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Volcanic forcing of El Niños

The winter of 1982-1983 was arguably one of the strongest El Niños of the last century. As in other years, the climate oscillation, which begins every three to 11 years with a warming in the western Pacific, brought drought and fire to some parts of the world and torrential rain and floods to others. That winter, the natural disasters led to the deaths of more than 2,000 people and the displacement of hundreds of thousands more.

Large ash plumes from equatorial volcanoes, such as El Chichón in Mexico, can likely double the chances of an El Niño the following winter. The steaming crater of El Chichón volcano is seen here in January 1983, less than a year after a major explosive eruption formed a new crater that later partially filled with a steaming, acidic lake. Photo by Bill Rose, courtesy of Global Volcanism Program at Smithsonian National Museum of Natural History.


The previous spring, from late March to early April of 1982, El Chichón volcano erupted in three major explosive bursts — the most destructive eruptions in Mexico's history. The plumes reached up to 20 kilometers high and ejected an estimated 10 million tons of sulfur dioxide aerosols into the stratosphere, where within weeks, they were distributed around the globe.

The timing of these two events spurred research to connect large volcanic eruptions with the onset of an El Niño. Now, climatologist Julian B. Adams of the University of Virginia and colleagues report in the Nov. 20 Nature that the events are likely linked in certain cases. By comparing paleoclimate reconstructions of El Niños from tree rings, corals and ice cores to proxy records of volcanic activity, they have established that the chances of an El Niño-like event occurring after a large tropical eruption are double what they would be by chance alone.

"The new paper suggests that there is apparently some influence from volcanic aerosols on the Pacific ocean-atmosphere system, but the authors are careful to describe these perturbations as El Niñolike," says Stephen Self, a volcanologist at Open University in Milton Keynes, United Kingdom, who studies the atmospheric impacts of volcanism.

Adams' team is not the first to propose such a connection. After El Chichón, the late Paul Handler, a physicist at the University of Illinois, Urbana-Champaign, put forth the theory that El Niños were associated with volcanic eruptions.

Since then, several researchers have tried to confirm the link between volcanism and the onset of an El Niño, but they all encountered the same problem: Not all eruptions were followed by El Niños, and not all El Niños were preceded by eruptions. In fact, a 1995 study showed that the 1982-1983 El Niño had slightly preceded the El Chichón eruptions. Additionally, historic accounts of El Niños and volcanic emissions that were made prior to the invention of scientific instruments were not reliable enough to tease apart the specific circumstances under which the association did occur.

The new Nature study shows a small but statistically significant signal, which appears as a multi-year response after large tropical eruptions: a warming, or El Niño-like event in the first three years, followed by a cooling, or La Niña-like event, over the next three years. "It's a careful study that presents a much more reasoned and convincing case than earlier work about a teleconnection between El Niños and volcanic eruptions," Self says.

The findings are important for researchers attempting to accurately predict El Niños in order to reduce their human and economic costs and also to climatologists studying how future climate change might affect these events.

The study suggests, somewhat counterintuitively, that volcanic aerosols ejected near the equator reflect incoming solar radiation, causing an atmospheric cooling, but also producing a warming in the tropical Pacific. The authors say that because oceanic and atmospheric circulations are linked, the decrease in solar radiation triggers a complex chain of events that is similar to the normal behavior of El Niños and La Niñas. "The same mechanisms which lead to natural oscillations cause the surface temperatures in the eastern and central Pacific to react oppositely to what might typically be expected," says co-author Michael Mann of the University of Virginia.

Alan Robock, director of the Center for Environmental Prediction at Rutgers University and editor of the Journal of Geophysical Research-Atmospheres, and who also studies volcanic eruptions and climate, says that he would like to know what the physical mechanism is that produces this response and "see it reproduced in climate models."

Mann says a follow-up study will show that the Cane-Zebiak theoretical model, the first to successfully predict El Niños, "exhibits the same behavior in response to tropical volcanic radiative forcing as found in our study."

Sara Pratt
Geotimes contributing writer


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