7 Conclusions

The Alfred Wegener Institute climate model, AWI-CM, described in this study, contributes to the diversity of climate models with the unstructured mesh approach for its sea ice-ocean component. Biases in AWI-CM tend to be less pronounced than in models contributing to the previous Climate Model Intercomparison Project 5, as shown by objective performance indices. Even though some long standing biases such as a too zonal pathway of the North Atlantic current, the cold bias over the North Atlantic subpolar gyre or the warm bias west of Africa are still present in AWI-CM, especially Southern Ocean sea surface temperature and the atmospheric temperature above, sea ice concentration around Antarctica as well as North Atlantic ocean temperature profiles are well represented. Furthermore, there is an excellent agreement of the Arctic sea ice thickness in the past years for which observations are available. Therefore, AWI-CM results are a solid contribution to the CMIP6 project. Sea ice-ocean models on unstructured meshes have matured (now contributing to CMIP6) and can be used at high resolutions enabled through excellent scalability characteristics. Our results support the notion that some of the climate change features are robust against model formulation. However, there are some important features that deviate from other CMIP simulations:
1) Despite the smaller Arctic sea ice decline trend compared to observations, as early as starting between 2025 and 2030 there are isolated years with virtually sea ice free Arctic summers (1 Million km2 sea ice extent or less) independent of climate change mitigation efforts. Only after 2050 mitigation efforts start to play a substantial role and Arctic sea ice can recover to some extent in the SSP126 scenario with strong mitigation efforts.
2) The AMOC decreases by around 25% until the end of the 21st century according to the AWI-CM SSP585 scenario simulation, which is less than the multi-model average value of 40% calculated from CMIP5 models and Earth System Models of Intermediate Complexity (EMICs, Weaver et al., 2012).
The AWI-CM model data is available through the Earth System Grid Federation (ESGF) and includes not only the DECK and ScenarioMIP experiments (Eyring et al., 2016; O’Neill et al., 2016) with AWI-CM-1-1-MR (Semmler et al., 2018) described in the present study. At the time of writing, AWI-CM results from the Polar Amplification Model Intercomparison Project (PAMIP, Smith et al., 2019) and results with AWI-CM-1-1-HR from the High Resolution Model Intercomparison Project (HighResMIP, Haarsma et al., 2016) are available as well (Semmler et al., 2017; Semmler et al., 2019). Furthermore, data publications of AWI-CM simulations are planned for OMIP (Griffies et al., 2016) and PMIP (Kageyama et al., 2018).