1 Introduction

Around 50 institutions worldwide are participating in the current sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6, Eyring et al., 2016). The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany, contributes for the first time to CMIP with the novel Finite Element Sea Ice-Ocean Model (FESOM) coupled to the atmosphere model ECHAM6 developed at Max-Planck-Institute (MPI) for Meteorology in Hamburg. The novelty of FESOM lies in the use of global unstructured meshes that only few institutions worldwide are employing at this stage (e.g. Petersen et al., 2019; Korn, 2017). The unstructured-mesh approach allows putting a particular focus on dynamically active regions such as the North Atlantic Current, the Southern Ocean and the tropics while using relatively coarse resolution elsewhere. For the set of “Evaluation and Characterization of Klima” (DECK) and ScenarioMIP experiments a mesh with local refinement of up to 8 km in the North Atlantic Current and the Southern Ocean is used. Coupling the unstructured ocean model FESOM to ECHAM6, which is also used for the MPI-ESM contribution to CMIP6, offers the unique opportunity to investigate the influence of an alternative ocean model formulation on the results which will be exploited in further research.
Many models that participated in CMIP3 and CMIP5 have common descent and share ideas and code with each other (Masson and Knutti 2011, Knutti et al, 2013). This leads to a clustering of results based on model ”genealogy” and challenges the assumption of model independence. The ocean part of the AWI-CM is a new unstructured mesh model. It is thus based on a different dynamical core compared to most of the models contributing to CMIP6. Although many parameterisations in FESOM are similar to conventional structured-grid ocean models, and although the ECHAM model has already participated since CMIP3 in the CMIP efforts (Stevens et al., 2013), it can be argued that the use of an unstructured-mesh sea ice-ocean model is an important contribution to the diversity of the CMIP6 ensemble. Large-scale characteristics dominated by the formulation of the atmosphere, such as the equilibrium climate sensitivity, are not expected to be influenced too much by the ocean formulation. In contrast, the ocean has the potential to modulate the transient evolution and regional patterns of the response considerably. This can lead to differences in projected changes of coupled phenomena such as the El Niño-Southern Oscillation (ENSO) as well as sea ice in polar regions.
The aim of this paper is to present the main characteristics of the AWI-CM in the context of the CMIP6 project based on an evaluation of selected atmosphere, ocean, and sea ice parameters for present-day climate as well as for future climate. The evaluation of the unstructured mesh ocean component compared to the traditional mesh ocean component of Max Planck Institute for Meteorology (MPIM) is beyond the scope of this study and will be the topic of a collaborative publication with the MPIM.
In section 2, a brief model description is given along with a summary of the performed DECK and ScenarioMIP simulations, following the CMIP protocol. In section 3 remaining model drift and imbalances are analyzed. Section 4 describes biases in our present-day simulations for some important atmosphere, sea-ice, and ocean variables. The climate change signal is analyzed in detail in section 5. Finally, a discussion of the results and conclusions are presented in sections 6 and 7.