Félicitations à Floriane Sudre qui a soutenu sa thèse le 14 mars 2024

Sur le sujet suivant : Past and future variability of ocean fronts in the Mozambique Channel and influence on ecosystems



La composition du jury de thèse :

M. Vincent ROSSI - Directeur de thèse : vincent.rossi@mio.osupytheas.fr
M. Boris DEWITTE - Directeur de thèse : bxd.dewitte.legos@gmail.com
M. Lionel RENAULT - Rapporteur : lionel.renault@ird.fr
M. Michael ROBERTS - Rapporteur  : mike.roberts@mandela.ac.za
Mme Anne MOLCARD - Présidente : anne.molcard@mio.osupytheas.fr
Mme Marina LEVY - Examinatrice : marina.levy@locean.ipsl.fr
Mme Véronique GARÇON - Invitée : veronique.garcon.legos@gmail.com


Résumé :

Fronts are ubiquitous ocean structures delineating transitions between waters of distinct properties. Previous studies have shown how they can be major actors of climate variability as well as hotspots of biodiversity, thereby drawing a significant cross-disciplinary interest. Fine-scale fronts in particular, lasting for a few days to a few weeks over spatial scales of tens to hundreds of kilometers, are difficult to study on multidecadal timescales. Their short life-cycle and unpredictability makes them elusive to in-situ observations, while their fine spatial scale involves hefty instrumental and computational costs in satellite and modeling studies. The Mozambique Channel (MC) is an ideal laboratory to study those fine-scale fronts due to its energetic flow and rich biodiversity. The MC is also sensibly-sized which enables high-resolution multidecadal modeling analyses. This thesis thus aims to propose a description of where and when fronts are most frequent in the MC and how this variability responds to projected climate warming.

We first analyze satellite and modelled sea temperature data to describe the seasonal variability of thermal fronts in the MC over the past decades and up to 1000 m. Thermal fronts are identified via statistical thresholding as the top 30% of the thermal gradient distribution in the MC. We show that horizontal thermal gradients peak below the surface mixed layer, suggesting that they are largely associated to dynamics of the ocean interior rather than to mixed-layer processes. We also find that front dynamics are regionally dependent, and we argue that the seasonal variability of thermal fronts in the MC is driven by both the mesoscale eddy activity and regional upwelling cells. 

Observational trends exhibit a significant change in the eddy activity within the MC in response to climate warming. Hence, and in light of the first results, we hypothesize that changes in eddy activity would result in significant changes in front activity as well. We thus perform a dynamical downscaling experiment using a high-resolution regional model to explore how eddy and front variability respond to projected warming in the MC. We find that both eddy and front activities are projected to increase in the northern MC and decrease south of 17°S, suggesting that the thermal fronts most affected by the warming perturbation are those forming on the edge of eddies. 
The regions of the MC most affected by the projected climate warming in turbulence and fronts overlap key habitats for marine megafauna. While several studies have shown that fine-scale fronts structure plankton ecosystems, little is known about their interactions with highly mobile marine megafauna. Investigating the relationship between marine megafauna and ocean fronts involves both high-resolution front maps and animal tracking data. To this end, we propose a ready-to-use and publicly-available high-resolution observational front dataset focused on the Southwest Indian Ocean. This dataset provides two complementary front diagnostics: thermal gradients (on a 1 km grid, over 2003-2020) and attracting Lagrangian Coherent Structures (on a 1.56 km grid, over 1994-2020). Fronts are then identified with statistical thresholds, which are included into the dataset for relevant ecoregions.
The comprehensive examination of past and future variability of fine-scale ocean fronts presented in this thesis leads to significant implications for marine ecosystems and fine-scale biogeochemical fluxes within the MC, which are yet to be determined. The findings brought by this research can contribute to developing conservation areas that are both aware of ocean fronts and adaptive to climate changes.

Keywords : mesoscale, fine scale, eddies, ocean fronts, Indian Ocean, Mozambique Channel

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