PhD position: The role of the upper ocean for ocean heat storage and climate


Supervisors:

J-B Sallée and Casimir de Larvergne

Project description:

Climate change is traditionally measured by the change in global mean surface temperature, related to the so-called climate sensitivity (transient, or at equilibrium). Since the early days of climate sciences, the questions of what is and what sets Earth climate sensitivity have been two central challenges. They are still much debated in the scientific community today, because the feedback mechanisms that determine climate sensitivity are complex and difficult to quantify.

The ocean plays a central role in setting the rate of global surface warming because of its enormous heat capacity relative to the atmosphere. In particular, the rate at which heat is transferred from the surface to deeper ocean has been shown to be an important source of spread in simulations of 21st century climate change (Boé et al., 2009). The main aim of this project is to investigate the role of the upper ocean in constraining the amount of heat that can be absorbed by the global ocean, thereby influencing Earth climate sensitivity.

In this project, we will investigate (i) upper ocean processes that set the rate at which heat is sequestered in the deeper ocean (the ocean heat uptake efficiency); (ii) how the representation of upper ocean processes and properties relates to the spread in ocean heat uptake efficiency in the current generation of climate models; (iii) the potential to better constrain future ocean heat uptake by combining climate model simulations with historical in-situ observations of the upper ocean; (iv) the relationship between ocean heat uptake and Earth climate sensitivity.

The rate at which heat is transferred from the surface to deeper ocean is controlled by the ocean thermocline ventilation and deep convection. The first task of the PhD project will be to probe models of ocean and climate to relate global ocean heat uptake efficiency to mixed-layer depth and thermocline stratification. This analysis will draw upon simple conceptual models of ocean and climate that have investigated ocean thermocline ventilation since the early days of oceanography, as well as upon state-of-the-art coupled climate models. In particular, we will explore the inter-model spread of climate models to seek underlying relationship between their representation of the upper ocean (mixed-layer depth, stratification) and ocean heat uptake efficiency. Particular attention will be given to the Southern Ocean, which is a known key region for the ventilation of the global deep ocean. Sensitivity analysis will be conducted by running targeted NEMO runs, or by using existing runs to better understand the mechanisms explored from climate models and related them to future changes.

Building on the analysis of climate models and a recent synthesis of the observed changes in upper-ocean structure (Sallée et al. 2021), we will look for observation-based constraints on ocean heat uptake efficiency. Changes with time of the ocean heat uptake efficiency, following ocean and climate warming, will be considered.

Finally, the link between the project and earth climate sensitivity will be dependent on the advancement of the other steps. It will either be approached as a discussion point of the thesis, or more formally by dwelling into Equilibrium Climate Sensitivity and Transient Climate Sensitivity analysis to be link with representation of upper ocean and global ocean heat content.

Beyond their role for heat uptake, mixed-layer depth and stratification are crucial ingredients shaping marine ecosystems, oceanic carbon uptake and oxygenation (Kwiatkowski et al., 2020). These are key themes studied at LOCEAN and within the PROTEO team, so that the student will have opportunities for broader interactions and collaborations.

References:
  • Boé, et al (2009), Deep ocean heat uptake as a major source of spread in transient climate change simulations, Geophys. Res. Lett., 36, L22701, doi:10.1029/2009GL040845.
  • Li et al., 2019, Comparing Ocean Surface Boundary Vertical Mixing Schemes Including Langmuir Turbulence. J. Adv. Model. Earth Syst. 11, 3545–3592.
  • Kwiatkowski et al., 2020, Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections. Biogeosciences 17, 3439–3470. doi:10.5194/bg-17-3439-2020.
  • Sallée et al., 2021, Summertime increases in upper ocean stratification and mixed layer depth, Nature, accepted.

Further details can be obtained by contacting Jean-Baptiste Sallée (jean-baptiste.sallee@locean.ispl.fr)

Application:

Please concact Jean-Baptiste Sallée (jean-baptiste.sallee@locean.ispl.fr) with a CV before the end of april, and as soon as possible. The supervisors will then select a candidate, who will then go through a competetive selection process of Sorbonne Université, from May to June. Final decision from the university will be made early July.

Starting date:

October 2021

Location:

The succesfull applicant will be employed by and student at Sorbonne University, and based in the university department called LOCEAN. LOCEAN is a joint research laboratory gathering scientists from: (i) Sorbonne Université, (ii) Muséum National d’Histoire Naturelle, (iii) Centre National de la Recherche Scientifique and  (iv) Institut de Recherche pour le Développement. It houses roughly 150 staff, two thirds of which being either researcher, university lecturer/professor, Ph.D. students or post-doctoral fellows. LOCEAN is part of the IPSL consortium, and is based at the campus of Sorbonne Université, in the heart of Paris.

Salary range: 

The salary will match the phD salary set by the French government which is around 1800 euros per month (before tax). 

Indication of the skills required: 

CHARACTERISTICS: DESIRABLE REQUIREMENTS:
Skills / Experience Very good competence of analysis of large ocean observation databases or numerical models, and experience of conducting innovative studies based on those to develop understanding of ocean dynamics. Excellent understanding of ocean and climate dynamics
Qualifications Master in oceanography, meteorology, climate dynamics, applied mathematics, or a relevant scientific discipline; or equivalent degree
Numerical ability Ability to apply advanced numerical and statistical methods to analysis of model output or observations.
Computer / IT skills Experience of running scientific code (e.g. Matlab, python, Fortran) and skills in advanced computing techniques.
Managerial ability Ability to manage project workflow.
Communication skills Fluent in written and spoken English language. Ability to present research effectively in written and oral form. (No criteria in ability to speak in French)
Decision Making Ability to make sensible decisions under guidance from the team
Interpersonal skills Able to work as part of a team within LOCEAN-IPSL and to engage with external collaborators. Flexible and adaptable