Volcanism disrupts northern precipitation

Earth’s tropical belts transport heat and moisture poleward, which can supply rainfall to temperate latitudes. During the mid 20th century, the width of the belts, or Hadley cells, shrank as their poleward edges moved equatorward. Since about 1980, however, the northern belt has been expanding (see the article by Thomas Birner, Sean Davis, and Dian Seidel, Physics Today, December 2014, page 38 ). Raquel Alfaro Sánchez of the Ecological and Forestry Applications Research Centre in Barcelona, Spain, and her colleagues turned to past observations to help explain the tropical belt’s changing width.

Earlier simulations of the circulation hypothesized that when a volcano erupts, sulfur and other aerosols ejected into the atmosphere reduce the incoming solar radiation, which cools the air and tends to contract the tropical belt. Using the width of tree rings from five regions across the globe (see the article by Toby Ault and Scott St. George, Physics Today, August 2018, page 44 ), the researchers reconstructed the latitude position of the Hadley circulation in the Northern Hemisphere over the past 800 years. Their analysis showed that the tropical belt contracted by 0.25 to 1.56 degrees latitude about a year after the eruption of a tropical or midlatitude volcano. Observed and simulated eruptions can also trigger El Niño events, which strengthen the subtropical jet stream that brings heat and moisture to places like the western US and eastern Europe: Both experienced wetter-than-average conditions after volcanic events and a consequent surge in tree growth.

Alfaro Sánchez and colleagues also looked at the decades when their reconstructed tropical belt showed the greatest expansion, the longest period of which lasted from 1568 to 1634 CE (highlighted in yellow in the figure). Although they aren’t sure what caused that expansion, the time period coincides with large drought events and societal upheaval in regions in what are now the US, Turkey, and China. Early English colonists landed on Roanoke Island in 1585 and mysteriously disappeared shortly thereafter. The Celâlî rebellion against the Ottoman Empire from 1596 to 1610 may have been exacerbated by famine. And a similar chain of events may have contributed to the Ming dynasty’s final collapse in 1644. (R. Alfaro-Sánchez et al., Nat. Geosci., 2018, doi:10.1038/s41561-018-0242-1 .)