OREGON STATE UNIVERSITY

Nature Study: Scientists make important discovery about equatorial ocean cooling

10/12/2009

CORVALLIS, Ore. – 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 that were thought to warm ocean surface waters, actually cool them.

The implications are significant, researchers say. Climate models that project global atmospheric circulation don’t accurately account for the effects of these tropical instability waves, while at the same time they have an enormous impact on sea surface temperatures near the equator, according to Jim Moum, a professor of oceanography at Oregon State University and principal investigator on the project.

“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,” Moum said. “Our measurements indicate that, instead of warming, TIWs cool equatorial waters by mixing from below. So it appears we have a problem that needs to be straightened out.

“It may help explain in part why some of these circulation models haven’t been as accurate as we had hoped they would be.”

Results of the study are being published this week in Nature Geoscience.

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.

“The water is moving in all directions, at different velocities and creating a spiral, eddy-like structure,” Moum said. “It is a very unique environment and we are just beginning to understand the processes that make it work.”

The research team, from OSU and the University of Washington, discovered this through a systematic and comprehensive series of shipboard measurements in the fall of 2008 using specialized instrumentation developed at OSU.

Tropical instability waves have been hard to characterize in detail because of the remote location and the sheer number of measurements required. Complicating matters, the scientists say, is that these tropical instability waves – though considered annual – don’t actually occur every year. Their intensities differ, as do their duration.

And no one is sure what causes these waves in the first place.

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

Moum said that these waves are hard to study because they require intense measuring of the vertical water column over a period of several weeks, and moorings established at the equator to gauge potential El Nino and La Nina events aren’t yet calibrated to measure the mixing of water. Moum and his colleagues are working on that project.

The last major attempt to monitor these tropical instability waves came in 1991, but when researchers arrived at the equator they discovered it was one of the years when the TIWs simply weren’t there.

“They tend to not be there during El Nino years,” Moum said, “but we don’t know why.”

One of the challenges in doing the 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.

“These sea surface temperatures form the lower boundary of the atmosphere, which then responds to those temperatures, so 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.

“So 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.”

The research was funded by the National Science Foundation. Other authors on the study include Alexander Perlin, Jonathan Nash and Philip Wiles of OSU’s College of Oceanic and Atmospheric Sciences; and Ren-Chieh Lien and Michael Gregg of the University of Washington.