"The El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on the planet," says NCAR scientist Caspar Ammann. "When thinking about long-term climate, we must ask whether this system itself undergoes changes, perhaps in response to changes in radiative forcing or in the background climate itself. Our findings, based on two reconstructions, suggest that it indeed might."
When a volcano erupts in the tropics, its aerosol emissions spread into the stratosphere across the northern and southern hemispheres, reflecting some of the sun's heat back toward space and thereby cooling the Earth's atmosphere. This cooling alters the interaction between the oceans and atmosphere, possibly encouraging a warming response in the Pacific Ocean as the massive body of water attempts to restore an initial equilibrium.
"Our results suggest that the atmospheric cooling from an eruption may help nudge the climate system towards producing an El Niño event," said Michael Mann, an environmental scientist at the University of Virginia. The study results will appear in the November 20 issue of the journal Nature.
"This research illustrates the value of paleoclimate studies that draw on research from disparate fields to uncover connections," said David Verardo, director of NSF's paleoclimate program, which funded the research. "Studies of modern climate conditions gleaned from thermometers and barometers can only get you so far. Challenging the conventional wisdom, as this research does, is necessary to achieve a comprehensive understanding of Earth's climate," he said.
Some scientists had previously noted that during the 20th century, El Niño events-the periodic warming of sea surface temperatures in the equatorial Pacific-tended to follow the eruption of volcanoes in the tropics. But that 100-year period, the only time span for which reliable instrumental records were kept, was considered too short a duration to substantiate a link between the two phenomena. The connection was thought to be coincidental. "So we turned to the paleoarchives for a longer history," Mann said. "We actually didn't expect the relationship to hold up in the long run."
The scientists instead found that, when looking back over a 350-year period, as far back as paleorecords allow, there was credible evidence that volcanic activity in the tropics may play a significant role in the occurrence of El Niño events. "We now have a long record showing that the relationship between volcanic eruptions and an increased probability of El Niño events continues to hold up over several centuries," Mann said. "It's probably not just a fluke."
Mann, Ammann, and UVa scientist Brad Adams used the paleoclimate records stored in ice cores, corals, and tree ring records to reconstruct El Niño events. They used independent ice-core volcanic dust evidence to reconstruct volcanic activity back to the early 1700s.
The paleoclimate records are called 'proxy records' because they are not direct measurements of current climate and ocean conditions, but instead are reconstructions of past conditions gleaned from the physical, biological, or chemical records or, "signatures," stored in natural archives in the environment. Using these records, the scientists were able to precisely identify the years when eruptions occurred and the years when El Niño events occurred.
When they counted, year by year, the separate events and brought them together for comparison, they found that there was a nearly one-in-two chance that an El Niño event will occur after a volcanic eruption in the tropical zone, roughly double the normal probability. "I wouldn't call this a tight connection - it's not a one-to-one relationship," Mann said, "but it appears that the eruption of a tropical volcano nudges the climate towards a more El Niño-like state."
El Niño is a prominent altering factor on world climate, affecting weather patterns for months and years, often causing drought and severe weather in different parts of the world. "We seek to understand how El Niño responds to changes in natural factors such as volcanic activity in part, so we can potentially better understand how El Niño might respond to more recent human influences on climate," Mann said.
Adams added that the findings might help oceanographers and atmospheric scientists to make better probabilistic forecasts of El Niño activity. "This is not a strictly predictive tool, but it may help in anticipating the odds that an El Niño event might occur in a given period," Adams said.
The National Oceanic and Atmospheric Administration also sponsored the research.
NSF Program Contact: Dave Verardo, firstname.lastname@example.org.