Scientific and Applied rationales

 Ocean Salinity relates the Earth global water cycle to ocean circulation. An 86% of the total evaporation and 78% of total precipitation take place over the ocean. Salinity is modified through processes that increase or decrease the fresh water amount in the ocean surface, mainly precipitation and evaporation, but also river discharges, and freezing and melting in polar regions. These changes in surface salinity are transferred to the deep ocean and spread to other regions by advection and diffusion mechanisms. This generates slight differences in dissolved salt content between the different water masses that are sufficient to play a major role in ocean dynamics and in the relationship between the ocean and the Earth’s climate, e.g. by creating density gradients that drive ocean currents able to transport huge amounts of heat and water modulating the climate over the continents.

Salinity is especially relevant in some key processes like the dense water formation at high latitudes, where high salinity waters from the subtropical Northern Atlantic Ocean are brought northwards by the Gulf Stream and then, in contact with the very cold and less saline arctic waters, form dense water masses that sink and push the three-dimensional ocean conveyor belt, the global thermohaline ocean circulation. Atmospheric circulation at tropical latitudes drives fresh water evaporated from the Atlantic towards the Pacific, and then contributes to keep the high salinity needed for the continuity of the global thermohaline circulation. The surface salinity in tropical oceans is strongly modulated by precipitations, upwelling processes, freshwater input by  large rivers (Amazon, Orinocco, Congo, Niger, Gange, Brahmapoutra, Yellow river, ...) as well as by surface current advection and vertical mixing. Knowing the salinity distribution at global scale and monitoring its seasonnal and interannual variability is crucial to better understand the ocean’s role in the climate system, regulated by this circulation and water and heat fluxes between atmosphere and ocean.

The El Niño / Southern Oscillation (ENSO) is the major source of interannual climate variability, and its prediction and occurrence can have an enormous socio-economic impact. It has been shown that sea surface salinity (SSS) information would play little role in the statistical nowcast of ENSO, but a significant role in the 6-12 month predictions. At these lags, positive SSS anomalies off the Equator have the potential to modify the subsurface stratification of the western Pacific as they are subducted westward. In this region, the most prominent feature related to salinity is the existence of a “barrier layer”  that isolates the mixed layer from the entrainment of cold water from below. Thus, salinity stratification helps to preserve a warm anomaly, increases the fetch of westerly winds, and leads to the ocean-atmosphere coupled instability leading to an ENSO event. On the other hand, the eastern edge of the warm pool is distinguished by a sharp SSS gradient, but by a weak Sea Surface Temperature (SST) gradient.

SSS is then a variable that not only, through its link to the evaporation minus precipitation balance, can provide valuable estimations of rainfall over the oceans, one of the less known components of the Earth’s water cycle, but is fundamental in other processes that force our global climate system.

 Legend: An example of Measurements of CO2 fugacity ( fCO2) in surface seawater and atmosphere as well as salinity across  the Amazon River plume (Kortzinger, JGR, 2003) .
In addition, Rivers can have a significant impact on the hydrography and biogeochemistry of large ocean areas. This is especially true for the Amazon River, by a considerable margin the largest river in the world. Like in most aquatic ecosystems, respiration exceeds autochthonous gross primary production in the Amazon. The resulting negative net ecosystem production is fueled by (particulate and dissolved) organic carbon leaking from the terrestrial system and causing the Amazon, like most rivers, to be highly supersaturated in CO2 In general in river plumes, very strong correlations  thus exhist between sea surface salinity and CO2 fugacity: therefore, measuring surface salinity at global scale from space shall better help constraining our estimates of C02 fluxes at the ocean surface (sequestration & release).
 Legend: A SeaWiFS chlorophyll a image depicting relatively lifeless midocean water (dark blue) that contrasts sharply with the highly productive waters influenced by the nutrient-rich Amazon outflow. Note also the plume of high-chlorophyll water spreading northwestward from the mouth of the Orinoco River (from Fratantoni and Glickson, 2002).
As importantly, in many river-plume influenced ocean area (Tropical Atlantic, Bay of Bengale, Yellow sea, Baltic,..),  bio-optical properties of the ocean surface are strongly linked to the river freswater input dilution in the surrounding saltier ocean water masses. Tracing temporal changes in the salinity field patterns at the ocean surface around the world largest river mouth will help better understanding mechanisms responsible for primary productivity and photo-bleaching.
Finally, recent  research results indicate that the most destructive hurricanes in the Tropical Atlantic may be influenced by ocean–atmosphere interaction with the warm Amazon and Orinoco freshwater plumes just prior to reaching the Caribbean. Monitoring the Amazon and Orinoco Rivers spread outward into the western equatorial Atlantic Ocean might bring insight into their possible role as active participants in hurricane maintenance and intensification.
Moreover, Hurricanes can dump hundreds of trillions of gallons of freshwater on the ocean surface. See image below. 
Credit :NASA/Goddard Space Flight Center Scientific Visualization Studio
Hurricane Isabel 2003 Rain Accumulation: this image shows rain accumulation from Hurricane Isabel from September 6 through 20, 2003 based on data from the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis. The accumulation is shown in colors ranging from green (less than 50 mm of rain) through red (200 mm or more).