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Tropical Instability Waves

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Scientists studying a little-understood phenomenon in the equatorial Pacific Ocean called “tropical instability waves” have made a surprising discovery: These north-south subsurface waves thought to warm ocean surface waters, actually cool them.

“The assumption for years has been that these TIWs warm sea surface temperatures at the equator by moving warm water from off the equator, toward the equator,” according to Jim Moum, a professor of oceanography at Oregon State University and principal investigator on the project. “Our measurements indicate that, instead of warming, TIWs cool equatorial waters by mixing from below.”

The mixing of waters near the equator is an extraordinarily complex series of processes. Winds blowing toward the west drive a South Equatorial Current, which in turn pushes water in a westerly direction where it piles up against the continents. This creates a large west-to-east pressure gradient creating the Equatorial Undercurrent, which flows eastward, but beneath the South Equatorial Current. Tropical instability waves—which propagate westward, but have large north-south currents—complicate the current structure, Moum said.

One of the challenges in doing this research is that mixing of the water can occur on a scale of a few centimeters, to tens of meters, Moum said. But this is buried in a structure of currents and bands of water that span hundreds of kilometers.

The cooling effect was discovered through a systematic and comprehensive series of shipboard measurements in the fall of 2008 using specialized instrumentation developed at OSU.

“What we found,” Moum said, “is that their impact on sea surface temperatures can be enormous—cooling the sea surface by an order of several degrees within just a couple of weeks. Understanding how tropical instability waves work is an important step toward improving global circulation models.”

“Since sea surface temperatures form the lower boundary of the atmosphere, which then responds to those temperatures, there are tremendous implications for global atmospheric circulation,” Moum said. “The impacts of changing equatorial surface temperatures truly are global. The Madden-Julian oscillation, for example, occurs in the Indian Ocean and has impacts on what happens in the Gulf of Mexico.

“Understanding how these processes work is an important step,” Moum added. “It is the only way that models of the atmosphere-ocean circulation can be properly calibrated.”

Article courtesy of the College of Earth, Ocean, and Atmospheric Sciences 2010 Research Highlights

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