The map above shows salinity near the ocean surface as measured by the Aquarius instrument on the Satélite de Aplicaciones Científicas (SAC)-D satellite. The data depicted shows average salinity from May 27 to June 2, 2012, in a range from 30 to 40 grams per kilogram, with 35 grams being the average. Lower values are represented in purples and blues; higher values are shown in shades of orange and red. Black areas occur where no data was available, either due to the orbit of the satellite or because the ocean was covered by ice, which Aquarius cannot see through.
Technically, Aquarius measures the emissivity or “brightness temperature” of the surface waters, notes Gary Lagerloef, Aquarius principal investigator, based at Earth and Space Research in Seattle. Land masses have a higher emissivity than the ocean, so any measurement close to land tends to be skewed by its brightness. Over time, the Aquarius research team should be able to calibrate the measurements and develop mathematical tools to better distinguish the salt signal. But for now, the measurements are so new that the team is still working on the big picture of ocean salinity.
Aquarius is the first NASA instrument specifically designed to study surface ocean salinity from space, and it does so at a rate of 300,000 measurements per month. It uses three passive microwave sensors, called radiometers, to record the thermal signal from the oceans’ top 10 millimeters (about 0.4 inches).
“An overarching question in climate research is to understand how changes in the Earth’s water cycle—meaning rainfall and evaporation, river discharges and so forth—ocean circulation, and climate link together,” said Lagerloef. Most global precipitation and evaporation events take place over the ocean and are very difficult to measure. But rainfall freshens the ocean’s surface waters, and Aquarius can detect these changes in saltiness. “Salinity is the variable we can use to measure that coupling. It’s a critical factor, and it will eventually be used to improve climate forecasts.”