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: step forcing experiment with abrupt doubling of CO2 concentrations A step forcing experiment with instantaneous doubling of CO2 concentrations at t=0. The experiment is performed with MPI-ESM model, coarse resolution (CR: T31). It consists of an ensemble with 20 runs over 2000 years, starting from a se top perturbed initial conditions representative of pre-industrial CO2 levels. The experiment is aimed at computing the Green’s function of observable, whose convolution with the time evolution of a forcing allows for prediction of the long-term evolution of the forcing in response to a given forcing. This in the context of the linear response theory (see Lembo et al. 2020).
