Multi-resolution simulations with the AWI Climate Model (AWI-CM)

The recently established AWI Climate Model (AWI-CM), a coupled configuration of the Finite \nElement Sea Ice-Ocean Model (FESOM) with the atmospheric model ECHAM6, uses a novel \nmulti-resolution approach: Its ocean component builds on a finite element dynamical core \nsupporting unstructured triangular surface grids, allowing to distribute the grid points in a \nflexible manner. This allows to concentrate resolution in dynamically important regions, with a \ncontinuous transition zone to the coarser resolution in other areas. The model is an ideal tool to \nstudy the influence of explicit resolution of smaller scales in dedicated experiments. The unique \n– spatially seamless – approach might also be of benefit when it comes to temporally seamless \nprediction, bridging the gap between numerical weather prediction and climate models. \nA first benchmark set-up of AWI-CM with moderate resolution in the atmosphere (T63) and \n25km in key ocean areas, e.g. around the equator, achieved a similar overall simulation \nperformance in a long control simulation compared to well-established CMIP5 models. In \nparticular, the (isotropically) increased equatorial resolution considerably increased the realism \nof TIW activity and ENSO-related variability compared to standard resolutions. \nThe potential of AWI-CM is further exploited within the EU project PRIMAVERA in the \nHighResMIP of CMIP6, where we plan to contribute simulations with eddy-resolving resolutions \n(1/12° or 9-10 km) in key areas of the global ocean, such as the Gulf Stream-North Atlantic \nCurrent region, the Agulhas retroflection zone, or the Arctic basin. First simulations show \ndistinct improvements with respect to the development of deep temperature and salinity biases \nin the North Atlantic Ocean and an overall improvement of surface biases. At even higher \nresolutions of 4.5 km locally in the Arctic, linear kinematic features emerge in the simulated sea \nice distribution with potentially strong impacts on air-sea fluxes in the coupled system. Although \nthe tested set-ups are computationally very demanding (with numbers of grid points \ncomparable to a regular 0.25° grid), the throughput is high at about 8 simulated years per day \nbecause of high scalability. In addition, we are about to finish the development of a finite \nvolume version of the ocean model code (FESOM 2). It is already faster than the original FESOM \nversion by a factor of two to three, which will further enlarge the set of computationally feasible \napplications.