**5. Conclusions**

A first attempt of assimilating satellite salinity to model a major river plume has been presented. The latest version of SMOS, a satellite salinity product, was recently (2017) processed with an updated technique to reduce errors near the coast. This allows more frequent coastal data, which could be employed in a DA system of the Congo River plume. The ROMS Angola Basin configuration with IS4D-Var assimilated SSH, SMOS or SMOS SSH combined for four months.

The metrics applied to assess the assimilation analysis revealed that the SMOS observations were successfully assimilated during each month and maintained improvements throughout a subsequent forecast. The SMOS structure was well replicated within the model with regards to the plume area, distance to the centre of mass, orientation and average salinity within. Independent Argo float salinity profiles were available during the months analysed. The MAE of the Argo floats during the majority of months analysed reduced as compared to the control and persistence with the assimilation of SMOS. There are limitations to the current model, which are most likely due to a higher horizontal resolution required to resolve the upstream plume mixing. However, it has been shown that the current generation of satellite salinity observations can be successfully used to constrain a large river plume such as that produced by the Congo River. This successful assimilation stage could be the first step in producing operational forecasts of river plumes in the future.

**Author Contributions:** L.P. and R.T. conceived and designed the experiments. L.P. performed the experiments, analysis and wrote the paper. R.T provided insights and guidance, as well as contributing to the writing-review and editing of the paper.

**Funding:** This research was funded by the Natural Environment Research Council (NERC) award reference 1660325.

**Acknowledgments:** This study was supported by the Natural Environment Research Council (NERC). The authors would like to thank the Imperial College High Performance Computing Service (10.14469/hpc/2232) for providing computational resources. The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.altimetry.fr/duacs/). Argo float data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean

Observing System (http://doi.org/10.17882/42182). We thank two anonymous reviewers whose constructive comments improved the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
