Beyond Forcing Scenarios: Predicting Climate Change through Response Operators in a Coupled General Circulation Model

Abstract:

At least since the "cold-start problem” (Hasselmann et al. 1993), the climate response to external forcing has been often considered a promising application for the response operators. The so-called “impulse-response” approach has been tackled empirically, as well as from a theoretical point of view. Recently, the Ruelle’s response theory (RRT) has provided a solid mathematical background for application of these methods in a wide range of forced non-equilibrium systems, where the forcing can influence the statistics of the system’s internal variability. Here, we consider a coupled global climate model, MPI-ESM v1.2 to show how the linear response is able to assess the role of fast (atmospheric) and slow (oceanic) modes of the response. Over a wide range of timescales (from decadal to millennial), we investigate the predictability of the near-surface temperatures, the ocean heat uptake, and some features of the large-scale oceanic circulation, such as the Atlantic Meridional Overturning Circulation (AMOC) and the Antarctic Circumpolar Current (ACC). We also consider a key regional aspect of the response, analyzing the temperatures in the North Atlantic. For the first time, the RRT is here applied to a state-of-the-art model, that is typically used for the assessment of long-term climate projections, providing indications for an efficient setup of model experiments with appropriate forcing scenarios.