How is Salinity Measured?

The Aquarius mission, launched on 10 June 2011, is the first mission with the primary goal of measuring sea surface salinity (SSS) from space. Data from Aquarius will play a large role in understanding both climate change and the global water cycle.
 
In the open ocean the range of salinity is generally from 32 psu to 37 psu.  Along with temperature, it is a major factor in contributing to changes in the density of seawater and therefore ocean circulation. Salinity is the key to understanding the global water cycle. 97% of the Earth's free water resides in the oceans.  The water cycle is dominated by precipitation and evaporation. Precipitation over the oceans account for 12 Sverdrups (Sv = 1 million m³s^-1), with an additional Sv flowing into the oceans from terrestrial runoff.  Evaporation over the ocean is equal to 13 Sv.  In contrast precipitation and evaporation over land account for 3 Sv and 2 Sv respectively. Sea surface salinity can be used to measure the difference of these two processes, with excess precipitation resulting in lower salinity and excess evaporation yielding higher sea surface salinity. SSS is a measure of ice melt at high latitudes with glacial and sea ice melt causing a freshening of the surface waters
 
While sea surface temperatures have been measured from space for over 3 decades, the technology to measure sea surface salinity from space has only recently emerged.  Sea surface density, a driving force in ocean circulation and a function of temperature and salinity will finally be measurable every month on a  global scale. As the oceans have 1100 times the heat capacity of the atmosphere, the  ocean circulation becomes critical for understanding the transfer of heat over the Earth and thus understanding climate change.