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.