Benefits of dynamically modelled river discharge input for ocean and coupled system.

River freshwater input is crucial in modelling global ocean. Most ocean models used in CMEMS services rely on climatological river discharge data with various deficiencies, which can lead to biased simulated ocean states. The Copernicus Emergency Management Service (CEMS) Global Flood Awareness System (GloFAS) provides state-of-the-art global flood forecasts and downstream river discharge. A GloFAS-ERA5 global river discharge reanalysis dataset has been produced using the same system, modelled by routing runoff from ECMWF’s (European Centre for Medium- Range Weather Forecasts) atmospheric reanalysis ERA5 via a river network. As a global gridded data set that covers from 1979 to near-real-time, GloFAS-ERA5 reanalysis can provide an improved and standardized input of land freshwater input for global, regional and coastal ocean models. Evaluation results suggest that the overall performance of this new river discharge reanalysis is reasonably good in general when verified against a global network of 1801 discharge observation stations. A new method has been developed for conversion of the GloFAS-ERA5 reanalysis data into land freshwater input for the NEMO ocean model. This method has been tested with a climatology of GloFAS-ERA5 river discharge. Compared to the DRAKKAR climatology of land freshwater input (BT06, hereafter) used by most CMEMS services, this new data set has an increased global mean value of ~1.33 Sv, but with reduced seasonal variations. Assessment of this GloFAS-ERA5 land freshwater input has been carried out with the operational ECMWF ocean analysis system-OCEAN5, driven by the same ERA5 atmospheric forcing. Evaluation of simulated ocean state against in-situ observations show improvements in regions affected by Amazon freshwater input when use GloFAS-ERA5 instead of BT06, by reducing a negative sea surface salinity bias in these regions. However, negative impact from switching to GloFAS-ERA5 land freshwater input is also visible in several regions, e.g. in the Maritime Continent and west coast of central America, which is associated with a large positive bias in the GloFAS-ERA5 river discharge at these regions. This issue can be mitigated by applying bias-correction to the GloFAS-ERA5 land freshwater input, and by adding extra vertical mixing in several affected regions that are close to the river mouth. Assessments of module simulated ocean Essential Climate Variables (ECVs) have been carried out to quantify the benefit of this realistic freshwater time series input. Improvements in climate signals like the Atlantic Meridional Overturning transports is also recorded.