GOSI9: UK Global Ocean and Sea Ice configurations

• Published in: Geoscientific Model Development 2025-01, Vol.18 (2), p.377-403
• Main Authors: Guiavarc'h, Catherine, Storkey, David, Blaker, Adam T, Blockley, Ed, Megann, Alex, Hewitt, Helene, Bell, Michael J, Calvert, Daley, Copsey, Dan, Sinha, Bablu, Moreton, Sophia, Mathiot, Pierre, An, Bo
• Format: Article
• Language: English
• Subjects: Analysis; Antarctic Circumpolar Current; Arctic sea ice; Bathymetry; Bias; Boundary conditions; Climate models; Configurations; Ice cover; Ice melting; Ice thickness; Modelling; Momentum; Numerical weather forecasting; Ocean; Ocean models; Oceanography; Oceans; Parameterization; Representations; Salinity; Salinity effects; Sea ice; Sea ice models; Sea ice thickness; Surface temperature; Viscosity; Weather; Weather forecasting
• ISSN: 1991-9603; 1991-959X; 1991-962X; 1991-9603; 1991-962X
• DOI: 10.5194/gmd-18-377-2025

The UK Global Ocean and Sea Ice configuration version 9 (GOSI9) is a new traceable hierarchy of three model configurations at 1, 1/4 and 1/12° 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 1/4° 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.