Winds high in stratosphere affect deep ocean currents
Evidence has emerged that changes in the winds high up in the stratosphere have a powerful effect on ocean currents a mile below the surface.
Periodic changes in winds 15 to 30 miles high affect what the University of Utah researchers call an Achilles heel in the North Atlantic, altering ocean circulation patterns and thus the Earth's climate.
"It is not new that the stratosphere impacts the troposphere," says professor Thomas Reichler. "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."
The team 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.
First, they found, 'stratospheric sudden warming' events occur when temperatures rise and 80mph polar vortex winds encircling the Artic suddenly weaken or even change direction. The changes last for up to 60 days, giving time for their effects to propagate down through the atmosphere to the ocean.
Meanwhile, the winds affect the speed of the Atlantic circulation pattern - known as Atlantic Meridional Overturning Circulation - which influences the world's oceans by moving water around the planet.
Sometimes, both events happen several years in a row in one decade - and then not for another ten years.
Key to the effect is a region south of Greenland called the downwelling region, where water can get cold and salty enough - and thus dense enough - to start sinking.
"This area where downwelling occurs is quite susceptible to cooling or warming from the troposphere," says Reichler. "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."
The team's simulations show a decadal on-off pattern of correlated changes in the polar vortex, atmospheric pressure oscillations over the North Atlantic and changes in sea circulation more than a mile beneath the waves.
"If we as humans modify the stratosphere, it may - through the chain of events we demonstrate in this study - also impact the ocean circulation. Good examples of how we modify the stratosphere are the ozone hole, and also fossil-fuel burning that adds carbon dioxide to the stratosphere," says Reichler.
"These changes to the stratosphere can alter the ocean, and any change to the ocean is extremely important to global climate."