Beyond Forcing Scenarios: Predicting Climate Change through Response Operators in a Coupled General Circulation Model: 1% annual CO2 increase ramp experiment
Project: The response theory as a tool for investigating climate predictability and scale separation - The Collaborative Research Centre TRR181 “Energy Transfers in Atmosphere and Ocean” is an inter-institutional project funded by the German Research Foundation (DFG). Its aim is contributing to a better understanding of the energy cycle of the atmosphere and oceans through its fundamental dynamical regimes, i.e. the small-scale turbulence, internal gravity waves and geostrophically balanced motion. More specifically, the final task is to reduce model inconsistencies and the resulting relatively large energy biases. The specific aim of the S subproject is to develop metrics and diagnostics, in order to quantitatively address model inconsistencies and eventual improvements, as a consequence of better parametrizations of currently poorly understood processes. In this respect, the statistical mechanical formalism of the response theory (see Ruelle et al., 2009 for a review) is crucial to predict the climate response and disentangle the role of individual forcings (Ghil, 2015). This is the natural front-end of the effort for a better implementation of models energetics (Lucarini et al., 2014), given that the forcings alter one or several components of the energy exchanges in the system, either directly or through feedbacks. The response theory is relevant for the TRR-181, also because it provides tools for the investigations of energy conversion through scales, providing hints on the separations of scales between atmosphere and oceans by means of the so-called “susceptibility function” (e.g. Ragone et al., 2015). The response theory is here applied to the MPI-ESM-1.2 coupled model, extending a previous study based on an atmospheric intermediate complexity model (Lucarini et al., 2016). The aim is here to encompass the long timescales spanned by the ocean's variability. Summary: MPI-ESM model (T31 resolution) v. 1.2: ramp forcing experiment with 1% CO2 concentration increase every year until doubling with respect to pre-industrial values. A ramp forcing experiment with linear increase in CO2 concentrations by 1% every year, starting from t=0: when doubled concentration is achieved, the concentration is kept steady until the end of the run (1000 years). The experiment is performed with MPI-ESM model, coarse resolution (CR: T31). It consists of an ensemble with 20 runs, starting from a set of perturbed initial conditions representative of pre-industrial CO2 levels (same as for the 2xCO2abrupt experiment, see doi:10.26050/WDCC/2xCO2abrupt). The experiment is aimed as a testbed for the Green’s functions computed via the 2xCO2abrupt experiment. This is a model application of the linear response theory, as described in Lembo et al. 2020.
