Remote sensing of sea surface salinity: challenges and research directions

Salinity is a key parameter that affects the surface, deep circulations, and heat transport of oceans. Sea surface salinity (SSS) represents the salinity at the ocean surface and impacts atmosphere - ocean interactions and vertical ocean circulation. To monitor SSS, three passive microwave radiometers with an L-band (1.4 GHz) have been launched since 2009. The scientific need for SSS retrieval and estimation has grown in recent years; however, the operational retrieval of SSS via satellite remote sensing still faces significant challenges. This study provides a review of satellite-based SSS retrieval methods and guidelines to encourage future research. This paper introduces satellite-derived SSS research trends and summarizes the representative SSS satellite sensors and their retrieval methods. The limitations and challenges of satellite-derived SSS are then discussed. The errors from the retrieval algorithms, discrepancies in the spatio-temporal scales of in situ and remote sensing, and limitations of the satellite-derived SSS are then detailed. Finally, our paper provides suggestions for the future directions of SSS remote sensing in five ways: mitigation of measurement errors, improvement of currently available SSS products, enhancement of the usage of in situ data, reconstruction of three-dimensional salinity information, and synergetic uses of multi-satellite missions.