Sea Surface Salinity

SMOS, 2010, from:
This lecture is also available as part of an audio-narrated PowerPoint presentation.
The ocean is roughly 3.5% salt and the concentration of dissolved salts in the ocean is referred to as salinity, which varies across the globe and over time.
Salinity plays an important role in how the Earth system functions and is critical to many aspects of the ocean, from circulation to climate to the global water cycle.
The concentration of salt on the ocean surface — the part of the ocean that actively exchanges water and heat with Earth's atmosphere — is a critical driver of ocean processes and climate variability.••Because ocean surface salinity varies from place to place and over time, it is used to trace the ocean's role in Earth's water cycle. About 86% of global evaporation and 78% of global precipitation occur over the ocean. By measuring changes in ocean surface salinity caused by these processes, as well as changes caused by melting ice and river runoff, satellites provide important information about how the Earth's freshwater moves between the ocean and atmosphere and around the globe.

Thermohaline circulation

Together with temperature, salinity controls the density of seawater, determining whether it sinks or floats. Salinity has a major effect on the flow of deep ocean currents that move heat from the tropics to the poles and affect global climate.

Basic principle

The relationship between conductivity and salinity allows for remote sensing of salinity from space. As the conductivity of ocean surface waters changes (with salinity), there are minute detectable changes in the “brightness” of the surface in microwave emissions which are detectable by microwave radiometer.
For accurate measurements, it’s necessary to make adjustments for the sea surface temperature and roughness, the intervening atmosphere and ionosphere, and galactic signals reflected off the sea surface.
Steps for salinity retrieval:
  • evaluation of brightness temperature (i.e. apparent temperature) at the sea surface by correcting for ionosphere, atmosphere and extra-terrestrial radiations
  • correction of roughness and sea surface temperature contributions
  • retrieval of salinity from brightness temperature
Microwave radiometers measure salinity within 1-2 cm of the surface
Spatial resolution : between 0.25º and 1º, depending on the sensor.
Not close to the coasts.
Global coverage: in 3 to 10 days.

Salinity Missions