Coupling a Nested NEMO Ocean to the UK Atmosphere: Development and Atmospheric Impacts

Simulating the earth system with both the atmosphere and ocean can be expensive and slow, especially while utilising uniform high resolution meshes and long simulation lengths. However, high resolution is desirable, especially in the ocean, as coarser resolution restricts representation of desired small-scale processes, such as mesoscale ocean eddies. A variable resolution (VR) approach, in which one or multiple high-resolution child grids are embedded within a coarser parent grid in a two-way manner, is an option to reduce computational expense by concentrating finer resolution only where it is needed. VR has rarely been explored in the ocean of coupled AOGCM, with little (if any) analysis of the effects of VR coupling at the interface between the atmosphere and ocean. Along with the scientific investigation, there is the additional question as to whether the incorporation of VR can lead to simulations that can be cheap enough to generate useful large ensembles. Here we focus on the development of a coupled global UK atmosphere to a variable resolution, NEMO ocean through the library AGRIF. With this system we intend to investigate the extent in which the atmosphere sees both the fine scale resolved eddies of the higher resolution AGRIF nest (or nests) and any spurious edge artifacts introduced at the nest boundaries, and the consequences for atmospheric variability. We currently have a nested AGRIF system with a single nest coupled to the atmosphere, and are investigating the validity and the robustness of the coupling. We present a brief explanation of how the nested ocean is coupled to atmosphere, some results from our investigations of the coupling and our future goals for the coupled model.