212 Observing and modelling anomalous temperature of Sargassum aggregations

Job title:

212 Observing and modelling anomalous temperature of Sargassum aggregations

Company:

Centre National d’Etudes Spatiales

Job description

25-212 Observing and modelling anomalous temperature of Sargassum aggregationsPostuler25-212 Observing and modelling anomalous temperature of Sargassum aggregations

• Doctorat, 36 mois
• Temps plein
• Indifférent
• Maitrise, IEP, IUP, Bac+4
• Oceanography

PostulerMissionSince 2011, there has been a massive proliferation of Sargassum in the tropical Atlantic, with significant ecosystem and socio-economic consequences. In the context of forecasting and research into the causes of the phenomenon, LEGOS has pioneered the development of transport/growth/sinking models for Sargasso (Jouanno et al. 2021, 2023). While we are now able to represent seasonal variations and recent proliferation, there are many areas for improvement to gain a better understanding of inter-annual variations and longer-term trends.A key factor to consider is the specific temperature of the rafts. Indeed, the growth characteristics of Sargassum have been shown to have a first-order dependence on temperature, with an optimal range for growth appearing to be around 26-28°C and potentially accelerated mortality above 30°C. On the other hand, in-situ and satellite observations have shown that the temperature of the rafts differs from that of the environment (up to +1.5°C; Gulick et al. 2023). This warming could be related to the absorption of solar radiation by the algae, but also to a limitation of vertical mixing by the Sargassum biomass, although it is not yet clear which process is dominant. In addition, this warming is likely to cause thermal stress in the aggregations through threshold effects and contribute to the massive seasonal decline in biomass around July-August each year, determining the amount of Sargassum that will seed the following year’s bloom.The specific aim of this PhD project is to improve our understanding of the sensitivity of Sargassum raft temperature properties and its consequences on basin scale distribution through modulation of growth/mortality characteristics. This work will be based on joint Sargassum detection and surface temperature estimates at high resolution using Landsat imagery and environmental conditions obtained using multi-sensor observations. It will be combined with process studies based on Sargassum modelling (fine scale with CROCO-NH and basin scale with NEMO-Sarg).The doctoral project will be carried out in three stages:1. Produce a joint dataset of surface temperature and Sargassum detections from Landsat thermal and optical sensors (TIRS and OLI) over all available acquisitions in the North Tropical Atlantic Basin (5°S-50°N). The Sargassum detections will be based on a classical index approach coupled with learning algorithms that have proven effective for other missions (S2-MSI, Venus-VM5). The surface temperatures will be validated by in-situ observations from drifting buoys deployed within the rafts.2. Relate raft temperatures to environmental conditions (raft size, ocean-atmosphere heat flux, sea state, mixed layer depth, salinity) using a multi-sensor approach. This satellite approach will be complemented by an analysis of fine-scale CROCO-NH simulations of Sargassum aggregations (currently being developed at LEGOS), in particular to gain a better understanding of temperature changes on a diurnal scale and the controlling processes (mixing, air-sea fluxes, solar absorption). A parameterization of the raft temperature under varying environmental conditions will be proposed.3. Investigate the consequences of raft temperature anomalies on the large-scale distribution of Sargassum. To this end, we will mainly rely on sensitivity tests with the NEMO-Sarg model , in which a raft temperature parameterisation will be integrated.The PhD project is a contribution to the TOSCA SARGAT project (Pelagic Sargassum aggregations and transport). In-situ drifting buoy with temperature sensors embedded in Sargassum mats are expected to be deployed near the Lesser Antilles by CLS as part of the SARGCOOP2 project (submitted), and complementary funding will be sought for further deployments in other regions (Mexico, Florida). The project will rely on remotely sensed ocean color Sargassum detections (MODIS, S3-OLCI, S2-MSI, GOES), which will allow us to estimate real growth/mortality rates at the basin scale in order to validate the model. The improvement of NEMO-Sarg, and in particular a better representation of the mortality characteristics under warm temperature conditions, will benefit the Sargassum seasonal forecast operated by Mercator Ocean as part of the SCO-SESAM project. This PhD project will be particularly valuable in the context of the future TRISHNA mission (scheduled for 2026), which will provide new infrared data of sea surface temperature at a high temporal frequency in coastal areas.-Jouanno et al (2023). Skillful seasonal forecast of Sargassum proliferation in the Tropical Atlantic. Geophysical Research Letters, 50, e2023GL105545.-Jouanno et al. (2021). A NEMO-based model of Sargassum distribution in the Tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0), Geosci. Model Dev.-Gulick et al. (2023). Holopelagic Sargassum aggregations provide warmer microhabitats for associated fauna. Sci Rep 13, 15129.For more Information about the topics and the co-financial partner (found by the lab !);contact Directeur de thèse –Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 14th, 2025 Midnight Paris time !ProfilMSc in oceanography or remote sensing. Experience in teledetection and/or numerical modeling. Good skills for team work and rigorous analysis.

Expected salary

Location

Toulouse

Job date

Wed, 05 Feb 2025 06:56:31 GMT

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