The Green’s Function Model Intercomparison Project (GFMIP) Protocol

Abstract

The atmospheric Green’s function method is a technique for modeling the response of the atmosphere to changes in the spatial field of surface temperature. While early studies applied this method to changes in atmospheric circulation, it has also become an important tool to understand changes in radiative feedbacks due to evolving patterns of warming, a phenomenon called the “pattern effect.’ To better study this method, this paper presents a protocol for creating atmospheric Green’s functions to serve as the basis for a model intercomparison project, GFMIP. The protocol has been developed using a series of sensitivity tests performed with the HadAM3 atmosphere-only general circulation model, along with existing and new simulations from other models. Our preliminary results have uncovered nonlinearities in the response of the atmosphere to surface temperature changes, including an asymmetrical response to warming vs. cooling patch perturbations, and a change in the dependence of the response on the magnitude and size of the patches. These nonlinearities suggest that the pattern effect may depend on the heterogeneity of warming as well as its location. These experiments have also revealed tradeoffs in experimental design between patch size, perturbation strength, and the length of control and patch simulations. The protocol chosen on the basis of these experiments balances scientific utility with the simulation time and setup required by the Green’s function approach. Running these simulations will further our understanding of many aspects of atmospheric response, from the pattern effect and radiative feedbacks to changes in circulation, cloudiness, and precipitation.

Circus Tents, Convective Thresholds, and the Non‐Linear Climate Response to Tropical SSTs

“…Preliminary work as part of the Green's Function Model Intercomparison Project (GFMIP, Bloch‐Johnson et al. (2023)) has also demonstrated similar non‐linearities in five other GCMs, suggesting our results are not model‐specific (B. Zhang et al., 2022). This does not mean the Green's function approach is without merit, but suggests that future work should focus on mapping the TOA response across multiple ΔSST values for each location and understanding the responses in isolation before combining them so as to minimize the risk of introducing compensating errors.…”

“…This does not mean the Green's function approach is without merit, but suggests that future work should focus on mapping the TOA response across multiple ΔSST values for each location and understanding the responses in isolation before combining them so as to minimize the risk of introducing compensating errors. This is currently being undertaken in a multi‐model context as part of the GFMIP project (Bloch‐Johnson et al., 2023). A particular focus of future work should be on understanding how SST perturbations alter the distribution of subcloud moist static energy, particular over the perturbed region, and understanding what factors set the shape of the tropical “circus tent” and its response to forcing.…”

Surface Temperature Pattern Scenarios Suggest Higher Warming Rates Than Current Projections

“…We find that a warming in the equatorial West Pacific has a greater impact on the SW feedback and MSCD (nonlocal) than a warming in the tropical East Pacific (local). We do not expect our results to qualitatively depend on the model we are using, since the GFs in other studies are spatially similar to ECHAM6 (Bloch‐Johnson et al., 2023; Dong et al., 2019; Zhang et al., 2023; Zhou et al., 2017).…”

The Green's Function Model Intercomparison Project (GFMIP) Protocol

“…The analysis in this article were performed using a Jupyter notebook running Julia 1.7.1 (Bloch-Johnson, 2023). The notebook uses data stored in the JLD2 format (Bloch-Johnson et al, 2023), both available at Zenodo.…”

“…The analysis in this article were performed using a Jupyter notebook running Julia 1.7.1 (Bloch-Johnson, 2023). The notebook uses data stored in the JLD2 format (Bloch-Johnson et al, 2023), both available at Zenodo. Boundary conditions for running GFMIP experiments, as well as more resources, are available at https://gfmip.org.…”