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Volcanoes that blast gases high into the atmosphere not only change global temperatures but also influence flooding in unusual ways, Princeton researchers have found.

In an article in the journal Nature Geoscience, the researchers report that major of flooding depending on the location of the volcano and the dispersal of its plume. The patterns mostly divide along the line of the equator.

When a volcano's plume is generally contained in one hemisphere, flooding decreases in that hemisphere and increases in the other hemisphere. The pattern most strongly affects the tropical regions and demonstrates little to no effect on other regions.

Volcanoes that create plumes affecting both hemispheres show a different pattern. These eruptions decrease flooding in the tropics in both hemispheres, while increasing flooding in arid regions.

For the study, the researchers examined three major eruptions: the 1902 eruption of Santa Maria in Guatemala, whose plume was concentrated in the northern hemisphere; the 1963 eruption of Agung in Indonesia, whose plume was concentrated in the southern hemisphere; and the 1991 eruption of Pinatubo in the Philippines, with a more symmetric plume.

Global air currents play a key role in the impact on flooding

Gabriele Villarini, one of the principal researchers, said the key to the patterns lies in global air currents. Trade winds that encircle the globe meet at the equator in a region called the Inter-Tropical Convergence Zone. The converging winds create a split along a line that generally follows the equator.

The zone forms a weather band in tropical regions on both sides of the equator in which warm, moisture-laden water rises, producing heavy rainfall. The change between summer and winter shifts the line north and south, causing the rainy and dry seasons normally experienced in much of the tropics.

Major volcanic eruptions shift this pattern, said Villarini, a professor of civil and environmental engineering and the High Meadows Environmental Institute. The volcanoes blast gases, most importantly sulfur dioxide, into the stratosphere. In this region of the upper atmosphere, the sulfur gas oxidizes and becomes tiny, suspended particles.

These aerosols scatter incoming sunlight and absorb heat radiating from Earth. This simultaneously cools the Earth at surface level and warms the stratosphere, which affects air circulation. Previous scientific studies have demonstrated the effect on global temperature, and related techniques have been proposed for geoengineering projects to combat global warming.

The Princeton team found that the changes in air circulation resulting from the eruptions change the position of the Inter-Tropical Convergence Zone, causing it to shift north or south away from the hemisphere experiencing the eruption. This shift directly alters rain patterns. The zone, with its moisture-laden air, shifts away from the eruption, causing greater rain and heavier flooding in the corresponding tropical region.

Villarini said the effects of the increased rainfall are generally strongest in the year after the eruption and lessen after several years.

Agung and Santa Maria impacts were both split at the equator

The researchers examined the Santa Maria (1902) and Agung (1963) eruptions because their plumes were confined to single hemispheres. As a result, the sulfur aerosols disproportionately concentrated in that hemisphere, shifting air currents and pushing the Inter-Tropical Convergence Zone further into the other hemisphere.

After the Agung eruption in the southern hemisphere, 50% of stream gauges saw reduced peak flows (a measure of river flooding) in the tropical regions of the southern hemisphere in the first year after the eruption. Stream gauges in the tropics of the northern hemisphere saw an increase of about 40% in peak flows.

The Santa Maria eruption in the northern hemisphere was followed by a 25% increase in sites with peak flooding in the southern hemisphere's tropics, and a 35% increase in sites with decreased flows in the northern tropics. Additionally, Santa Maria saw increased higher peak floods in arid and temperate regions in the northern hemisphere. The researchers said about 25% of sites in those regions saw increases in the two years after the eruption.

Volcano plumes that straddle the equator depress flooding in the tropics and increase it in dry regions

The aerosol plume from the 1991 Pinatubo eruption spread about evenly across both hemispheres, the researchers found. Unlike the other two eruptions, Pinatubo decreased flooding in tropics in both hemispheres. Peak flows dropped at 20% of sites in the southern tropics and at 35% of sites in the northern tropics.

Arid regions showed the opposite effect. The researchers found that extremely dry regions experienced an increase in peak flows at about 35% of the sites on both sides of the equator following the Pinatubo eruption. Hanbeen Kim, lead author of the paper, said this increase is possibly due to a different air circulation mechanism called the monsoon-desert coupling.

In this pattern, air sinks over Asian monsoon regions and rises over nearby arid regions. The rising air pulls moisture upward, causing greater rainfall in the arid areas.

The researchers found that for eruptions spread across both hemispheres, like Pinatubo, shifts in the Inter-Tropical Convergence Zone do not play a major role. Instead, they said, changes in flooding are caused by cooling and related atmospheric circulation changes, such as those over desert regions.

Villarini said that by demonstrating the major effect of volcanic eruptions on flooding worldwide, the research shows the importance of understanding how changes in climate can have important effects beyond their immediate results. He said that scientists and political leaders should understand these impacts when assessing the risks of changes in the climate.