Huge swaths of ocean in the Southern Hemisphere have never been measured for saltiness. And some scientists say that mapping the amount of dissolved salt content and how it’s distributed in the world’s oceans is a key step toward giving us a better grasp of extreme weather events.
A satellite called Aquarius has been designed to fill this missing piece of the climate puzzle and is scheduled to launch into space from the Vandenberg Air Force Base on June 9. The $287 million instrument has been built to take fantastically precise measurements of the ocean's surface salinity: where it's salty and where it's not so salty. Scientists will use the data to create a detailed global salinity map.
From space, Aquarius can detect the equivalent of 1/8 of a teaspoon of salt in a one gallon bucket of water, an amount so highly concentrated it's invisible to the taste buds.
The ocean covers 70 percent of the earth. Fresh water is constantly getting sucked out of the ocean by evaporation and rising as water vapor into the atmosphere, where it condenses and then gets dumped back into the ocean as rainfall.
"What we're really interested in is how changes in global water cycles over the ocean affect circulation and how that affects climate," said Gary Lagerloef, the mission's principal investigator.
Water expands as it heats and condenses as it cools, so the saltier and colder that seawater is, the denser it is. When water density is unevenly distributed, it sets up differences in ocean pressure, which triggers ocean currents and stirs winds, Lagerloef said.
Salinity serves as an important indicator for the amount of evaporation and rainfall occurring over the ocean, said William Large, director of the climate and global dynamics division of the National Center for Atmospheric Research (NCAR.) More evaporation makes seawater saltier and more dense; more rain, and the salt levels drop. Monitoring changes in salinity, scientists say, will help us understand the water cycle, which plays an powerful role in regulating climate.
“All the water that we have evaporates from the ocean,” said Large, who will study the data that Aquarius sends back to earth. “What precipitates back onto the ocean shows up as low salinity areas. And it’s that water that doesn’t go right back onto the ocean and hits land that really matters to people.”
High density water also has a sinking component, meaning it causes elements like carbon dioxide and chlorofluorocarbons to sink into the ocean, sometimes staying underwater and out of the atmosphere for thousands, even tens of thousands of years, Large added.
“Clearly, if the ocean had not been taking up carbon dioxide as it has been, the carbon dioxide levels in the atmosphere would be higher than they are now,” he said.
For roughly the last century, salinity has been measured from ships and automated buoys. Modern oceanography determines salinity by measuring the electrical conductivity of the seawater, which varies according to the amount of salt.
The satellite instrument, which uses both a radiometer and radar, will measure radio emissions coming off the sea surface in microwave frequencies. But measuring salinity is tricky, much trickier than measuring ocean temperature, because it emits such a small signal. And this signal gets easily lost among the noise of other signals, such as temperature, windspeed and light. The challenge for scientists was to isolate a wavelength that measures salinity without getting interference from these other signals.
“It’s a great technical challenge,” Large said. “That’s why one doesn’t know quite how it’s going to do until you go out and do it.”
Salinity varies only slightly in the ocean – from 32 parts per thousand to 37 parts per thousand, averaging 35 parts per thousand.
“So there’s a very small range of variation of salinity,” Lagerloef said. “That means we have to make a very, very precise measurement with this radiometer.”
The European Space Agency launched a satellite called Soil Moisture and Ocean Salinity, or SMOS more than a year ago, which uses a slightly different technique to measure surface salinity, but focuses primarily on soil moisture. Once Aquarius is flying, the two teams will work together to combine measurements.
If all goes as planned, Aquarius will start collecting data about three weeks after launch. That data will have to be verified against ground measurements. And then the team will start creating maps. First objective, scientists say, is a comprehensive, high-resolution map of the ocean’s salinity distribution.
Then, they’ll measure salinity over the seasons, and eventually, changes in salinity from year to year. The mission is a joint undertaking between NASA and the Argentine space agency, CONAE.
“Using this information to improve our ability to monitor and forecast El Nino will be the next big thing we’ll tackle with this,” Lagerloef said. “Then looking into this longer term, we want to understand climate change.”