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Thèse « A climate perspective on pelagic Sargassum distribution: scenarios for proliferation and coastal vulnerability by 2100  » (F/H)

    Description

    The PhD thesis will be carried out at CNRM (Centre National de Recherches Météorologiques) in the IOGA (Interactions Océan-Atmosphère-Glace) Team of the GMGEC department, with regular exchanges with the LEGOS Dynotrop team (and possibly long-term visits of several months), made possible by the geographical proximity of the two sites in Toulouse.

    https://meteofrance.com/carte-didentite-de-meteo-france


    The Large Scale Meteorology and Climate Department (GMGEC, Toulouse) focuses on understanding scale interactions, interactions between the various components of the climate system, the response of the climate system to anthropogenic forcing, and sources of variability and long-term predictability. These activities are carried out in particular through the modelling of climate, ocean, marine biogeochemistry, atmospheric composition and air quality at global and regional scales, participation in model intercomparison exercises such as CMIP or CORDEX and their analysis, impact studies and the detection-attribution of observed climate change.

    At CNRM, the production of long-range, high-resolution NEMO-PISCES simulations, which are a prerequisite for Sargasso simulations, is part of S. Berthet’s participation to different climate modelling intercomparison initiatives, as the Coupled Model Intercomparison Project (CMIP, that feeds IPCC reports), the Global Carbon Project (Friedlingstein et al., 2023), GCP- N2O (Tian et al., 2020; 2024’s update in revisions) and RECCAP2 (Resplandy et al., 2023). At LEGOS the thesis is part of Sargassum observations and modelling activities carried out by J. Jouanno (SCO SESAM project with CLS and MERCATOR for the operationalization of the seasonal prediction model developed at LEGOS) and the TOSCA SARGAT project, which deals with the question of transport and quantification of coastal stranding. These projects will provide a collaborative framework and environment conducive to the progress of the thesis. Interactions are also expected with the ANR BIOMASS (PI. L. Berline) and the collaborative network set up by J. Jouanno as part of the ANR FORESEA, in particular with the LARGE in Guadeloupe. Computing resources will be requested for Irene SKL at the TGCC- CEA and for Belenos at Météo France, two machines with which the project leaders have experience.

    Missions

    Projet Européen Horizon Europe OptimESM (Optimal high resolution Earth System Models for exploring future climate change ; grant agreement No101081193)

    The European Horizon Europe "OptimESM" project aims to develop new climate models that combine high resolution with an unprecedented description of key physical and biochemical processes, and to provide long-term climate projections that better support policy and societal needs by providing guidance on regional climate change at different levels of global warming, the risk of abrupt Earth system changes at these warming levels, and the regional impacts resulting from such events.


    This PhD project aims to investigate the recent causes, but also the future of the massive proliferation of Sargassum in the tropical Atlantic. This pelagic macroalga has been present in the subtropical gyre and the Gulf of Mexico for centuries, but recent changes have led to its proliferation in a new area called the Great Atlantic Sargassum Belt, exposing new areas to its presence (Wang et al. 2019 ; Jouanno et al., 2024). Currently, the spread of pelagic Sargassum in the Atlantic is a concern for coastal populations in the Caribbean, Gulf of Mexico and West Africa. The development of a transport/physiology/stranding model of the Sargassum biomass (NEMO-Sarg, Jouanno and Benshila, 2020, Jouanno et al. 2021, Jouanno et a. 2023) has allowed realistic and multi-annual simulations to be carried out at the basin scale and to reproduce the tipping point that occurred in 2009/2010 in response to a strong NAO event, extending the range of the algae. The challenge now is to put this expansion into a climate perspective and to understand why NAO events in the past 70 years have not triggered such an expansion and whether other areas could be exposed in the future. 

    Our experience of modelling Sargassum distribution has revealed the very high importance of forcing NEMO-Sarg with accurate physical fields (currents, temperature, wind) and realistic nutrient fields. Until now, the strategy has been to use reanalysis (ERA5, GLORYS12, BIOMER) to force the Sargassum model over the recent period (Jouanno et al., 2021, 2024). The challenge now is to move on to climate scales. One hypothesis is that an eddy permitting resolution is needed to correctly represent circulation in the subtropical gyre, the near equatorial system and the Caribbean Sea.  Producing global physical-biogeochemical simulations at climate scale and intermediate resolution (1/4°) with a reasonable coast is now allowed by a new hybrid version of the oceanic component of CNRM-ESM2-1 (CNRM Earth System Model). This hybrid version has been developed jointly by CNRM, Cerfacs and Mercator-Ocean (Berthet et al., 2019; Bricaud et al., 2020; Maisonnave, 2020; Maisonnave et al., 2021; Maisonnave and Berthet, 2022). It is based on a 1/4° configuration of the ocean and sea ice model NEMO4.2-SI3 coupled to an externalized version of the marine biogeochemical model PISCESv2 at a lesser resolution (¾° horizontal resolution). This new externalised online coarsening protocol uses a new and dedicated version of the OASIS-MCT3 coupling interface developed by CERFACS, which allows the exchange of 3D fields . In this perspective, we propose to produce long term runs and projections with this new global hybrid model and combine them with NEMO-Sarg to provide insight into the current and future evolution of Sargassum biomass in the Atlantic Ocean.



    Based on this prototype, the PhD will examine the Sargassum seaweed issue in the context of climate change. It will delve into the factors that triggered the recent Sargassum tipping point, explore potential future scenarios, and assess the vulnerability of coastal regions. It is hoped that the PhD's findings will provide valuable insights for policymakers and stakeholders in developing effective strategies to manage and adapt to Sargassum blooms, and for the scientific community to better understand the changes that these massive strandings are likely to cause in tropical ecosystems. The specific objectives are as follows:

    1. Impact of climate variability on connectivity and biogeochemistry of the tropical Atlantic: The PhD will assess the ability of the hybrid ocean component of CNRM-ESM2-1 to represent the response of this region to climate variability in terms of circulation and biogeochemical variability.

    2. Sargassum in the present climate: The PhD will investigate the role of climate change (evolution of the circulation, atmospheric extreme event, surface warming, long term nutrient variability) and climate variability in participating to the 2010/2011 Sargassum tipping point. This will contribute to help consolidate and validate the modelling strategy to provide meaningful long term forecasts in (3).

    3. Global-scale proliferation scenarios for the period up to 2100: The PhD will explore the Sargassum response to different climate change scenarios (low and high emission), to anticipate potential expansions or retractions of the proliferation zone (in the Atlantic but also in other regions of the globe). This will provides knowledge on the risk of abrupt changes and tipping points with respect to Sargassum proliferation.

    The PhD will be carried out at CNRM in close collaboration with LEGOS.

    References:

    • Berthet, S., Séférian, R., Bricaud, C., Chevallier, M., Voldoire, A., & Ethé, C. (2019). Evaluation of an online grid-coarsening algorithm in a global eddy-admitting ocean biogeochemical model. Journal of Advances in Modeling Earth Systems, 11, 1759–1783. https://doi.org/10.1029/2019MS001644 

    • Bricaud, C., Le Sommer, J., Madec, G., Calone, C., Deshayes, J., Ethe, C., Chanut, J., and Levy, M., 2020: Multi-grid algorithm for passive tracer transport in the NEMO ocean circulation  model: a case study with the NEMO OGCM (version 3.6), Geosci. Model Dev., 13, 5465–5483, https://doi.org/10.5194/gmd-13-5465-2020

    • Jouanno, J et al. (2021a). A NEMO-based model of Sargassum distribution in the Tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0). Geoscientific Model Development, 14(6), 4069-4086.

    • Jouanno, J. et al. (2021b). Evolution of the riverine nutrient export to the Tropical Atlantic over the last 15 years: is there a link with Sargassum proliferation?. Environmental Research Letters, 16(3), 034042.

    • Jouanno J., Benshila R. (2020). Sargassum distribution model based on the NEMO ocean modelling platform (0.0). Zenodo. [software]: https://doi.org/10.5281/zenodo.4275901. (last access 01/02/2023).

    • Jouanno, et al. (2023). Skillful seasonal forecast of Sargassum proliferation in the Tropical Atlantic. Geophysical Research Letters, 50(21), e2023GL105545.

    • Jouanno et al. (2024), Drivers of the pelagic Sargassum tipping point. Under review Nature Communications Earth and Environment.

    • Maisonnave, E., 2020: Ocean/Biogeochemistry macro-task parallelism in NEMO , Working Note, WN/CMGC/20/31, CECI, UMR CERFACS/CNRS No5318, France

    • Maisonnave, E., Berthet, S. & Séférian, R., 2021: OASIS based grid coarsening of TOP-PISCES biogeochemistry in the NEMO ocean model: performance , Technical Report, TR/CMGC/21/201, CECI, UMR CERFACS/CNRS No5318, France

    • Maisonnave, E. & Berthet, S., 2022: Biogeochemistry coarsening in NEMO 4.2 , Technical Report, TR/CMGC/22/86, CECI, UMR CERFACS/CNRS No5318, France

    LIEU D'EXERCICE DU POSTE
    Centre National de Recherches Météorologiques, 42 av. Gaspard Coriolis, 31057 Toulouse Cedex 01, France (https://www.umr-cnrm.fr/spip.php?rubrique89)

    with long-term visits at LEGOS, 14 av. Edouard Belin, 31400 Toulouse (https://www.legos.omp.eu/)

    Profil

    The candidate should hold a Master degree in oceanography, (geo)physics, climate/environmental sciences, applied math, computer science or a related field.

    We are looking for a creative personality, who will enjoy solving scientific problems independently as well as in team efforts. Strong interests in climate variability and impacts, analyses of large climate datasets, but also technical aspecs regarding the use of complex models on supercomputers, and good programming skills (especially FORTRAN90, Unix shell, knowledge of Python) will be an advantage.

    The candidate should have a good command of spoken and written English.

    Please forward :

    [1] a curriculum vitae (including a description of their Master internship, computing skills and different language practice...)

    [2] a brief statement of research interests

    [3] names and contact details (email + telephone number) of two academic referees

    Expected starting date is January 1st, 2025.

    Applications should be sent by email no later than November 15th, 2024.

    For further information, please contact : Sarah Berthet (CNRM, sarah.berthet@meteo.fr) or Julien Jouanno (LEGOS, julien.jouanno@ird.fr).