Forcing and feedback in the MPI‐ESM‐LR coupled model under abruptly quadrupled CO₂

Abstract

[1] Radiative feedback mechanisms associated with temperature, water vapor, cloud, and surface albedo change determine climate sensitivity to radiative forcing. Here we use the linearized radiative kernel-technique in combination with a Gregory analysis to determine the strength and structure of feedbacks, as well as direct and adjusted CO 2 forcings in the coupled Max Planck Institute Earth System Model at base resolution (MPI-ESM-LR). We show that the combined Kernel-Gregory approach yields an elegant separation of surface temperature-dependent feedbacks from contributions to radiative forcing by fast adjustments. MPI-ESM-LR exhibits a relatively large cloud adjustment of nearly 2 W m 22 in direct response to quadrupled CO 2 , with positive cloud adjustment evident throughout the tropics, subtropics and over most landmasses whereas midlatitude storm tracks contribute negatively. The model features a nonlinear regression of radiation imbalance to global mean surface temperature change, resulting in a significantly increasing effective climate sensitivity after about 20 years which is approximately at temperatures 4-5 K above preindustrial. This feature is not uncommon among climate models and is relevant for future climate projections. We analyze the contribution of the individual feedback processes to this behavior and discuss possible origins such as differential ocean warming patterns associated with deep-ocean heat uptake or state dependencies of the feedback processes.Citation: Block, K., and T. Mauritsen (2013), Forcing and feedback in the MPI-ESM-LR coupled model under abruptly quadrupled CO 2 , J. Adv. Model. Earth Syst., 5, 676-691,

Surface and top-of-atmosphere radiative feedback kernels for CESM-CAM5

“…Example code for applying the kernels is available at https://github.com/apendergrass/cam5-kernels. Soden et al (2008), and MPI-ESM-LR control state kernels and years 21-150 of abrupt carbon dioxide quadrupling simulations from the same model (Block and Mauritsen, 2013 We apply the radiative kernels to the CESM largeensemble integrations to diagnose the top-of-atmosphere radiative feedbacks. The changes in surface and tropospheric temperature, water vapor mixing ratio, surface albedo, and cloud radiative effect are calculated from 30-year averages for each month from each ensemble member, 1976-2005 and 2071-2100.…”

“…Instead, it may be caused by differing upper tropospheric temperature amplification between the CMIP3 and CESM largeensemble simulations or due to underlying differences in the radiation codes. Block and Mauritsen (2013) report radiative feedbacks using MPI-ESM-LR kernels applied to abrupt carbon dioxide quadrupling experiments with the same model (they compare kernels calculated from different base states and apply them to short transient response and more developed long-timescale response; we compare with their control base-state kernels applied to long-timescale climate response because this is most similar to our application). There is remarkably close agreement for all feedbacks, excepting only the water vapor feedback.…”

“…Other kernels include those from CAM3 and more recently from the MPI-ESM-LR model (Block and Mauritsen, 2013). The only publicly available model-based surface radiative kernels are from ECHAM5 (Previdi, 2010) and MPI-ESM-LR, which is a more recent version of the ECHAM model (discussed in Fläschner et al, 2016).…”

“…Reanalysis-based kernels generated with ERA-Interim and RRTM are also available (Huang et al, 2017). Not all kernels have been validated to test the accuracy to which total radiative fluxes from a model simulation can be recovered with kernel calculations; examples that have been validated against model calculations of radiative flux due to doubling of carbon dioxide include Shell et al (2008), Block and Mauritsen (2013), and Huang et al (2017).…”

Climate feedback efficiency and synergy

“…Possible non-linearities due to statedependencies of the feedback mechanisms, which become relevant typically when models are forced harder with e.g. quadrupled CO 2 (Jonko et al 2012;Block and Mauritsen 2013) are beyond our scope.…”