Utah research: Stratospheric winds shape ocean currents, but how?

Above the Arctic, stratospheric winds blow counterclockwise around the North Pole at 80 mph, occasionally slowing and even reversing direction.

New research from the University of Utah shows these changes in the so-called polar vortex, 15 to 30 miles high, alter deep ocean currents, which have long been known to affect climate and weather occurring where we live — in the troposphere.

These findings suggest climate science should pay closer attention to the stratosphere in constructing the models used to predict climate change, according to Thomas Reichler, an associate professor of atmospheric science at the U.

Already well documented are the stratosphere's effects on the troposphere — the band of weather-bearing air below six miles — and the troposphere's effect on ocean currents, and vice versa. But the new U. findings are counterintuitive because the ocean's thermal mass should insulate it from rapid changes, and yet currents seem to change in response to a distant phenomenon.

"It's quite surprising to find relatively subtle events high up in the stratosphere would have an impact on the ocean," Reichler said. "The atmosphere that high is very thin, while the ocean is very heavy. How can that be?"

The how remains an open question, but 30 years of weather and ocean data, plus 4,000 years' worth of computer simulations, suggest a causal chain across the globe's ocean-atmosphere system, according to Reichler's conclusions published this week in the journal Nature Geoscience.

Climate science has tended to overlook high-altitude phenomena because it is widely presumed that oceans have a powerful effect on climate, according to Simon Wang, assistant director of Utah State University's Utah Climate Center. But Reichler's findings suggest the reverse is also true, Wang said, and even minor atmospheric changes can trigger large-scale interactions that could drive a half-century of climate change.

Reichler believes geography may be the key. The hair trigger lies in the North Atlantic just south of Greenland where the Gulf Stream drives high-salt-bearing water from the tropics. It cools on the journey north, causing it to sink.

"This area where downwelling occurs is quite susceptible to cooling or warming from the troposphere," he said. "If the water is close to becoming heavy enough to sink, then even small additional amounts of heating or cooling from the atmosphere may be imported to the ocean and either trigger downwelling events or delay them."

Creating a massive convection, this downwelling between Europe and North America influences currents all over the globe, not just in the Atlantic, Reichler said.

"How exactly do these changes in the stratosphere propagate down and affect the surface? We don't have a good answer yet," Reichler said. "That they propagate down isn't new. What's new is that it impacts the ocean in ways that seem to be important."

While changes in the polar vortex are associated with warming, he stressed that they are natural occurrences. The slowdown occurs in winter, on average, every second year, but there is no regular pattern.

Reichler's U.-funded study was co-written by doctoral student Junsu Kim, along with atmospheric scientist Elisa Manzini and oceanographer Jürgen Kröger, both with Germany's Max Planck Institute for Meteorology.

bmaffly@sltrib.com