We conduct a series of eight 45-day experiments with a global storm-resolving model (GSRM) to test the sensitivity of relative humidity R in the tropics to changes in model resolution and parameterizations. These changes include changes in horizontal and vertical grid spacing as well as in the parameterizations of microphysics and turbulence, and are chosen to capture currently existing differences among GSRMs. To link the R distribution in the tropical free troposphere with processes in the deep convective regions, we adopt a trajectory-based assessment of the last-saturation paradigm. The perturbations we apply to the model result in tropical mean R changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The generated R spread is similar to that in a multi-model ensemble of GSRMs and smaller than the spread across conventional general circulation models, supporting that an explicit representation of deep convection reduces the uncertainty in tropical R. The largest R changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The R in the moist tropical regions is disproportionately sensitive to vertical mixing processes within the tropics, which impact R through their effect on the last-saturation temperature rather than their effect on the evolution of the humidity since last-saturation. In our analysis the R of the dry tropical regions strongly depends on the exchange with the extra-tropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions.

• This work documents ICON-ESM 1.0, the first version of a coupled model based 19 on the ICON framework 20 • Performance of ICON-ESM is assessed by means of CMIP6 DECK experiments 21 at standard CMIP-type resolution 22 • ICON-ESM reproduces the observed temperature evolution. Biases in clouds, winds, 23 sea-ice, and ocean properties are larger than in MPI-ESM. Abstract 25 This work documents the ICON-Earth System Model (ICON-ESM V1.0), the first cou-26 pled model based on the ICON (ICOsahedral Non-hydrostatic) framework with its un-27 structured, icosahedral grid concept. The ICON-A atmosphere uses a nonhydrostatic dy-28 namical core and the ocean model ICON-O builds on the same ICON infrastructure, but 29 applies the Boussinesq and hydrostatic approximation and includes a sea-ice model. The 30 ICON-Land module provides a new framework for the modelling of land processes and 31 the terrestrial carbon cycle. The oceanic carbon cycle and biogeochemistry are repre-32 sented by the Hamburg Ocean Carbon Cycle module. We describe the tuning and spin-33 up of a base-line version at a resolution typical for models participating in the Coupled 34 Model Intercomparison Project (CMIP). The performance of ICON-ESM is assessed by 35 means of a set of standard CMIP6 simulations. Achievements are well-balanced top-of-36 atmosphere radiation, stable key climate quantities in the control simulation, and a good 37 representation of the historical surface temperature evolution. The model has overall bi-38 ases, which are comparable to those of other CMIP models, but ICON-ESM performs 39 less well than its predecessor, the Max Planck Institute Earth System Model. Problem-40 atic biases are diagnosed in ICON-ESM in the vertical cloud distribution and the mean 41 zonal wind field. In the ocean, sub-surface temperature and salinity biases are of con-42 cern as is a too strong seasonal cycle of the sea-ice cover in both hemispheres. ICON-43 ESM V1.0 serves as a basis for further developments that will take advantage of ICON-44 specific properties such as spatially varying resolution, and configurations at very high 45 resolution. 46 Plain Language Summary 47 ICON-ESM is a completely new coupled climate and earth system model that ap-48 plies novel design principles and numerical techniques. The atmosphere model applies 49 a non-hydrostatic dynamical core, both atmosphere and ocean models apply unstruc-50 tured meshes, and the model is adapted for high-performance computing systems. This 51 article describes how the component models for atmosphere, land, and ocean are cou-52 pled together and how we achieve a stable climate by setting certain tuning parameters 53 and performing sensitivity experiments. We evaluate the performance of our new model 54 by running a set of experiments under pre-industrial and historical climate conditions 55 as well as a set of idealized greenhouse-gas-increase experiments. These experiments were 56 designed by the Coupled Model Intercomparison Project (CMIP) and allow us to com-57 pare the results to those from other CMIP models and the predecessor of our model, the 58 Max Planck Institute for Meteorology Earth System Model. While we diagnose overall 59 satisfactory performance, we find that ICON-ESM features somewhat larger biases in 60 several quantities compared to its predecessor at comparable grid resolution. We empha-61 size that the present configuration serves as a basis from where future development steps 62 will open up new perspectives in earth system modelling. 63

Volcanic aerosol forcing has been reported in the literature to be less effective in changing the earth’s surface temperature than CO2 forcing. This implies a different feedback strength, and therefore different contributions from individual feedback mechanisms. We employ the CMIP6 version of MPI-ESM to understand the reasons for these apparent differences in the ability to change the surface air temperature. Using a highly idealized eruption scenario and comparing it to a doubling and a halving of CO2 concentration, we identify key reasons for changes in the magnitude of the feedback parameter. We show that the “efficacy” [Hansen et al. 2005] of volcanic aerosol forcing depends strongly on the method and the time scale used to calculate it. We argue that the seemingly established result of a lower-than-unity efficacy of volcanic aerosol forcing might only hold under the specific methodological choices other authors have made, but not in general. Furthermore, we find qualitative differences between the cooling and warming simulations, but strong similarities between the 0.5xCO2 and the idealized eruption cases. This hints towards processes, which are not forcing agent-specific, but specific to the sign of the forcing. A pronounced curvature in the N(T) plot (“Gregory plot”) for the cooling scenarios makes the computation of feedback through regression even more sensitive to subjective choices than in the 2xCO2 case. We disentangle the role of ocean heat uptake efficacy and atmospheric feedback processes in the framework of the pattern effect.

This work documents the ICON-Earth System Model (ICON-ESM V1.0), the first coupled model based on the ICON (ICOsahedral Non-hydrostatic) framework with its unstructured, isosahedral grid concept. The ICON-A atmosphere uses a nonhydrostatic dynamical core and the ocean model ICON-O builds on the same ICON infrastructure, but applies the Boussinesq and hydrostatic approximation. The oceanic carbon cycle and biogeochemistry is represented by the HAMOCC6 module and the terrestrial biogeophysical and biogeochemical process are integrated in the new JSBACH4 module. We describe the tuning and spin-up of a base-line version at a resolution typical for models participating in the Coupled Model Intercomparison Project (CMIP). The performance of ICON-ESM is assessed by means of a set of standard CMIP6 simulations. Achievements are well-balanced top-of-atmosphere radiation, stable key climate quantities in the control simulation, and a good representation of the historical surface temperature evolution. The model has overall biases, which are comparable to those of other CMIP models, but ICON-ESM performs less well than its predecessor, the MPI-ESM. Problematic biases are diagnosed in ICON-ESM in the vertical cloud distribution and the mean zonal wind field. In the ocean, sub-surface temperature and salinity biases are of concern as is a too strong seasonal cycle of the sea-ice cover in both hemispheres. ICON-ESM V1.0 serves as a basis for further developments that will take advantage of ICON-specific properties such as spatially varying resolution, and coupled configurations at very high resolution.

An atmospheric composition feedback mechanism modulates the global equilibrium climate sensitivity (ECS) through changes in the tropical upper-tropospheric and lower-stratospheric (UTLS) water vapor. The feedback mechanism is caused by the acceleration of the Brewer-Dobson circulation. This process changes the ozone (O$_{3}$) concentration, resulting in a drier and cooler UTLS region than without O$_{3}$ changes. Thus, the planetary long-wave emissivity increases, and the ECS decreases. However, the BDC alone already provides dynamical cooling through the tropical stratospheric upwelling, potentially impacting the ECS. Here, we analyze the implications of this effect from a tropical clear-sky perspective, applying a one-dimensional radiative-convective equilibrium (RCE) model that explicitly accounts for the adiabatic cooling by the BDC and includes an interactive representation of O$_{3}$. We study how increasing upwelling modifies the change of the tropical energy budget resulting from a doubling of CO$_{2}$. An increase in upwelling reduces the tropical ECS mainly through an increased tropical energy export related to the adiabatic cooling. The atmospheric composition feedback through O$_{3}$ contributes less than 30\% to the tropical ECS reduction. Due to the dominance of the energy export, any impact on the global ECS will depend on the redistribution of the energy in the extratropics. We show that GCMs simulate similar changes of the tropical energy export under increased upwelling which corroborates that the findings obtained with the RCE approach bear relevance for the global climate.