PHD defense: « Influence of salinity on the coupled ocean-atmosphere dynamics in the tropical Atlantic Ocean »

– Manon GÉVAUDAN – Soutenance de thèse – LEGOS, Toulouse, France –

Jury

Gilles Reverdin (LOCEAN) : Reviewer
Gregory Foltz (NOAA) : Reviewer
Sabrina Speich (LMD) : Examiner
Gaëlle de Coëtlogon (LATMOS) : Examiner
Hervé Giordani (CNRM) : Examiner
Sébastien Masson (LOCEAN) : Examiner
Julien Jouanno (LEGOS) : PhD Supervisor
Fabien Durand (LEGOS) : PhD Supervisor

Abstract

The tropical Atlantic Ocean presents a highly contrasted surface salinity, with low surface salinity in the western and central parts of the basin. This low salinity is due to an important freshwater supply from large rivers such as the Amazon, Orinoco and Congo, and from heavy precipitation in the intertropical convergence zone. This results in a strong salinity stratification that may influence vertical mixing in the upper ocean, and thus the sea surface temperature (SST) and air-sea fluxes. The aim of this thesis is to investigate the influence of salinity – and especially the strong salinity stratification – on the tropical Atlantic Ocean climate. 

In order to account for possible feedbacks of the salinity stratification on the atmosphere, a 1/4° coupled ocean–atmosphere model of the tropical Atlantic is developed. A series of sensitivity experiments is conducted to assess the impact on SST and air-sea fluxes of increasingly detailed processes:  the total salinity stratification in the present and in a future climate, the Amazon and Orinoco rivers, and the Amazon extreme floods.

A consistent mechanism emerges. The presence of strong salinity stratification leads to a warming of the ocean surface and an increase in convection and precipitation, but the warming is limited by a negative feedback from the atmosphere, which consists in an increase of the latent heat loss and a reduction in shortwave radiation reaching the ocean surface. The final change in SST is determined by the balance between the warming due to the vertical mixing and the cooling due to the atmospheric feedback, leading to an increase in SST of 0.5°C maximum. The influence of salinity stratification has a marked seasonality, with much weaker impact in winter because of a deeper mixed layer at this time. SST changes of opposite sign are finally observed in the cold tongue region, related to changes in the thermocline depth, and remote influence of the salinity stratification.

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Zoom link: https://cnrs.zoom.us/j/96700598474?pwd=LzRBUDVxait3MEE0Tm1qOCtlY1BxQT09

Meeting ID: 967 0059 8474
Password: XV80Ep