Description: Ocean model biases such as the North West corner cold bias connected to the location of the Gulf Stream path, the warm bias in upwelling zones, the warm bias in the Southern Ocean, and model drift like the deep ocean warm bias which tends to peak in around 800 to 1000 m depth in the Atlantic Ocean are issues common among state-of-the-art ocean models. These issues are often amplified when the ocean model is coupled to an atmosphere model to perform climate simulations. Furthermore, unrealistic freezing of the Labrador Sea is an issue in various climate models. With the unstructured mesh approach in our Finite Element Sea ice Ocean Model (FESOM) we are able to systematically investigate the benefits of local refinement of the ocean model grid both in an uncoupled set-up (sea-ice ocean only) as well as in a fully coupled climate model (atmosphere- land-sea ice-ocean). While the horizontal ocean model resolution is 25 km on average in the finer grids, we refine the grids in some key areas to up to 5 km. Therefore we can explicitly resolve ocean eddies and simulate eddy-mean flow interactions in these key areas. The atmosphere-land component of our AWI-CM (Alfred Wegener Institute Climate Model) is ECHAM6-JSBACH developed at the Max-Planck-Institute for Meteorology in Hamburg, Germany. Here we present results of century-long uncoupled and coupled simulations on ocean model grids with different local refinements while keeping the atmosphere resolution constant in the coupled simulations. Results indicate that high horizontal resolutions in key regions such as the Gulf Stream / North Atlantic Current area or the Agulhas Stream can reduce biases such as the North West corner cold bias and the deep ocean model drift.

Description: We have conducted a series of atmosphere-only and coupled model experiments on time scales from weather to climate and with different methods to address the question how the large scale circulation of the Northern mid-latitudes is affected by the shrinking Arctic sea ice as well as by the overlying atmosphere. A major pathway has been found from the Barents Sea / Kara Sea area to Eastern Europe and Northern Asia and a secondary one from the Canadian Arctic into North America. In contrast, the atmosphere above ocean areas is less affected by the Arctic. A recurring response feature to declined Arctic sea ice is the slowdown and southward shift of the jet stream with less cyclone activity north of it leading to around 0.5 K colder conditions over some limited regions of North America and North Siberia in winter consistent with a negative Arctic Oscillation index. This happens despite the tendency of less intense cold advection due to the warmer Arctic in cases of anomalous northerly flow. It should be noted that for robust responses large ensemble simulations are needed due to low signal-to-noise ratio. In this respect it has been proven helpful to perform simulations in a Numerical Weather Prediction setting as the short simulation time enables us to easily run ensembles of several hundreds of realizations. Furthermore, in such a setting the initial response to a suddenly changed Arctic sea ice cover can be studied giving us hints how anomalies in the atmosphere develop. Coupled model simulations hint at no discernable influence of shrinking Arctic sea ice on the ocean on time scales of a year while on decadal to centennial time scales the ocean starts to react with possible feedbacks to the atmosphere. Due to less and thinner sea ice cover the momentum flux into the ocean increases which spins up the Beaufort Gyre. This response propagates towards the North Atlantic as an increased outflow through the Fram Strait, which drives increased volume transport into the Barents Sea, thus fostering the Atlantification of the basin. The response is not confined to the interior of the Arctic and our results suggest that it may reach as far south as the North Atlantic Current as a combined response to the dynamical ocean adjustment triggered within the Arctic and, secondarily, to the atmospheric weakening of the westerly winds.

Description: The AWI climate model AWI-CM consists of the Finite Element Sea-ice Ocean Model (FESOM) developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven and the ECHAM6 atmosphere land model developed at the Max Planck Institute for Meteorology in Hamburg. With the innovative flexible mesh structure of the FESOM model it is possible to highly resolve key ocean regions such as the western boundary currents or the Antarctic Circumpolar Current. In relatively coarse resolutions the model has proven to be of comparable quality as the CMIP5 models when measured with objective performance indices while in finer resolutions long-standing biases such as the North Atlantic deep ocean bias could be improved considerably. The AWI climate model will be part of CMIP6 including HighResMIP and production simulations have recently been started. Furthermore, at the moment the AWI-CM is being coupled to the Parallel Ice Sheet Model PISM to investigate the stability of the West Antarctic Ice Sheet when considering ice shelf – ocean interactions at very high resolutions (5 to 10 km) locally around Antarctica while leaving the ocean resolution in the order of 100 km in the dynamically less active subtropical areas.