Volcanic eruptions can have an enormous influence on global climate from year to year. Most notably, the sulfuric acid particles from eruptions reflect sunlight and absorb energy radiating from Earth, warming the lower stratosphere while cooling the troposphere. Despite its importance, the atmospheric response to volcanic eruptions is not well characterized. Only two large tropical eruptions have occurred since the beginning of the satellite era in 1979, and climate models often seem to contradict observations of those eruptions. In a new study, McGraw et al. work to explain this apparent discrepancy.

After the eruptions of El Chichón in 1982 and Mount Pinatubo in 1991, the belt of westerly winds surrounding Antarctica expanded toward the equator. Because of the sample size of only two events, previous studies were unable to conclude whether there was a causal relationship between the eruptions and the atmospheric changes (called a negative Southern Annular Mode, or SAM) that followed.

By analyzing observations of the Mount Pinatubo and El Chichón eruptions alongside 207 model simulations, the team found that the atmospheric response to volcanic eruptions is a positive SAM and that the concurrent El Niño during both eruptions may be to blame for why a negative SAM was observed. Their research also shows that the positive SAM response to volcanic eruptions is stronger during La Niña, when the equatorial Pacific Ocean is cooler than usual.

The statistical significance of the scientists’ results relies upon their use of many different simulations. Because internal climate variability is large, the Southern Hemisphere’s atmospheric response to volcanic eruptions could be obscured in any single simulation. Between internal variability and the effects of El Niño and La Niña, the discrepancies between climate models and observations can be accounted for. Earth’s atmosphere is a complicated system, and the response to major tropical volcanic eruptions like Pinatubo, although significant, does not overwhelm its other complexities. (Geophysical Research Letters, doi:10.1002/2016GL069835, 2016)

—Leah Crane, Freelance Writer