Stratosphere wind changes affect seas and climate, study shows
Changes in the winds high in the stratosphere affect the seas and climate, according to a new study. The research from the University of Utah study suggests changes in winds from 15-30 miles up hit at a vulnerable "Achilles heel" in the North Atlantic and alter mile-deep sea circulation patterns that change the weather. The findings have been published in the online journal Nature Geoscience.
Senior author Thomas Reichler says, "We found evidence that what happens in the stratosphere matters for the ocean circulation and therefore for climate."
It was already realised that changes in the stratosphere, between 6-30 miles above Earth, affect what happens in the troposphere, which stretches from Earth's surface up to six miles or about 32,800 feet up and where weather occurs.
It was also known that global circulation patterns in the seas, caused mainly by changes in water temperature and saltiness, influence global climate.
Associate professor of atmospheric sciences at the university, Thomas Reichler, says, "It is not new that the stratosphere impacts the troposphere. 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."
Thomas Reichler carried out the research with University of Utah atmospheric sciences doctoral student Junsu Kim, atmospheric scientist Elisa Manzini and oceanographer Jà ¼rgen Krà ¶ger, from the Max Planck Institute for Meteorology based in Hamburg, Germany. The study was financed by the University of Utah.
The team used weather observation data supercomputer simulations of 4,000 years of weather to reveal a surprising association between decade-scale, alterations in stratospheric wind patterns called the polar vortex, and rhythmic differences in deep-sea circulation patterns.
There are two main changes:
These can happen several years running or miss a decade, so adding this decade-scale effect into climate modelling is important in forecasting decade-to-decade climate changes that are difference from global warming, says Thomas Reichler.
"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. These changes to the stratosphere can alter the ocean, and any change to the ocean is extremely important to global climate," he explains.
"The North Atlantic is particularly important for global ocean circulation, and therefore for climate worldwide. In a region south of Greenland, which is called the downwelling region, water can get cold and salty enough - and thus dense enough - so the water starts sinking."
The area is the most important region of seawater downwelling on Earth. The sinking of cold, salty water "drives the three-dimensional oceanic conveyor belt circulation. What happens in the Atlantic also affects the other oceans."
"This area where downwelling occurs is quite susceptible to cooling or warming from the troposphere. 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," he says.
Due to that sensitivity, Thomas Reichler has named the sea south of Greenland "the Achilles heel of the North Atlantic."
During the winter months the stratospheric Arctic polar vortex turns counterclockwise around the North Pole, which means powerful 80-mph winds are around 60 degrees north latitude. They are even stronger than jet stream winds that travel at less than 70 mph in the troposphere. But every few years the stratospheric air is suddenly and dramatically altered and the vortex becomes warmer and weaker and can even go clockwise.
"These are catastrophic rearrangements of circulation in the stratosphere," and the weaker or reversed polar vortex persists up to two months. Breakdown of the polar vortex can affect circulation in the troposphere all the way down to the surface," Thomas Reichler says.
His research investigated whether changes in stratospheric polar vortex winds make the sea warmer or colder and how the sea is affected.
Scientists knew that the wind changes affected the North Atlantic Oscillation, low pressure centred on Greenland and high pressure over the Azores to the south. The pattern can reverse or fluctuate.
As the changing pressure patterns are over the sea downwelling area near Greenland, it remains to be confirmed if the pattern affects the downwelling and the global oceanic circulation conveyor belt.
The computer simulations reveal a decadal on-off pattern of correlated alternations in the polar vortex, atmospheric pressure changes above the North Atlantic and differences in sea circulation over one mile under the waves. Observations are consistent with the pattern revealed in computer simulations.
In the 1980s and 2000s, several stratospheric sudden warming events made polar vortex winds weaker. In the 1990s, the polar vortex remained strong.
Thomas Reichler and his team used global ocean observations from various studies to reconstruct the way the conveyor belt ocean circulation behaved during the same 30-years.
"The weakening and strengthening of the stratospheric circulation seems to correspond with changes in ocean circulation in the North Atlantic," Thomas Reichler says.
To reduce uncertainties about the observations, the researchers used computers to simulate 4,000 years of atmosphere and ocean circulation.
"The computer model showed that when we have a series of these polar vortex changes, the ocean circulation is susceptible to those stratospheric events," he explains.
To further back up the findings, the researchers combined 18 atmosphere and ocean models into one giant simulation and saw similar outcomes.
The researchers say the study suggests there is "a significant stratospheric impact on the ocean."
"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."
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