The SSM is an implementation of the Finite Volume Community Ocean Model (FVCOM). FVCOM was developed by the University of Massachusetts-Dartmouth, USA, by Chen et al. (2003) and is still actively being developed. FVCOM is a three dimensional (3D) ocean model using the finite volume method to discretise the governing equations in integral form and solve them numerically using the finite difference method over the computational grid. FVCOM uses an unstructured irregular grid in the horizontal plane, composed of triangular elements which can vary in shape and size.

Below is a summary of the datasets used to force the Scottish Shelf Waters Reanalysis. More information is available at www.tinyurl.com/SSW-Reanalysis.

Open boundary forcing along the Atlantic boundary:
The Atlantic boundary was forced by the Met-Office European North West Shelf ocean physics reanalysis for temperature, salinity, residual (non-tidal) current speed, and residual sea surface height (SSH) (O’Dea et al., 2012, 2017; https://doi.org/10.48670/moi-00059) were combined with hourly tidal current speeds and SSH from TPXO9-atlas harmonic tides (Egbert and Erofeeva, 2002).

Open boundary forcing along the Baltic boundary:
The Baltic boundary was forced by the Swedish Meteorological and Hydrological Institute (SMHI) hourly Baltic Sea Physics Reanalysis data for SSH and residual current speeds. These were supplemented with tidal currents from TPXO9-atlas tidal harmonics (Egbert and Erofeeva, 2002). The Baltic Sea Physics Reanalysis (https://doi.org/10.48670/moi-00013) data were used for daily temperature, salinity, residual current speed, and SSH.

Atmospheric forcing:
ERA5 atmospheric data (Hersbach, 2020) were used for surface forcing of mean sea level pressure, precipitation, evaporation, surface air temperature, thermal/solar radiations and wind velocity.

Fresh water forcing:
Climatological daily river volume fluxes from the Hydrological Predictions for the Environment over Europe (E-HYPE) dataset from the SMHI (Arheimer et al., 2020) were used for fresh water forcing of the SSW-RS domain. This dataset was combined with the Grid-to-Grid (G2G) model from the Centre for Ecology and Hydrology to include river fluxes outside of Scotland and Northern Ireland. The combined dataset of Scottish-Northern Irish rivers with Northwest European Shelf rivers contains daily volume flux and temperature for 912 rivers in the SSW-RS domain.

Data Assimilated:
Daily sea surface temperatures were assimilated from the Ocean Data Analysis System for Marine Environment and Security for the European Area (ODYSSEA) satellite dataset (Autret et al., 2019; https://doi.org/10.48670/moi-00153).

Bathymetry:
The Atlantic Margin Model 1.5 km gridded bathymetry was interpolated to the unstructured SSM grid. These bathymetry data originated from EMODnet (https://emodnet.eu), adjusted from lowest astronomical tide (LAT) to mean sea level (MSL) vertical reference datum (Graham et al., 2018).

Version 3.02 release notes:
- New hybrid sigma layer scheme. The vertical water column is resolved using 20 standard terrain following sigma layers (each representing 5% of the water column) in areas shallower than the 120 m isobath. In areas deeper than the 120 m isobath, the water column is resolved with 5 fixed layers at the surface and 2 fixed layers at the bottom. The surface layers are 3, 6, 7, 7 and 7 m thick and reside at 1.5, 6, 12.5, 19.5, 26.5 m depth. The bottom layers are both 6 m thick and reside 3 and 9 m above the bottom. The remaining 13 sigma layers are of equal spacing for the mid depths.
- An updated version of the Atlantic Margin Model 7 km (AMM7) for non-tidal lateral boundary forcing is used. The new AMM7 uses ECMWF ERA5 atmospheric forcing which is consistent with the SSW-RS atmospheric forcing.
- The reanalysis has been extended to 27 years, 1993 to 2019.
- A small change was made to the SST assimilation. Data is now assimilated with a lower limit of 100 m and the pycnocline is resolved in 1 m bins. For 2019, version 5 of ODYSSEA SST is used while version 4 is kept for the rest of the dataset.

References:
Arheimer, B., Pimentel, R., Isberg, K., Crochemore, L., Andersson, J. C. M., Hasan, A., & Pineda, L. (2020). Global catchment modelling using World-Wide HYPE (WWH), open data, and stepwise parameter estimation. Hydrology and Earth System Sciences, 24(2), 535–559. https://doi.org/10.5194/hess-24-535-2020
Autret, E., Piollé, J. F., Tandéo, P., Prévost, C. (2020) ODYSSEA Level 4 Sea Surface Temperature Reprocessing over the European North West Shelf / Iberia Biscay Irish Seas Reprocessing SST-ATL-SST-L4-REP-OBSERVATIONS-010-026, Tech. Rep. 1.4, https://catalogue.marine.copernicus.eu/documents/PUM/CMEMS-SST-PUM-010-0...
Chen, C., Liu, H., & Beardsley, R. C. (2003). An Unstructured Grid, Finite-Volume, Three-Dimensional, Primitive Equations Ocean Model: Application to Coastal Ocean and Estuaries. Journal of Atmospheric and Oceanic Technology, 20(1), 159–186. https://doi.org/10.1175/1520-0426(2003)020<0159:AUGFVT>2.0.CO;2
Egbert, G. D., & Erofeeva, S. Y. (2002). Efficient inverse modeling of barotropic ocean tides. Journal of Atmospheric and Oceanic Technology, 19(2), 183–204. https://doi.org/10.1175/1520-0426(2002)019<0183:EIMOBO>2.0.CO;2
Graham, J. A., O’Dea, E., Holt, J., Polton, J., Hewitt, H. T., Furner, R., Guihou, K., Brereton, A., Arnold, A., Wakelin, S., Castillo Sanchez, J. M., & Mayorga Adame, C. G. (2018). AMM15: a new high-resolution NEMO configuration for operational simulation of the European north-west shelf. Geoscientific Model Development, 11(2), 681–696. https://doi.org/10.5194/gmd-11-681-2018
Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Muñoz-Sabater, J., Nicolas, J., et al. (2020). The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146: 1999–2049. https://doi.org/10.1002/qj.3803