Sensing Salt on the Sea - No More Grainy Pictures

June 8, 2011 - via

Scientists have long known that the saltiness of sea water is critical to ocean circulation patterns, to the Earth's water cycle and to global climate. While their theory was clear, however, what they could actually see of these shifting levels of sea surface saltiness was a not-so-clear picture patched together from scattered buoys and uneven shipboard data old and new.

With the launch of a NASA rocket set for Thursday at Vandenburg Air Force Base in California, a sensitive new microwave instrument aboard an internationally developed satellite is about to bring this picture up to high definition.

The first satellite to scan the salty surface of the seas was the European Space Agency's Soil Moisture and Ocean Salinity instrument launched in 2009. That instrument captures images of emitted microwave radiation around the frequency of 1.4 GHz and concentrates on polar regions. SMOS carried the first-ever, polar-orbiting, space-borne, 2-D interferometric radiometer. NASA's Aquarius mission will cover the entire global ocean on a weekly basis using three radiometers sensitive to 1.413 GHz and a scatterometer that corrects for the ocean's surface roughness.

From 408 miles in space, the sensors of the Aquarius instrument will detect subtle differences in salt content of ocean surface water as differences in thermal brightness in the microwave band. A saltier area of ocean emits a brighter thermal signal to the Aquarius sensors, which are able to detect changes as small as two parts per 10,000.
"If you took a pinch of salt and put it in a gallon of water, we could detect that sensitivity from 408 miles above the Earth," Aquarius project manager Amit Sen said in a NASA podcast. "That's quite a feat by itself."

Among other advances, the satellite-borne measurements will fill in gaping holes in data from the southern hemisphere oceans, key areas of ocean circulation where monitoring is especially sparse. They will also develop more detailed profiles of changes underway in the central North Atlantic, where salinity has been increasing. And researchers will get a more detailed look at the Nordic and Labrador Seas, where cold, salty, dense water accumulates and sinks to the depths, part of a global conveyor that transports heat from the equator toward the poles.

The new readings also will supply important information about the exchange of energy and water vapor between the ocean and the atmosphere, where the processes of evaporation and precipitation account for 80 percent of the planet's water cycle.

All of this new information is expected to eventually find its way into computer models that simulate global ocean and atmospheric circulation, improving forecasts of the future of our changing climate.

Principle Investigator Gary Lagerloef, a scientist at the independent lab Earth & Space Research in Seattle, sees a new frontier in satellite-based ocean research.

"We'll see the ocean in a whole different light," he told NASA's Alan Buis. "When the first Earth science satellites launched in the 1970s, we saw ocean eddies for the first time and got our first glimpse of the tremendous turbulence of the ocean. With Aquarius, we're going to see things we don't currently see. It's as though the blinders will be removed from our eyes."

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Author:John D. Cox