A recent study in Northeast Asia determined that ash from volcanic eruptions can be a significant suppressor of tropical cyclone formation because the ash has a cooling effect on the ocean. However, instead of looking to the sky to make their discovery, the researchers found their answers in the rings of local trees.

The study, appearing in the June 25 issue of Science of the Total Environment, provides some of the first robust evidence that volcanic eruptions influence the formation of these devastating storms over a large part of the world's biggest continent.

Tropical cyclones — a scientific term encompassing hurricanes and typhoons — currently affect about 35% of the world's population, and the scope of the damage they cause is expected to increase in the coming years in response to climate change. 

The researchers used the levels of a specific oxygen isotope present in rings from 63 trees in a Korean national park to reconstruct a 350-year history of activity of tropical cyclones in South Korea and the surrounding ocean.

This isotope, δ¹⁸O, has been proven to be a good indicator of tropical cyclone activity because it is present in significantly smaller quantities in rainfall from tropical cyclones than in regular precipitation. This results in less of the isotope being absorbed into the tissue of trees in typhoon years.

The team's reconstruction proved highly accurate. While parts of it are difficult to verify due to historically patchy records of tropical cyclones, the researchers compared their findings to detailed records of cyclones from 1950-2005 and found that their predictions were 96.4% accurate.

The findings confirm that tree rings hold much more information than the age of the tree.

“Tree rings basically reflect everything that is affecting trees," said first author Jan Altman, a researcher from the Institute of Botany of the Czech Academy of Sciences. “Tropical cyclones induce significant damage to trees, so some will have lower growth for several years as they invest all the energy to heal this damage, and you can see this in the rings.”

Using tree rings to track the activity of tropical cyclones is nothing new to Altman and his colleagues. They have used this method in their research as far back as 2013, when they published a study examining how tropical cyclones influence the growth of trees.

However, in this study the researchers wanted to use tree rings to predict some of the factors that contribute to the formation of these storms over long periods of time, something that is still a mystery to scientists.

“We know from the short-term meteorological and satellite observation that tropical cyclone activity is rapidly changing. However, we do not know if these changes are exceptional on a long-term scale,” Altman told The Academic Times.

When the researchers compared their reconstruction of cyclone activity with records of known major volcanic eruptions, they consistently found that levels of the oxygen isotope in question were elevated in the two years immediately following a volcanic eruption, suggesting a suppression of cyclone activity.

The researchers propose that this suppression stems from cooling in the atmosphere and oceans caused by volcanic ash.

“It is known that large volcanic eruptions affect the worldwide climate. Specifically, they have a cooling effect as the sulfates in the air reduce the amount of light penetrating the stratosphere,” Altman said. “This cooling effect also reduces the sea surface temperature. One of the main conditions for the formation of cyclones is that sea surface temperature is relatively high, so the cooling effect induced by large volcanic eruptions reduces cyclone activity.”

One point of surprise for the researchers was how well their data fit this model, because their findings contradicted the conclusions of recent studies that rejected the ocean-cooling hypothesis, including one from 2019 in npj Climate and Atmospheric Science.

Another 2019 study, published in PNAS, did find that volcanic eruptions can affect tropical cyclone formation, but not on a global scale.

So while these findings do shed some light on the relationship between volcanic eruptions and tropical cyclones, these events and their connection to the wider climate are still largely shrouded in mystery.

“In future research we would like to provide more complex stories from more localities to provide a better understanding of the large-scale processes at play in tropical cyclones,” Altman said. “Although the processes are very complex, we believe that such findings can improve our knowledge of the effect of the large volcanic eruptions on global climate.”

The paper “Large volcanic eruptions reduce landfalling tropical cyclone activity: Evidence from tree rings” will be published in the June 25 issue of Science of the Total Environment. The authors of the study were Jan Altman, Jiri Dolezal and Nela Maredova, Czech Academy of Sciences; Matthias Saurer and Kerstin Treydte, Swiss Federal Institute for Forest, Snow and Landscape Research; Jong-Suk Song, Andong National University; and Chang-Hoi Ho, Seoul National University.

The headline of this story has been updated to better reflect the scope of the research.