Ocean Salinity in the first 10 meters below the sea surface is measured since approximately 1874, the date for the first oceanographic campaigns. Historical SSS knowledge is comming mostly from bucket measurements and thermosalinograph (TSG) measurements onboard research vessels or ships of opportunity but also from moored buoys (The Global Tropical Moored Buoy Array),
In 2002, F. Bingham et al have examined historical distributions of SSS observations in the World Ocean ATlas 1998 database.
observations; and red, >1000 observations. No color means less than 10 observations. From F. Bingham et al, 2002.
Concerning the 1.3 million SSS observations in 1°x1° cell since 125 years, it was found by these authors that:
Since the beginning of 2000, the density of SSS measurements continuously increased, in particular because of the growth of ARGO floats deployements.
Given that the later floats reach the surface approximately once every 10 days and that ~3000 sensors are at sea now, we get by this network system approximately ~300 SSS measurements/10 days over the global ocean.
In addition, in the frame of the Global Drifter Array program, several surface drifters are now equiped to measure SSS. A large number of these drifters were recently deployed and their data qualified by LOCEAN/IPSL team in the context of SMOS cal/val activites.
In the context of SMOS mission, a french calibration/validation project entitled "GLOSCAL" project is funded by CNES and lead by F. Gaillard at the Ifremer/Laboratoire de Physique des Océans (LPO). F. Gaillard and her team, collect the SSS data from the Coriolis database, qualify them, standardize them at a 3m depth, and generates objectively-analyzed fields from the in situ measurements.
Bingham , F., S. Howden, and C. Koblinsky, Sea surface salinity measurements in the historical database, J. Geophys. Res. , 107(C12), 8019, doi :10.1029/2000JC000767, 2002.