Stéphane Dolléac
Titre : Can we really estimate trends in phytoplankton abundance over the 25 year ocean colour satellite era ?
Abstract : Phytoplankton abundance is crucial to the natural carbon cycle and the sustainability of marine ecosystems. However, its slow natural variability and its response to anthropogenic climate change remains poorly understood. Since 1997, numerous algorithms have been developed to estimate chlorophyll concentration, a proxy of phytoplankton abundance, from ocean colour remote sensing. These tools have enabled 25 years of continuous, global-scale observations, providing invaluable insights into the variability of phytoplankton. However, analysing recent trends using these datasets presents significant challenges. Temporal inconsistencies and discontinuities in satellite time series – resulting from sensor transitions or decommissioning – introduce biases that complicate trend detection. Here, we present an inter-comparison framework designed to address these limitations by analysing multiple ocean colour remote sensing products. We implemented a robust statistical approach to minimize the influence of temporal discontinuities on trend detection. These corrections largely improved inter-product agreement on chlorophyll trends and provide novel insights into recent phytoplankton dynamics. The next step is now to try to remove slow natural variability to attribute the remaining trends to climate change.

Corentin Gentil
Abstract : Western boundary currents transport large masses of water, usually poleward from the equator. Therefore, they contribute to heat redistribution at a global scale, and are essential to understanding the Earth’s climate. These currents are boundary layers, and are generally smaller than the mesh size of the models used to predict future climates (at low resolution). Their numerical representation and physical understanding are therefore necessary when attempting to predict future climates. This presentation will briefly introduce western boundary currents, and then introduce a toy model derived from the Boussinesq equations that shows the interaction between a western-edge current and a topographic slope, a key physical phenomenon for understanding the detachment of these currents along the coast.

https://ird-fr.zoom.us/j/91940354222?pwd=aXD6AtUxDsmmbIOua7MP9NSrCfBc14.1
ID de réunion: 919 4035 4222
Code secret: 652866

TT

10 ans d’aventures dans les eaux Sénégalaises : la bouée-observatoire Melax, sentinelle de l’upwelling des Canaries, célèbre sa première décennie !

Melax, « Eclair en wolof, est l’unique bouée-observatoire du fameux upwelling des Canaries et a 10 ans ! Cette plateforme de mesure marine déployée sur le plateau Sénégalais est le fruit d’une collaboration du LOCEAN, du Laboratoire de Physique de l’Atmosphère et de l’Océan Siméon Fongang (LPAOSF) de l’Université Cheikh Anta Diop de Dakar, du Centre de Recherche Océanographique de Dakar-Thiaroye, de l’IRD, et de la météorologie nationale sénégalaise. Ce groupe franco-sénégalais, réuni au sein du Laboratoire Mixte International ECLAIRS, a installé Melax au sud de Dakar à environ 30 km au large de la côte, par 35 mètres de profondeur. Elle est équipée de capteurs de dernière génération qui mesurent des paramètres clés de la physique et la biogéochimie marine, tels que les vents, les radiations, l’humidité, les précipitations, la température, la salinité, les courants marins, l’oxygène dissout de fond et, depuis peu, la chlorophylle de surface. Ces données à haute fréquence et long terme, dont une partie est transmise en temps réel, sont essentielles pour améliorer les prévisions marines et climatiques, et notre compréhension de l’upwelling côtier. Cette année, Melax devient une pièce maîtresse du tout jeune Observatoire National du Littoral Sénégalais : ses données seront précieuses pour mieux prédire les extrêmes qui affectent la pêche artisanale ou intensifient l’érosion côtière, pour mesurer les effets du changement climatique, et pour anticiper les impacts des pollutions marines liées à l’essor des activités industrielles côtières. 

Au cours de cet « annivers-séminaire », l’équipe du projet reviendra sur les 10 années d’aventure Melax, avec un court-métrage, des interventions de protagonistes, des illustrations de résultats scientifiques, pour échanger avec vous sur le travail de terrain et de labo ayant permis d’assurer cette longévité, et sur les perspectives pour les 10 prochaines années! 

Alban Lazar, Christophe Noisel, Vincent Echevin, Eric Machu et Juliette Mignot.

https://ird-fr.zoom.us/j/91940354222?pwd=aXD6AtUxDsmmbIOua7MP9NSrCfBc14.1

ID de réunion: 919 4035 4222
Code secret: 652866

Formation SIFAC

Formation SIFAC

Formation SIFAC

Abstract : In the Arabian Sea phytoplankton blooms are dominated by diatoms that contribute to sedimentary phytodetritus flux supplying major elements (C, N, Si) and food for benthic biota and hence are important to address. This study analyzed for the first time diatom frustules (diversity and abundance) including organic matter and ancillary parameters using the core top sediments from the central (64 °E) and eastern Arabian Sea (western Indian continental shelf), the area profoundly influenced by atmospheric forcing (monsoons) and oxygen minimum zone (OMZ) with high spatial variability. The signature of atmospheric forcing was also reflected in the sedimentary profiles and diatom diversity. Significantly higher diatom frustules abundance and organic carbon content were noticed in those areas (north of the central Arabian Sea and eastern Arabian Sea) where natural nutrient enrichment occurs via open-ocean upwelling, winter convection, and lateral advection supporting large diatom-dominated phytoplankton blooms and intense OMZ. A total of 40 diatom genera were found with two centric (Thalassiosira and Coscinodiscus), and one pennate (Nitzschia) diatoms as major contributors. Large-sized diatoms like Coscinodiscus and Rhizosolenia that could escape grazing and sink consequently due to higher ballasting. In the south (13, 11 °N) of the central Arabian Sea, lower organic carbon as well as less abundance of frustules were found as this area mostly remains nutrient-poor dominated by small-medium sized phytoplankton. Relatively smaller chain-forming centric (Thalassiosira) and pennate diatoms (Pseudo-nitzschia, Fragilaria, Nitzschia, etc.) were found throughout the transect with higher abundance in the south. The euphotic diatom diversity from the existing literature was compared with the frustule diversity from the sediments, suggesting not all diatoms make their way to the sea bed. Such distinct spatial north-south variability in diatom frustule size as well as abundance could be attributed to cell size, grazing, and water column mineralization rates related to OMZ

https://ird-fr.zoom.us/j/91940354222?pwd=aXD6AtUxDsmmbIOua7MP9NSrCfBc14.1
ID de réunion: 919 4035 4222
Code secret: 652866
Contact at LOCEAN in charge of the seminar: alicepietri@locean.ipsl.fr

Abstract : Léna Champiot-Baillard
Title: Warming and sea ice loss drive consistent CMIP6 projections of increasing Arctic Ocean primary production
Abstract : The Arctic Ocean is experiencing profound environmental changes due to climate change, impacting Net Primary Production (NPP) and its spatial and temporal distribution. NPP is broadly projected to increase, due to less sea ice, more light availability and warming. In this study, we analyze NPP trends in the Arctic Ocean at a regional scale using CMIP6 low mitigation (SSP5-8.5) simulations. Our objective is to assess how NPP evolves throughout the 21st century in different regions of the Arctic and to identify the key drivers of these changes, including sea ice retreat, warming, and changes to nutrient availability. We used a multi-model approach to study the projections of NPP, focusing on different Phytoplankton Functional Types (PFTs), such as diatoms and miscellaneous phytoplankton and investigated the role of physical and biogeochemical constraints including light, nutrient and temperature limitation. Additionally, we assess model uncertainties and their regional variations to better understand the robustness of future projections. Our results indicate a significant increase in Arctic Ocean NPP, mainly driven by the reduction in sea ice cover and longer ice-free seasons. However, spatial heterogeneities emerge, with some regions showing stronger NPP increases than others, depending on local nutrient dynamics and water column stratification. Notably, the relative contribution of different phytoplankton groups evolves over time, with smaller phytoplankton becoming more and more dominant in response to changing environmental conditions.

Cosme Mosneron-Dupin
Title : Le tour de la mer de Weddell en quatre-vingts jours, Edition Allemande
Abstract : Après quatre ans passés au fond de l’océan Austral, il est temps de récupérer les mouillages SO-CHIC. Pour cette mission, notre narrateur embarque à bord du Polarstern, un brise-glace de recherche allemand. À son bord : des scientifiques allemands, un équipage allemand… et surtout, un cuisinier allemand.
Notre narrateur survivra-t-il à ce séjour linguistique ? Les mouillages seront-ils récupérés ?
Plongez dans cette épopée homérique où se mêlent banquise, aventure, sciences et émotions. Qui sait, il y aura peut-être des photos de bébés manchots.
Convient à tout public.

https://ird-fr.zoom.us/j/91940354222?pwd=aXD6AtUxDsmmbIOua7MP9NSrCfBc14.1
ID de réunion: 919 4035 4222
Code secret: 652866

Contact at LOCEAN in charge of the seminar: margaux.perhirin@locean.ipsl.fr

Mixing and dispersion in wave-driven nearshore models

Rip currents and surf eddies are horizontal structures, typically ranging from 10 to 100 meters in scale, that form in the nearshore zone as a result of vorticity generated by wave breaking. This presentation will give an overview of a few years of research on this frontier of oceanography and ocean modeling. Originally designed as a coastal model with a regional scope, CROCO has, over the past decade, been adapted to simulate processes in the fine-scale nearshore zone. This evolution began with the use of 3D wave-averaged equations and later benefited from the implementation of a new non-hydrostatic solver, enabling the explicit resolution of nearshore waves. These advancements have allowed the model to uncover previously unknown 3D coastal dynamics, particularly the alteration of 2D turbulent cascade and the emergence of a vertical shear instability that gives rise to novel eddy structures we refer to as mini-rips. As intermediate-scale phenomena, these structures may represent a missing link in explaining the mixing patterns observed in the surf zone, with consequences for dispersion in the deeper ocean. Ultimately, wave-resolving 3D models provide a more accurate representation of nearshore processes at a reasonable computational cost. In addition, wave-resolved modeling is expected to play a key role in refining the parameterizations used in simpler wave-averaged models of greater spatial and temporal scope.

https://ird-fr.zoom.us/j/93649770932?pwd=Ub0bIJPADFrk5btAKRvJqWSq4k2T0r.1

Le LOCEAN a effectué son Dialogue Objectifs Ressources avec les tutelles le 26 mars dernier. Cet exercice vise à présenter aux tutelles nos besoins (en personnels IT, C/EC et moyens financiers/équipements) pour mettre en œuvre notre projet scientifique validé par l’HCERES.  C’est un exercice important, que nous avons préparé collectivement, et qui nous demande de nous projeter à un horizon de 3-5 ans, pour identifier les compétences/expertises du laboratoire qui sont menacées, notamment du fait des départs en retraite, et prioriser les besoins du laboratoire pour mettre en œuvre son projet scientifique.

Nous proposons de vous refaire rapidement la présentation faite aux tutelles à cette occasion (~30 min), de vous résumer les recommandations des tutelles dont nous disposons à ce stade, puis de discuter collectivement de ces recommandations et de la manière de les mettre en œuvre (30 min). Ce séminaire sera le point de départ d’une réflexion approfondie et au long cours du laboratoire sur la définition de ses priorités scientifiques, sur son identité, et sur son attractivité, objectifs du présent mandat.

En espérant vous y voir nombreux,

Jean-Benoit et le codir

https://ird-fr.zoom.us/j/91828884548?pwd=kPybvK60aE2MHW2qgg3wnjKt13rNyb.1

François Challet

Abstract : Mesoscale eddies play a crucial role in the Arctic Ocean to maintain the stratification. Past studies have shown that most of the Arctic Ocean was baroclinically unstable, with two main vertical modes centered either in the halocline or the Atlantic Water (AW) layer. In the Eurasian basin, studies have focused on the role and variability of warm AW eddies transporting heat to the interior. In this study, we use an eddy-resolving version of the NEMO model in the Arctic to highlight the presence of AW eddies with significantly different properties in different regions of the basin. In all regions, cyclones and anticyclones with opposite properties are detected. Warm-core anticyclonic eddies are mostly found in the western Nansen Basin, where they can persist for longer than a year, never crossing the Gakkel Ridge. Further downstream along the boundary current, we show the presence of cold anticyclonic eddies likely forming at the front between the cold Barents Sea branch and the warm Fram Strait branch in the Eastern Eurasian Basin, and of anticyclonic eddies transporting both cold halocline anomalies and warm anomalies in the AW layer along the Gakkel Ridge. These three types of eddies present different seasonal and interannual variability patterns, suggesting that different drivers are responsible for their formation.

——————————————————————
Adrien Klotz

Title: Nearshore environment from the ocean to the shore: remote sensing monitoring and 3D hydro-morphodynamic modeling.

Abstract: Nearshore, the sea to land interface offers favorable development conditions for many living beings, including human communities and their related activities. The nearshore environment is among the most dynamic geomorphological environments on earth, putting it under pressure from climate change. From the reinforcement of strong weather events, such as storms, to sea-level rise, the large spatio-temporal variability of the processes governing the evolution of the nearshore environment requires monitoring with a high temporal frequency and fine spatial resolution. This presentation will first present the recent developments of remote sensing techniques – using satellites and Unmanned Aerial Vehicles (UAVs or drones) – to conduct large scale surveys of the nearshore environment including bathymetry mapping, sandbar detection and shoreline delineation. Combined with surveys, models are used to forecast the evolution of the nearshore. A focus on the 3D hydro-morphodynamic modeling of sandbars – one of the most common morphological features of the nearshore environment – using the model CROCO in its phase-averaged mode, will present its first application on a natural beach. The results demonstrate the ability of CROCO to predict the onshore and offshore migration of a sandbar when only wave condition forcing is changed, addressing a long time modeling challenge.

45-55-417 and zoom link : https://cnrs.zoom.us/j/99773209411?pwd=MqH7YEbJagutooSQAELgb2XkI4BFt8.1

Introduction to my previous work on biologging and seabirds
Abstract:
In this talk, I will introduce a series of studies in which I have explored various aspects of seabird behavior using bio-logging techniques. Topics include:
– Ocean wave observation using motion records of seabirds
– Take-off behavior of wandering albatrosses in relation to environmental conditions
– Deep-sea foraging behavior of king penguins observed by video logger
– Excretion behavior of flying seabirds
https://ird-fr.zoom.us/j/97485060550?pwd=8p9Vb9SZ8kUGN9IbNZkmb7q3nqSsQg.1