The Alfred Wegener Institute Climate Model (AWI-CM) participates for the first time in the Coupled Model Intercomparison Project (CMIP), CMIP6. The sea ice-ocean component, FESOM, runs on an unstructured mesh with horizontal resolutions ranging from 8 to 80 km. FESOM is coupled to the Max-Planck-Institute atmospheric model ECHAM 6.3 at a horizontal resolution of about 100 km. Using objective performance indices, it is shown that AWI-CM performs better than the average of CMIP5 models. AWI-CM shows an equilibrium climate sensitivity of 3.2°C, which is similar to the CMIP5 average, and a transient climate response of 2.1°C which is slightly higher than the CMIP5 average. The negative trend of Arctic sea ice extent in September over the past 30 years is 20-30% weaker in our simulations compared to observations. With the strongest emission scenario, the AMOC decreases by 25% until the end of the century which is less than the CMIP5 average of 40%. Patterns and even magnitude of simulated temperature and precipitation changes at the end of this century compared to present-day climate under the strong emission scenario SSP585 are similar to the multi-model CMIP5 mean. The simulations show a 11°C warming north of the Barents Sea and around 2 to 3°C over most parts of the ocean as well as a wetting of the Arctic, subpolar, tropical and Southern Ocean. Furthermore, in the northern mid-latitudes in boreal summer and autumn as well as in the southern mid-latitudes a more zonal atmospheric flow is projected throughout the year.

A regional Earth System Model has been implemented for the South Asia region. We investigate the effect of the marine biogeochemical feedback which affects the attenuation of the short-wave radiation upon the regional climate. In the experiment where the feedback is activated the average SST is lower over most of the domain. The greatest deviations (more than 1°C) in SST between the two runs occur in the summer period during the phytoplankton bloom. A significant cooling of subsurface layers occurs and the thermocline shifts upward compared to the Jerlov type absorption. The phytoplankton primary production and its deviation in the feedback-based simulation turned out to be higher, especially during periods of winter and summer phytoplankton blooms. The marine biogeochemistry feedback also affects the amount of precipitation in the model in particular during the monsoon season. The associated SST cooling leads to a reduction of the precipitation but affects it in different ways. In the Arabian Sea, the reduction of the transport of humidity across the equator leads to a reduction of the large scale precipitation in the eastern part of the basin, reinforcing reduction of the convective precipitation. In the Bay of Bengal, the feedback increases the large scale precipitation, contouring the decrease of convective precipitation. Thus, the main impacts of including the biogeochemical coupling in the Indian Ocean include the enhanced phytoplankton primary production, a shallower thermocline and decreased SST, with cascading effects upon the model ocean physics which further translates into altered atmosphere dynamics.

Deep water formation (DWF) in the North Western Mediterranean (NWMed) is a key feature of Mediterranean overturning circulation. Changes in DWF under global warming may have an impact on the regional and even on the global climate. Here we analyze the deep convection in the Gulf of Lions (GoL) in a changing climate using an atmosphere-ocean regional coupled model with a high horizontal resolution enough to represent DWF. We find that under the RCP8.5 scenario the NWMed DWF collapses by 2040-2050, leading to almost a 90% shoaling in the winter mixed layer depth by the end of the century. The collapse is mainly related to changes in sea water temperature and salinity of Modified Atlantic Water (MAW) and Levantine Intermediate Water (LIW) that strengthen the vertical stratification in the GoL. The stratification changes also alter the Mediterranean overturning circulation and the water, heat and salinity exchange with the Atlantic.

Substantial changes have occurred in the Arctic Ocean in the last decades. Not only sea ice has retreated significantly, but also the ocean at mid-depth showed a warming tendency. By using simulations we identified a mechanism that intensifies the upward trend in ocean heat supply to the Arctic Ocean through Fram Strait. The reduction in sea ice export through Fram Strait induced by Arctic sea ice decline increases the salinity in the Greenland Sea, which lowers the sea surface height and strengthens the cyclonic gyre circulation in the Nordic Seas. The Atlantic Water (AW) volume transport to the Nordics Seas and Arctic Ocean is consequently strengthened. This enhances the warming trend of the Arctic AW layer, potentially contributing to the “Atlantification” of the Eurasian Basin. Therefore, the Nordic Seas can play the role of a switchyard for the Arctic sea ice decline to influence the Arctic heat budget at mid-depth.

Using the depth (z) and density (ϱ) frameworks, we analyze local contributions to AMOC variability in a 900-year simulation with the AWI climate model. Both frameworks reveal a consistent interdecadal variability, however the correlation between their maxima deteriorates on year-to-year scales. We demonstrate the utility of analyzing the spatial patterns of sinking and diapycnal transformations through depth levels and isopycnals. The success of this analysis relies on the spatial binning of these maps which is especially crucial for the maps of vertical velocities which appear to be too noisy in the main regions of up- and downwelling because of stepwise bottom topography. Furthermore, we show that the AMOC responds to fast (annual or faster) fluctuations in atmospheric forcing associated with the NAO. This response is more obvious in the ϱ than in the z framework. In contrast, the link between AMOC deep water production south of Greenland is found for slower fluctuations and is consistent between the frameworks.