Quantification of salinity is hampered by the lack of time and space resolution of existing measurements and models. At present, skin salinity measurements are available every few days with limited spatial resolution. Daily skin salinity products are full of gaps, which some applications can’t tolerate. Modeled salinity derived in the ocean mixed layer differs from remote sensing data of ocean skin layer salinity to a large extent for certain regions. The cool skin is a conductive layer in the upper few millimeters of the ocean within which transport of salt is dominated by vertical diffusion under the condition of weak to moderate winds. A technique to derive ocean skin layer salinity from satellite-based data for daily and 101 to 102 km scales was developed.
A conceptual model is proposed to express sea surface salinity as a function of freshwater flux (evaporation minus precipitation) and vice versa. It is shown to provide good agreement with existing salinity observations. The predicted salinity agrees with Aquarius and SMOS (Soil Moisture Ocean Salinity) data within 0.2 PSU error. It also has the potential to enhance the capability of monitoring and modeling salinity as a data retrieval algorithm for remote sensing at improved spatial and temporal resolution.
The model is formulated using an idealized one-dimensional diffusion equation for the ocean surface layer. The key idea is to use a mathematical tool known as fractional calculus, which allows expression of freshwater flux in terms of sea surface salinity without solving for the vertical distribution of salinity through the mixed layer. The contrast between the horizontal length scale of freshwater flux (on the order of 102 km) and the vertical length scale of ocean skin layer (less than 1 mm thick) justifies the use of a (vertical) one-dimensional salinity model within a layer of infinitesimal depth for daily and 101 to 102 km scales as considered in this study.
The newly formulated freshwater model leads to a method for deriving sea surface salinity from evaporation and precipitation data. It is the first salinity-gradient-independent model linking freshwater flux and sea surface salinity. The model is also an innovative algorithm for retrieving missing satellite skin salinity data using global freshwater fluxes (and vice versa) for the study of oceanic hydrologic cycle. More information can be found at: https://onlinelibrary.wiley.com/doi/10.1002/2014GL06136/full.