SI3 regional configuration of the Arctic This is a configuration of the NEMO community ocean model based on the ORCA2_SAS_ICE reference configuration. The NEMO code is available from https://forge.nemo-ocean.eu/nemo/nemo. This configuration has a resolution of 1/36 degree and is a cut-out of the global 1/36 configuration: https://github.com/immerse-project/ORCA36-demonstrator. The code base is a pre-4.2.0 NEMO version, the model source code can be found in the file src_tar. Model setup Follow the instructions on https://sites.nemo-ocean.io/user-guide/index.html to download and install the NEMO model version 4.2.0. Swap the src directory for the one in the tar file src_tar. Compile the ORCA2_SAS_ICE reference configuration. Put the rest of the files in this zenodo archive in the EXP00 directory, except the namelist_cfg_for_DOMAINcfg file which goes into tools/DOMAINcfg along with the grid files to be downloaded later. The files provided include example configuration namelist files namelist_cfg and namelist_ice_cfg. The atmospheric forcing used is the Drakkar forcing set (DFS) version 5.2, year 2008. The atmospheric forcing is interpolated on-the-fly, using the weights files. The weights were calculated using the nemo WEIGHTS tool. For the ocean (bottom) boundary the World Ocean Atlas 2018 multidecadal monthly averages are used. The data is already interpolated onto the ARC36 grid. Interpolation was done using the SOSIE tool. Files provided are monthly averages of sea surface salinity and temperature. Finally, the model grid domain_cfg.nc needs to be created. Download the ORCA36 files from ftp://ftp.mercator-ocean.fr/download/users/cbricaud/BENCH-ORCA36-INPUT.tar.gz, see the ORCA36 demonstrator github page. The necessary files are the coordinates and bathymetry files. To cut out the Arctic domain use ncks -F -d y,7000,,1 in.nc out.nc. Put in tools/DOMAINcfg and use the DOMAINcfg NEMO tool to create the domain_cfg.nc file using the file namelist_cfg_for_DOMAINcfg as namelist_cfg. The resulting file is large (122GB) therefore executing in parallel mode is required. The individual processor files need to be merged into one, use the REBUILD_NEMO tool. Put the resulting domain_cfg.nc file into EXP00 and run NEMO following the instructions. The ARC36 configuration was set up and run on ARCHER2 using 594 NEMO processors and 12 XIOS processors. Animation The animation has been created from daily average of sea ice fraction from the 1/36° Arctic NEMO-SI3 model integrations with the EAP rheology. The animation has started on the 00:00 of the 1st January 2008 and carried out through the January. It shows a limited area of the model domain north of Fram Strait. The lower concentrations correspond to the opening leads, with “blurred” leads and some multiple signatures due to ice displacement and data averaging over 1-day periods. Acknowledgements: EU IMMERSE (Grant agreement ID: 821926), NERC APEAR project (NE/R012865/1, NE/R012865/2, #03V01461), part of the Changing Arctic Ocean programme; EU H2020 COMFORT (no. 820989); NERC PRE-MELT (NE/T000546/1), and LTS-S CLASS (NE/R015953/1). ARCHER UK National Supercomputing and JASMIN facilities.

The funding for the research came from the NERC Project "PRE-MELT" 15 (NE/T000260/1) and European Union's Horizon 2020 research and from the innovation programme under grant agreement No 821926 (project IMMERSE—Improving Models for Marine EnviRonment SErvices). The repository fulfils the public data access requirements of these projects.