Eddies have been overlooked for too long. These turbulent swirls of water, ranging in size from a few kilometers to hundreds of kilometers across, peel off large ocean currents and mix heat and carbon dioxide into deeper ocean layers, like cream stirred into coffee. They are the most energetic feature of the ocean, critical to getting climate models right—but also largely invisible to satellites, except when they happen to sweep up a massive bloom of green phytoplankton.
No longer. Eddies and, on land, the ebb and flow of rivers and lakes will snap into focus after the launch of the Surface Water and Ocean Topography (SWOT) satellite, a joint venture between NASA and CNES, the French space agency. Expected to launch no earlier than 15 December from Vandenberg Space Force Base in California, on a SpaceX Falcon 9 rocket, the $1.2 billion satellite carries an altimeter that can measure the height of a water surface to within a couple centimeters, allowing researchers to infer the movements that sculpt it. “The change we expect from SWOT will be quite dramatic,” says J. Thomas Farrar, a physical oceanographer at the Woods Hole Oceanographic Institution (WHOI).
For oceanographers it will be like slipping on a pair of eyeglasses, says Rosemary Morrow, a physical oceanographer at the Laboratory of Space, Geophysical, and Oceanographic Studies in Toulouse, France. The satellite will capture eddies as small as
7 kilometers across and cover nearly the entire globe every 21 days. On land, SWOT will be able to map the changing height of more than 6 million lakes, from the Great Lakes down to ponds, while also capturing flows in rivers wider than 100 meters. It will replace spotty, infrequent measurements from the ground and make the field of hydrology far more empirical, and global, than it ever has been. “It’s going to help us constrain how the water cycle works in the Arctic, Africa—places where we don’t have on the ground data,” says Tamlin Pavelsky, a hydrologist at the University of North Carolina, Chapel Hill, and co-lead of SWOT’s freshwater science team.
For nearly 4 decades, NASA and CNES have launched a series of radar altimeter satellites, which use reflected pulses of radar to measure water height. Those instruments have monitored the accelerating rise of global sea levels, a basic indicator of climate change. By measuring the ocean’s bulges and dimples, they also track the large-scale currents that sweep water around the planet. But the satellites’ coarse spatial resolution meant rivers and small eddies were out of reach.
SWOT gains a sharper view with the help of two 5-meter booms, each bearing an antenna to catch reflections of the radar signal SWOT pulses to Earth’s surface. The widely separated antennas give SWOT the resolution to measure the height of a patch of water just kilometers wide, rather than hundreds of kilometers, bringing small eddies into view.
Armed with the precise observations, hydrologists will be able to say how lakes and rivers change seasonally, and how short-term climatic drivers, such as El Niño, affect those rhythms. For marine ecologists, SWOT will be able to chart how the levels of the world’s major rivers drop each time a dam or weir interrupts them, and how severely that fragments aquatic habitats. It will also see the ripples that betray a river’s shallows and deep pools, a boon to studies of how rivers evolve. And SWOT will capture flood waters as they move down river, which should help flood modelers, although the measurements won’t be fast enough to help communities prepare.
Although SWOT is supposed to operate for just 3 years, its science team plans to look for correlations between the water flows it detects and features the ongoing Landsat missions see in visible light, such as changing lake and river widths. Those visible changes could serve as proxies for water levels, allowing researchers to continue to keep tabs on the planet’s flows, Pavelsky says. “Even once SWOT is gone, you can still continue that analysis.”
SWOT’s view of eddies may be its biggest payoff. For example, it will test predictions that thousands of small eddies stir the ocean at any one time, says Sylvia Cole, a physical oceanographer at WHOI. Eddies just a few kilometers across likely play a critical role in stirring heat and carbon into the oceans near the poles, Morrow says. They also drive the mixing in smaller seas, she says. “We’re probably underestimating the energy in the Mediterranean by 90% because we’re missing these smaller scale structures.”
At the coastlines, SWOT will provide a detailed picture of how hot spots of sea level rise in the open ocean influence coastal inundation, says Sönke Dangendorf, a physical oceanographer at Tulane University. It will also study another potential threat to coasts: small eddies that might warm nearby waters, fueling stronger hurricanes, Morrow says. “We’re trapping more heat not just at the surface, but also at depth.” These are important questions to answer, fast, as much of humanity lives along coasts, she adds. “Everything is felt more keenly in the coastal zone.”
Source: Science Mag