From the new NASA Aquarius satellite. According to RealClimate:
The patterns are not particularly surprising, there is higher salinity in the sub-tropical evaporative regions, lower salinity near the equator (because of the rain!), and particularly low salinity near big river outflows (the Amazon plume stands out clearly). However, as we noted earlier, the main interest is going to be in the variability.
In the surface ocean, there are strong salinity gradients across the major currents – water is much fresher on the west side of the Gulf Stream extension than on the east side for instance. Warm core eddies in the South Atlantic are saltier than surrounding sea. The western pacific warm pool is fresher than water in the East Pacific since it rains more there. One important thing to note is that temperature and salinity anomalies are often balanced – that is cold water is often fresh, and warm water is often relatively salty. Since these two factors have opposite impacts on the density it is difficult to judge whether water is more or less dense (which is key to the ocean circulation) just from the temperature alone.
This is important, for instance, in attempts to predict ocean circulation based on current conditions – if you only use temperature information (because we don’t yet have good salinity data in real time), then you can incorrectly assume density anomalies that are much too large, or even the wrong sign (this was one of the problems in the Keenlyside et al paper from 2008).