GOSI9: UK Global Ocean and Sea Ice configurations

The UK Global Ocean and Sea Ice configuration version 9 (GOSI9) is a new traceable hierarchy of three model configurations at 1, and based on version 4.0.4 of the NEMO code. GOSI9 has been developed as part of the UK's Joint Marine Modelling Programme (JMMP), a partnership between the Met Office, the National Oceanography Centre, the British Antarctic Survey, and the Centre for Polar Observation and Modelling. Following a seamless approach, it will be used for a variety of applications across a wide range of spatial and temporal resolutions: short-range coupled numerical weather prediction (NWP) forecasts, ocean forecasts, seasonal and decadal forecasts, and climate and Earth system modelling. The GOSI9 configurations are described in detail with a special focus on the updates since the previous version (GO6-GSI8). Results from 30-year ocean–ice integrations forced by CORE2 fluxes are presented for the three resolutions, and the impacts of the updates are assessed using the integrations. The upgrade to NEMO 4.0.4 includes a new sea ice model SI3 (Sea Ice modelling Integrated Initiative) and faster integration achieved through the use of partially implicit schemes that allow a significant increase in the length of the time step. The quality of the simulations is generally improved compared to GO6-GSI8. The temperature and salinity drifts are largely reduced thanks to the upgrade to NEMO 4.0.4 and the adoption of fourth-order horizontal and vertical advections helping to reduce the numerical mixing. To improve the representation of the Southern Ocean, a scale-aware form of the Gent–McWilliams parameterization and the application of a partial-slip lateral boundary condition on momentum in the Southern Ocean have been added, resulting in a stronger and more realistic Antarctic Circumpolar Current (ACC) transport and a reduction in the temperature and salinity biases along the shelf of Antarctica. In the Arctic, the representation of sea ice is improved, leading to a reduction in surface temperature and salinity biases. In particular, the excessive and unrealistic Arctic summer sea ice melt in GO6-GSI8 is significantly improved in GOSI9 and can be attributed to the change in the sea ice model and to the higher albedos that increased sea ice thickness.