Improvements to the Met Office’s global ocean–sea ice forecasting system including model and data assimilation changes

Abstract

The Forecast Ocean Assimilation Model (FOAM) is the Met Office’s operational, coupled ocean–sea ice system, which produces analyses and short-range forecasts at global and regional scales each day for various stakeholders, including defence, marine navigation and science users. This paper describes and evaluates the impacts of recent model and data assimilation (DA) updates on global FOAM when compared to its current operational version. The model updates include the use of the TEOS10 formulation for the seawater equation of state, with improved ocean model settings in the Southern Ocean and the implementation of a new sea ice model. Updates to the DA include an increase in the number of DA minimisation iterations, an improved specification of observation errors for sea surface temperature and sea level anomaly (SLA), and optimisations of the DA computational efficiency. Large-scale DA corrections for temperature have also been removed to prevent an inconsistent projection of the SLA DA signal onto large-scale temperature at depth. For 1-year runs at 1/12° resolution, the new FOAM system shows a 40 % improvement in observation-minus-background (OmB) statistics for SLA and subsurface temperatures relative to the current system in eddy-rich regions, which result in a similar level of improvement for ocean currents. To evaluate potential impacts on the pre-Argo period, 1-year experiments at 1/4° resolution are run withholding profiles of temperature and salinity observations in both new and current FOAM systems. Limited to the assimilation of only surface data, OmB statistics for SLA, temperature and salinity in the new FOAM system can reach improvements up to 90 % in the Southern Hemisphere relative to the current system, resulting in more temporally consistent ocean transport and heat content results. Therefore, it is expected that the model and DA updates will lead to more potential for use of FOAM reanalyses in climate studies, particularly in the pre-Argo period, and will provide improved ocean–sea ice initial conditions to FOAM as well as to the Met Office short-range and seasonal coupled ocean–atmosphere–land–sea ice forecasting systems.