High in the sky may affect something low in the deep ocean. This is far from an intuitive deduction. A University of Utah study suggests something amazing: Periodic changes in winds 15 to 30 miles high in the stratosphere influence the seas by striking a vulnerable Achilles heel in the North Atlantic and changing mile-deep ocean circulation patterns, which in turn affect Earth’s climate.
"We found evidence that what happens in the stratosphere matters for the ocean circulation and therefore for climate," says Thomas Reichler, senior author of the study published online Sunday, Sept. 23 in the journal Nature Geoscience.
Scientists already knew that events in the stratosphere, 6 miles to 30 miles above Earth, affect what happens below in the troposphere, the part of the atmosphere from Earth’s surface up to 6 miles or about 32,800 feet. Weather occurs in the troposphere. It is also where humans for the most part live.
Researchers also knew that global circulation patterns in the oceans — patterns caused mostly by variations in water temperature and saltiness — affect global climate.
"It is not new that the stratosphere impacts the troposphere," says Reichler, an associate professor of atmospheric sciences at the University of Utah. "It also is not new that the troposphere impacts the ocean. But now we actually demonstrated an entire link between the stratosphere, the troposphere and the ocean."
Reichler and colleagues used weather observations and 4,000 years worth of supercomputer simulations of weather to show a surprising association between decade-scale, periodic changes in stratospheric wind patterns known as the polar vortex, and similar rhythmic changes in deep-sea circulation patterns. The changes are:
Stratospheric sudden warming events occur when temperatures rise and 80-mph polar vortex winds encircling the Artic suddenly weaken or even change direction. These winds extend from 15 miles elevation in the stratosphere up beyond the top of the stratosphere at 30 miles. The changes last for up to 60 days, allowing time for their effects to propagate down through the atmosphere to the ocean.
Changes in the speed of the Atlantic circulation pattern — known as Atlantic Meridional Overturning Circulation — that influences the world’s oceans because it acts like a conveyor belt moving water around the planet.
Sometimes, both events happen several years in a row in one decade, and then none occur in the next decade. So incorporating this decade-scale effect of the stratosphere on the sea into supercomputer climate simulations or models is important in forecasting decade-to-decade climate changes that are distinct from global warming, Reichler says.
"If we as humans modify the stratosphere, it may — through the chain of events we demonstrate in this study — also impact the ocean circulation," he says. "Good examples of how we modify the stratosphere are the ozone hole and also fossil-fuel burning that adds carbon dioxide to the stratosphere. These changes to the stratosphere can alter the ocean, and any change to the ocean is extremely important to global climate."
In the 1980s and 2000s, a series of stratospheric sudden warming events weakened polar vortex winds. During the 1990s, the polar vortex remained strong.
Reichler and colleagues used published worldwide ocean observations from a dozen research groups to reconstruct behavior of the conveyor belt ocean circulation during the same 30-year period.
"The weakening and strengthening of the stratospheric circulation seems to correspond with changes in ocean circulation in the North Atlantic," Reichler says.
Tje study suggests there is "a significant stratospheric impact on the ocean," the researchers write. "Recurring stratospheric vortex events create long-lived perturbations at the ocean surface, which penetrate into the deeper ocean and trigger multidecadal variability in its circulation. This leads to the remarkable fact that signals that emanate from the stratosphere cross the entire atmosphere-ocean system."
For further information see Stratospheric Winds.
Vortex image by D. Reichler via University of Utah.