.

Alberto Carrassi

Nansen Environmental and Remote Sensing Center, Bergen, Norway

Titre: Attribution of climatic events using a data assimilation-based formulation of model evidence

Abstract: Data assimilation (DA) methods were oroginally designed for state estimation, but are starting to be increasingly applied to the model selection and attribution problems as well.

Probabilistic event attribution is the problem of assessing the probability of occurrence of an observed episode under different hypotheses: a notable example is the causal assessements about episodes of extreme weather or unusual climate conditions. Two quantities are computed: (i) the probability of occurence, p_i, referred to as factual, which represents the probability in the real world; and (ii) the probability, p_o, counterfactual, in an alternative world that might have occured had the forcing of interest been absent. The so-called fraction of attribuable risk (FAR) is then defined as the change in likelyhood of an event that is attribuable to the external forcing. 

The approach widely used to compute the FAR is very costly as it uses a large ensemble of model simulations, unconstrained from the observations, and is difficult to implement in a timely, systematic way in the aftermath of a climatic episode. We will show, as a proof of concept, that these obstacles are removed or mitigatedby using the FAR using DA, leading to an efficient DA-based approach to the attribution of climate related events.

Carrassi et al., 2017 have introduced a contextual formulation of model evidence (CME) that allows for estimating the two concurrent probabilities, p_o and p_i needed to compute the FAR. In particular, these authors have shown that the CME can be efficiently computed usin an ensemble Kalman filter with localization - a requirement for ensemble-based DA with high dimensional models - Metref et al., 2018 developped a new formulation of the CME using domain localization, the domain-localized CME (DL-CME). In this talk we will first define the CME and shows that it can be computed using state-of-the-art DA methods. We will later provides examples of its application to the model selection and to the attribution problems using low dimensional numerical models and the complexity global atmospheric SPEEDY model.

This presentation will be given in english.

Thierry Delcroix

LEGOS

Titre: ENSO et la catastrophe écologique de l'île de Pâques

Résumé: L’île de Pâques est une île chilienne située dans le sud-est de l’Océan Pacifique par 27°S et 109°W. Elle est particulièrement connue du grand public pour ses statues de pierre monumentales, les moai, emblème de l’île, dont la construction et la symbolique continuent à alimenter nombre d’hypothèses. L’île est également connue pour d’autres ‘énigmes’ scientifiques dont l’origine et la chronologie d’installation des premiers habitants, la quasi-disparition de l’ancienne civilisation Rapa Nui et l’extinction massive de la flore à partir du XVII^ème siècle. Une des hypothèses fréquemment avancée quant à cette extinction, parfois qualifiée de ‘catastrophe écologique’, fait état d’une surexploitation des ressources naturelles par les habitants, à valeur d’exemple de ce qu’il ne faut pas faire. D’autres hypothèses ont invoqué des causes climatiques, notamment l’influence du petit âge glaciaire, du minimum d’insolation de Maunder et, dans une moindre mesure, du phénomène El Niño Oscillation Australe (ENSO). C’est à cette dernière hypothèse, ENSO, que nous nous intéresserons dans cet exposé. Nous tenterons de démontrer, à travers l’analyse combinée d’observations climatiques instrumentales, d’indicateurs paléo-climatiques et de simulations numériques du dernier millénaire que les anomalies climatiques associées à ENSO auraient probablement pu, à elles seules, engendrer des déficits pluviométriques et hydriques conséquents, à même de contribuer aux changements importants de la flore de l’île. Nous en conclurons que facteurs anthropiques et climatiques (naturels) doivent donc impérativement être analysés de concert pour parfaire notre compréhension de cette ‘catastrophe écologique’.

Erwin Bergsma

LEGOS

Titre : Continues observation of nearshore depth profiles and storm-morphology

Résumé : This talk focuses on the use of a video-based remote sensing technique to obtain nearshore depth profiles and ultimately capturing storm morphology. Video-data has been collected over a period of 1.5 years and covers one of the most energetic European winters ever measured (2013-2014). The video-data is used to invert depths in the nearshore every hour. A spectral technique to determine wave number and frequency is used to estimate celerity and invert to depth using the linear dispersion relation for free surface waves. With conventional methods, depth measuring instruments won’t last under extremely powerful wave conditions during storms. The application of this video-based remote sensing technique during storms give a first quantitative measure for storm morphology. In addition, the 1.5 year hourly bathymetries result in a unique dataset of nearshore morphology under calm and extreme storm conditions, the data shows great morphological impact (erosion) of the extreme winter and subsequent recovery.

This presentation will be given in english.

Fabrice Ardhuin,

IFREMER / LOPS

Titre : Mesure des courants, dérive de glace et vagues par radar Doppler: la mission SKIM

Abstract :

We propose a new satellite mission that uses a near-nadir Ka-band Doppler radar to measure surface currents, ice drift and ocean waves at spatial scales of 20 km and more, across a 300 km swath, with snapshots at least every day for latitudes 75 to 82, and every few days otherwise. The use of incidence angles at 6 and 12 degrees allows a measurement of the directional wave spectrum which yields accurate corrections of the wave-induced bias in the current measurements. The instrument principle, algorithm for current velocity and mission performance are presented here. The proposed instrument can reveal features on tropical ocean and marginal ice zone dynamics that are inaccessible to other measurement systems, as well as a global monitoring of the ocean mesoscale that surpasses the capability of today's nadir altimeters. Measuring ocean wave properties facilitates many applications, from wave-current interactions and air-sea fluxes to the transport and convergence of marine plastic debris and assessment of marine and coastal hazards. SKIM has just been pre-selected by ESA to be Earth Explorer mission number 9. The choice for EE9 is between FORUM (Far infrared outgoing radiation) and SKIM, and it will be made in 2019 for a launch in 2025.

More information at: http://tinyurl.com/SKIMonRG  or Twitter: #SKIM4EE9

The ESA call for experts to be part of the Mission Advisory Group is open : http://missionadvice.esa.int/index.php 

 Yann Krien

Post-doctorant au LARGE (Laboratoire de Recherche en Géosciences et Energies) / Université des Antilles

Titre : Exemples de limites et de perspectives d'amélioration des modèles de submersion marine cyclonique

Résumé :

Avec 6 ouragans majeurs, dont certains d'une intensité exceptionnelle (ex: IRMA), la saison cyclonique 2017 a été particulièrement active en Atlantique Nord, et notamment dans les petites Antilles. Dans le cadre du projet FEDER C3AF (Conséquences du Changement Climatique sur les Antilles Françaises), des missions de terrain et des comparaisons entre modèles numériques et données de la mission CYGNSS de la NASA ont permis de mieux appréhender un certain nombre de limites associées aux modèles de submersion marine cyclonique. L'objet de cette présentation est de présenter quelques travaux en cours sur ces problèmes, ainsi que des perspectives d'amélioration. On abordera notamment 3 principales questions: 1-l'impact des ondes infragravitaires sur la submersion marine, 2-le problème de la représentation des champs de vent cyclonique, 3-la question des échanges de quantité de mouvement à l'interface atmosphère-océan. 

Robin Waldman,

CNRM

Présentation en français.

Titre : On the role of the chaotic mesoscale dynamics on the Mediterranean thermohaline circulation

Résumé:

The Mediterranean Sea is a semi-enclosed sea which displays a thermohaline circulation feeding anti-estuarine exchanges at its outer strait. Many of its key circulation features occur at the mesoscale: transports at straits, subbasin circulations and deep convection. Hence its variability is highly sensitive to the chaotic mesoscale dynamics. In particular, deep convection is the main driver of thermohaline circulation through the formation and subsequent spreading of deep waters. This phenomenon is believed to be inhibited by the restratification process driven by mesoscale eddies.

The aim of this presentation is to characterize several aspects of the Mediterranean thermohaline circulation and to study how mesoscale impacts their mean properties and their variability.

For that we performed ensemble eddy-permitting (7km) and eddy-resolving (2km) Mediterranean Sea simulations over the well-documented 2012-2013 period and the historical 1979-2013 period. We will first characterize deep convection from observations during the exceptional 2012-2013 field campaign MERMEX – HyMeX in the Northwestern Mediterranean Sea. We will then address how mesoscale dynamics impacts the mean properties and the chaotic variability of deep convection in that region. We will finally assess the respective roles of the mesoscale chaotic dynamics and external forcings in the interannual variability of the Mediterranean thermohaline circulation.

Xi Yu Xu,

Associate Professor, Key Laboratory of Microwave Remote Sensing,

National Space Science Center,

Chinese Academy of Sciences (CAS),

Beijing, China

Title : Evaluation of Coastal Sea Level of Jason-2 Altimetry Offshore Hong Kong

Abstract :

Satellite altimetry provides precise sea surface height measurements up to about 10 to 20 km from the coastlines. In the coastal zones, radar echoes reflected to the satellite become much more difficult to interpret because of land contamination. In parallel, the quality of geophysical corrections we need to apply to altimeter range measurements also degrades significantly. This leads to an important decrease in the number of valid data when approaching the coast. In the recent years, a number of groups have developed different approaches to overcome those difficulties, either by improving the altimeter geophysical corrections (e.g. tropospheric correction, ocean tide models, etc.) or by developing specific methodologies (called retracking) to extract accurate range measurements (distance between satellite and sea surface) from non-standard radar echoes (e.g., PISTACH and ALES retrackings). Thanks to these efforts, a number of experimental reprocessed coastal altimetry sea level data sets are now available. In this paper, we evaluate those available for the Jason-2 satellite mission. We focus on the Hong-Kong region and consider high-resolution (20-Hz) along-track data for a specific Jason-2 pass that crosses the coast nearby the Hong-Kong tide gauge. Six years and a half of data are analyzed, from July 2008 to December 2014 (orbital cycles 1-238). We compare sea surface height measurements obtained using the ALES retracker and the different retrackers of the PISTACH products with the official standard data (called Geophysical Data Records –GRD-) distributed by space agencies. We compare each near-coastal sea level estimate with data from the Hong Kong tide gauge (located 10km away). We also compute sea surface height bias and noise over both open ocean (>10km away from coast) and coastal zone (within 10km or 5km coast-ward). Finally, coastal sea level trends from the different retrackers are estimated over the 2008-2014 time span. The results show that, after outlier removal, in the coastal band, ALES performs better than the other retrackers considered in this study, both in terms of noise level and trend uncertainty. An interesting, but still preliminary result is that over the 6.5-year time span considered here, the altimetry-derived sea level trend is significantly larger within 5 km of the coastline than at larger distances from the coast.

Carine Tchamabi,

Post-doctorante, LEGOS

Titre : Simulations numériques et connectivité physico-biogéochimique des systèmes insulaires Noronha-Rocas dans l’Atlantique tropical

Résumé :

L’influence de la présence de l’archipel de Fernando de Noronha (FN, 3°51'S-32°25'W) et d’Atoll das Rocas (AR, 3°52'S-33º49'W) sur la structure physico-biogéochimique dans l’Atlantique tropical a été étudiée en utilisant des données in-situ, satellites et des techniques de modélisation numérique. Initialement, pour examiner les effets de la présence de ces îles sur la circulation et la structure thermohaline, nous avons employé le modèle hydrodynamique Regional Ocean Modeling System (ROMS) avec une haute résolution horizontale () pour simuler deux scénarios climatologiques distincts; avec la présence de FN et AR (Scenario I) et avec «l’enlèvement artificiel des deux îles» (Scénario NI). Les résultats de la simulation du Scénario I, validés avec les données d’observations, indiquent que les instabilités générées par la perturbation du flux par les obstacles (îles) provoquent un développement de tourbillons en aval de FN et AR. Ces structures de meso-échelle ont une influence sur les propriétés thermodynamiques des eaux autour de ces îles en favorisant un mélange vertical à la base de la couche de mélange qui se traduirait par un refroidissement de subsurface, particulièrement pendant la période de renforcement de la branche centrale du courant équatorial sud (cSEC). En accord avec les observations, les sorties du modèle biogéochimique (ROMS – PISCES, Pelagic Interactions Scheme for Carbon and Ecosystem Studies) montrent une nette amélioration de la concentration de la chlorophylle en aval de FN et AR. Ces résultats montrent également une augmentation de la concentration en nutriments; ce qui confirme l’importance de ces processus physiques. Comparés à l’expérience artificielle (Scénario NI), les résultats du Scénario I montrent que ces îles ont également une influence sur les paramètres du système de carbone. Les résultats du modèle (champ de vitesse) ROMS ont été aussi utilisés pour l’application du modèle individu-centré (ICHTHYOP) afin de simuler le transport de l’ichthyoplancton Dog Snapper (Lutjanus jocu), relâchés à FN. Nous avons évalué le pourcentage de rétention de larves à FN et recrutement à AR pour différentes simulations considérées. Le modèle confirme une connectivité larvaire entre ces deux îles.

Mots-clés : Simulation numérique, archipel de Fernando de Noronha, Atoll das Rocas, Atlantique tropical, courant équatorial sud ‘‘central’’

Olusegun Dada,

Department of Marine Science and Technology

Federal University of Technology, Akure, Nigeria

Title :Trends of Niger Delta Shoreline in the last 100 years

Abstract :

Deltaic coasts are dynamic geomorphic systems where continuous changes occur on diverse spatial and temporal scales, and these changes constitute an important aspect of their evolution. More important, they host critical economic infrastructures and diverse ecosystems as well as high concentrations of human population. However, they are highly vulnerable to the effects of global climate change and human interference which necessitate their continual monitoring for sustainable management.

This paper investigates trends of shoreline along the Niger Delta coast of the Gulf of Guinea, North Atlantic Ocean in the last ten decades (1923-2013) by means of topographic maps and satellite imagery in GIS environment. Further, in other to understand processes governing the multi-decadal shoreline changes in this wave-dominated delta re-analyzed wave data coupled with hydro-meteorological data were analyzed.

Findings reveal two distinct shoreline dynamics patterns/phases: first, mobility with high net retreat between 1923 and 1987 interval and; second, mobility with low net progradation from 1987 to 2013. During the first phase, shoreline retreat is mainly concentrated on the western and arcuate sections of the delta. In the second phase, all sections of the Niger Delta coast prograded seaward.

Rainfall and river discharge variations are consistent with shoreline change results: there is downward trend in river discharge between 1923 and 1987, and significant upward trend after 1987. Multivariate regression analysis show that interannual wave regime as well as mean longshore sediment transport rate are higher in the last three decades (1980-2013) than over the last 80 years (1900-1980) along this coast. This implies that high net pre-1987 (1923-1987) shoreline retreat shoreline of the delta is most likely influenced by weakening of river (water and sediment) discharges due to changing climate and human interference.

However, the paper concludes that changing wave regime and associated littoral transport off the Niger Delta acting in concomitant with fluvial discharge variability constitute a powerful and effective mechanism responsible for much of the Niger Delta shoreline changes.

Séminaire exceptionnel

Dr. Myriam Khodri

Chercheure, Laboratoire d'Océanographie et du Climat : Expérimentation et Approches Numériques (LOCEAN) / IPSL

Présentation en français.

Titre : The influence of volcanic forcing on Pacific Ocean inter-annual to decadal variability over the last centuries

Résumé :

The understanding of ENSO intrinsic dynamics has improved over recent decades, in particular with the identification of useful predictors. The understanding of ENSO response to external forcing is however primitive compared to our understanding of its internal dynamics. There is, for instance, neither a clear consensus on how anthropogenic forcing influences ENSO nor how it is influenced by volcanism. Some explosive tropical volcanic eruptions are sufficiently strong to inject aerosols into the stratosphere. Those aerosols backscatter incoming solar radiation, and reduce the global surface temperature by a few tenths of a degree Celsius for a couple of years. While this systematic, global effect of large tropical eruptions, is relatively well understood, their impact on regional seasonal phenomena such as the Asian monsoon or on natural modes of climate variability such as ENSO has not yet settled to a consensus. Observational in situ SST data sets dating back to 1882 indicate a large positive ENSO-like pattern following four out of five big eruptions during the historical period. While this is a small sample, and a warm event was already underway during two of these eruptions, longer ENSO-proxy records also suggest a more probable equatorial Pacific warming within 2 years after large tropical eruptions. Modelling studies have however reported contrasting responses to volcanism. There is hence a need to reconcile in situ observations and paleo-proxies, which suggest an El Niño during the 2 years that follow the eruption, and modelling studies that have reached no consensus on either the sign or the mechanism of the ENSO response to volcanism. 

In this presentation I’ll show that relying on SST anomalies relative to the tropical average (relative SST) allows us to reconcile models and observations. This metric reveals that an El Niño event tends to follow large tropical eruptions in observed SST data sets and in the Coupled Model Inter-comparison Project phase 5 (CMIP5) historical experiments. Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air–sea interactions in the Pacific, favouring an El Niño-like response. The general framework outlined above is useful to identify the core dynamics involved in post-eruption inter annual to decadal climate variability over the last millennium, and its applicability to adaptation to current variability and future change.

Mathieu Ardyna

Laboratoire d’Océanographie de Villefranche/CNES

Titre : Comprendre le contrôle environnemental de la biomasse et de la phénologie phytoplanctoniques dans l’océan Austral :  De l’utilisation des satellites aux flotteurs BGC-Argo.

Résumé : L'océan Austral (OA), sensible au changement climatique, connaît actuellement un rapide réchauffement et un adoucissement de surface. De tels changements hydrologiques peuvent altérer de façon significative la pompe biologique de carbone de l’OA (c.-à-d., l'efficacité de la production primaire et son transfert aux niveaux trophiques supérieurs et/ou sa séquestration en profondeur). Cependant, avant d’être en capacité de faire des prédictions, une meilleure compréhension de la biogéographie et des facteurs environnementaux contrôlant les dynamiques phytoplanctoniques (c.-à-d., la productivité et la phénologie) dans l'océan Austral est essentielle. Dans le cadre de cette étude, nous présentons une bio-régionalisation de l’OA à partir d'observations satellitaires et de flotteurs BGC-Argo, où l'on observe une gamme de trois ordres de grandeur de productivité. Les facteurs environnementaux qui contrôlent la phénologie et la productivité du phytoplancton semblent être complètement découplés. Un gradient latitudinal dans l'initiation des floraisons phytoplanctoniques apparaît clairement relié aux régimes de lumière, avec certaines exceptions dans les zones de marge de glace et soumises à de forts mélanges verticaux. Quant à la productivité du phytoplancton dans l’OA, elle est clairement associée à la fois aux régions peu profondes et, où la limitation du fer semble être moins prononcée. Ces résultats nous permettent d’avoir une vision globale dans l'espace et le temps des facteurs contrôlant les dynamiques phytoplanctoniques (c.-à-d., les éléments nutritifs, les régimes de lumière et de mélange vertical) qui sont d'un intérêt fondamental pour identifier et expliquer les changements potentiels dans les écosystèmes marins de l’OA.

Helmut Maske,

Professor, Département de Microbiologie Marine, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Mexico

Title : What makes the sub-oxic mesopelagic zones off Peru and Mexico?

Abstract :

Oxygen profiles in the ocean generally show a discontinuity between 100 and 1000m in the form of a minimum. In certain regions, Sub-oxic Mesopelagic Zones (SMZ) this minimum reaches anoxic conditions, for example off Peru or Mexico. Why/how are these SMZs forms? The governing process is the formation of organic particles (POM) in the surface layer that will sink and be oxidized in the mesopelagic layer. The rate of this process has to be sufficiently high to form and maintain the SMZ against the diapycnic diffusion isopycnic transport of oxygen.

Mexican and Peruvian SMZs show very different surface oceanographic conditions. The Peruvian SMZ lies in a region of strong coastal upwelling with high productivity that can maintain a high POM flux rate. Off Mexico the ocean is seemingly oligotrophic, highly stratified and the question is where the POM flux originates. I suggest that the Mexican SMZ is ‘cryptically’ productive made possible by a large regional uplift of the pycnocline. This uplift is providing the necessary inorganic nutrients to the euphotic zone to stimulate primary productivity and the POM flux.