[ES_JOBS_NET] Postdoc position on global ocean ventilation and climate change at Sorbonne University (France)

[Casimir de Lavergne]

Human influence on global ocean ventilation

The World Ocean has crucial climatic and life-supporting roles. We know that these roles depend critically on the “ventilation” of the deep ocean, i.e., the transport of tracers (heat, carbon, oxygen, nutrients) between the surface and the deep seas. However, too little is known about the processes shaping deep-ocean ventilation and its changes. The Southern Ocean is thought to have an outsized role in connecting surface and deep waters: about 60% of the total global ocean volume had its last surface contact or will have its next surface contact south of 40oS (de Vries and Primeau, 2011). A key unknown of the ventilation puzzle concerns the return path from the interior ocean to the Southern Ocean surface, which involves wind-driven vertical circulation as well as mixing processes, and which controls the rate at which the large carbon and nutrient deep-ocean reservoirs communicate with the surface (e.g. Le Queré 2009; de Vries 2017). The central objective of the project is to understand the drivers of this return path from the global deep ocean to the Southern Ocean surface, and their ability to change under on-going climate change.

Ocean ventilation can occur either through advection of material as part of the global ocean overturning circulation, or through diffusive mixing. While the resurfacing of deep waters in the Southern Ocean has been described as primarily driven by winds (Marshall and Speer, 2012), the local and large-scale impacts of interior mixing hotspots remain unclear. The first objective of this project will be to quantify the impact of varying interior mixing intensity versus winds on the resurfacing of deep waters in the Southern Ocean. This will be addressed by sensitivity experiments using varying mixing schemes in an existing configuration of the NEMO global ocean model (used in the IPSL climate model).

The second objective of the project will be to delineate the processes driving Southern Ocean overturning circulation change in the context of ongoing climate change. The ocean responds to climate change through modifications of heat, freshwater and momentum fluxes at its boundaries. Recent work has started to highlight the potential important contribution of thermodynamic drivers (change in air-sea heat fluxes) over mechanical drivers (change in momentum fluxes) in impacting some large horizontal current systems worldwide (Shi et al., 2021; Peng et al., 2022). But the relative importance of these drivers on the vertical circulation, as well as the time evolution of the respective role of each driver under past and future climate change remains a knowledge gap. This gap will be addressed in this project from a newly developed modeling framework, allowing to isolate the contributions of heat, freshwater and momentum fluxes at the ocean surface to drive ocean change (Silvy et al., 2022a). We will leverage existing simulations that have been run based on a large ensemble experiment (historical+SSP2-4.5) developed for CMIP6 with the IPSL climate model (Silvy et al., 2022a,b).

APPLICATION:

Please contact Jean-Baptiste Sallée (jean-baptiste.sallee at locean.ipsl.fr) and Casimir de Lavergne (casimir.delavergne at locean.ipsl.fr) with a CV before mid-December, and as soon as possible.

STARTING DATE:

As soon as possible

DURATION:

Depending on the starting date. End of the postdoc in October 2024 (due to funding availability). There are good prospects to continue employment through an alternative research position.

SUPERVISORS:

J-B Sallée and Casimir de Lavergne

Collaborators:

Y. Silvy (Uni. Berne), J. Mignot (LOCEAN), E. Guilyardi (LOCEAN)