Observation impact statement on satellite sea surface salinity data from two operational global ocean forecasting systems

Observing system experiments have been carried out with two operational ocean data assimilation systems to assess the impact of assimilating satellite sea surface salinity (SSS) data. The SSS data reduces the errors in tropical Pacific salinity by 4% and 8% in the Met Office and Mercator Ocean systems respectively with an impact down to about 50 m depth on average. Salinity errors were reduced by up to 30% in the tropical Atlantic in both systems. There were significant reductions in near-surface temperature and sea level anomaly errors in both the central tropical Pacific and Amazon outflow regions in the Met Office system. The experiment results and characteristics of the operational ocean forecasting systems lead to a new set of requirements for satellite salinity data. The product accuracy required to significantly improve real-time ocean analysis and forecasts should be less than 0.2 pss at a resolution of at least 50 km for daily products. Improvements to spatial resolution down to the order of 10 km would represent an even more significant breakthrough. To be assimilated in near-real time, products should be available within 24 h and estimates of the systematic and random components of the uncertainty should be provided with each measurement.