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).