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Calculating Evaporation from the Ocean

Scientists strive to unravel a mix of dynamic factors

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Illustration by Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution

Imagine you turn on the tap in the morning and water pummels out and spills over your sink. Later you go out to your garden, but water trickles feebly out of the hose. The water pump in your house is definitely not working the way it used to.

Scientists say something like that is probably happening in our planetary home. Climate change is gunning the motor of Earth’s water pump, driving more rainfall to already wet areas and less to drier regions.

To get a handle on how things will change, it would sure help if we could get a handle on how the motor is driving more moisture to the atmosphere. But figuring out how and why water molecules move between air and ocean, at present at least, is a formidable challenge for scientists.

“It’s the boundary between two turbulent fluids, the ocean and atmosphere; each is its own thing, basically chaotic and hard to calculate,” said Carol Anne Clayson, an oceanographer at Woods Hole Oceanographic Institution.

“Then the two are coupled: If something changes the sea surface temperature, for example, the atmosphere responds to it, and every atmospheric response changes the sea surface temperature,” she said. “We don’t have the computational power to simulate in a model all the physics that goes on—even if the interface between them were flat and never-changing.”

Which it is most definitely isn’t. The air-sea interface “is typically the most turbulent part of the ocean,” Clayson said. A dizzying mix of interrelated factors—waves, winds, water temperature and salinity, bubbles and spray, solar radiation, and others—each adds a layer of complexity that occurs over wide ranges of time (seconds to seasons) and space (millimeters to miles). [See illustration above.]

“Getting observations of what’s going on at the air-sea interface is not trivial, especially in extreme conditions such as high winds,” Clayson said. “It’s also difficult to simulate the air-sea interactions, especially in extreme conditions, in laboratory experiments in a wave tank. Current computers don’t have enough computational capacity to incorporate all the processes occurring, on all the spatial and temporal scales involved, to produce realistic simulations.”

Clayson uses satellite measurements in an effort to improve estimates of ocean evaporation rates. “But the problem is going to be solved by large groups of people applying a range of methods,” using more and improved data from satellites, sensors deployed in the ocean, and lab and at-sea experiments, she said. All of these will feed more details into models that can provide insights into how the climate and ocean are working and improve predictions of where and when rainfall is coming, or not coming, from the ocean in the future.

Carol Anne Clayson’s research is funded by the 
NASA and the National Science Foundation.

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