The UK Met Office Unified Model (MetUM) version 10.6 with a regional nested domain was used to carry out a suite of simulations of the atmosphere over the NE North Atlantic region. This set up of the MetUM used the Global Atmosphere 6 and Global Land 6 (GA6/GL6) configurations including the ENDGame dynamical core (Walters et al. 2017). One modification to the standard GA6/GL6 configuration was to include form drag in surface momentum exchange over sea ice, based on Lüpkes et al. (2012) and Elvidge et al. (2016), and now part of the GL8 configuration. This new scheme has recently been implemented in the operational forecasting suite following evidence of significant improvements in simulated fluxes of momentum and heat and consequently improvements to the representation of wind speeds and temperatures over-and-downwind of the marginal-ice-zone during Arctic CAOs (Renfrew et al. 2019a). In our set up the MetUM was run globally with an N320 longitude-latitude grid (0.56° x 0.375°, equivalent to 60 km by 42 km at the equator) and 70 vertical levels up to a height of 40 km. The Iceland and Greenland Seas nested domain was 200 x 210 grid points with a spacing of 0.072° x 0.072° (equivalent to 8 km by 8 km) centred on 70.8°N, 14.0°W.

The MetUM was run in atmosphere-only mode with SST and sea-ice fields prescribed at the lower boundary for both the global and regional nested domains. The SST and sea-ice data were taken from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) system (Donlon et al. 2012; Roberts-Jones et al. 2012) and re-gridded to match the respective resolutions of the global model and the nested domain. The lower boundary conditions were updated daily. Within this set up, the global model was re-initialised daily at 00 UTC by ERA-Interim reanalysis (Dee et al. 2011). After initialisation on the first day of the simulation, the nested domain was only forced at the lateral boundaries by the global model. This means the nested domain is able to spin up and maintain mesoscale structures, within a regional atmospheric circulation environment that is nudged towards reality on a daily basis. The nested domain is relatively small, so is strongly influenced by the lateral boundary conditions. All simulations were run across an extended winter period, 1st November to 30th April, for 20 seasons from winter 1990/91 to 2009/10.