Ocean heat and carbon storage under ambitious emission mitigation: Uncertainties due to the representation of ocean mesoscale eddies in a non-eddying Earth system model

• Applicant: Ivy Frenger, Ph.D.
• Subject Area: Oceanography
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 538865427

The ocean must be accounted for in climate mitigation efforts because of its absorption of anthropogenic carbon dioxide emissions (Canth) from fossil fuel burning and resultant excess heat (Hanth) in the Earth system. It is not well understood how ocean storage of Canth and Hanth will respond to ambitious mitigation of carbon dioxide (CO2) emissions consistent with the Paris Agreement of well below 2oC warming in 2100. Ocean storage of Canth and Hanth both are driven by ocean transport and mixing processes. Ocean phenomena at the mesoscale (scales of around 100 km, here referred to with "eddies") transport and mix waters across the ocean and thus also Canth and Hanth, and are known to impact ocean storage of Canth and Hanth under net-positive CO2 emissions. Yet, eddies are not resolved in IPCC (Intergovernmental Panel on Climate Change)-type Earth system models that are used to project climate. Instead, effects of eddy mixing and transports are "parameterized", that is accounted for based on ocean properties that are resolved, such as the large-scale ocean density structure and spatial gradients of carbon and heat. In ongoing work I find that an eddy parameterization is not capable of reproducing effects of resolving eddies on ocean storage of carbon and heat under net-positive emissions. Here I will explore uncertainty in ocean storage of Canth and Hanth, and global warming due to ocean mesoscale eddies in a non-eddying Earth system model under ambitious emission mitigation. To do so, I will modify eddy parameterizations in a non-eddying Earth system model. I will do so individually for eddy transport and mixing, effects that have been suggested to partially compensate, and focus on the importance of the vertical structure of eddy effects which has been suggested to be important. One criteria of evaluation of results will be if I can reproduce effects of resolving ocean eddies on ocean storage of Canth and Hanth. I will then test how changing the parameterizations affects ocean storage of carbon, heat, and global warming under ambitious emission metigation that includes net-zero and net-negative CO2 emissions in the second half of the century. To put results into perspective I will compare the impact of changing eddy parameterizations to the magnitude of natural variability, based on a small ensemble of simulations with three members. Outcomes of this project will be an improved grasp of uncertainty in climate projections due to the representation of ocean mesoscale eddies under net-zero and net-negative emissions, adding to a robust understanding of global warming under ambitious emission mitigation.

• DFG Programme: Research Grants