A recent study suggests that human activity may have decreased the number of tropical cyclones in the western North Pacific. A separate study predicts that the total number of intense tropical cyclones will decrease globally in the decades to come, but rise in certain regions. Both appeared in Geophysical Research Letters.
Human activity continues to expel large amounts of heat-trapping carbon dioxide into the atmosphere, influencing tropical cyclone activity. But Chiharu Takahashi, a research scientist at the University of Tokyo’s Atmosphere and Ocean Research Institute, and his colleagues note that aerosols of microscopic solids and liquid droplets can also have an impact on cyclone formation. For instance, desert dust can absorb sunlight and warm the atmosphere, while volcanic aerosols and human-generated sulfates can reflect sunlight back into space and cool the atmosphere.
Takahashi and his colleagues took note of a curious trend: the number of tropical cyclones in the southeastern part of the western North Pacific has decreased markedly over the past two decades. Hoping to find out why, they turned to aerosols.
Using global climate models, the researchers examined both tropical cyclone activity and industrial and volcanic sulfate aerosol emissions levels over space and time. Their research suggested that such aerosols may be responsible for roughly 60 percent of “the pronounced decrease in tropical cyclone generation in the southwestern part of the western North Pacific for 20 years from the early 1990s,” Takahashi says. He and his colleagues detailed their findings online Sept. 22 in Geophysical Research Letters.
How might sulfate aerosol emissions lower sea surface temperatures? Global climate models, the authors note, suggest that aerosols have the potential to increase the amount of vertical wind shear in the western North Pacific, which would suppress the formation of the warm cores of tropical cyclones. Another aerosol effect: a potential decrease in the circulation of air at low altitudes that contributes to tropical cyclones.
A separate study, published two weeks later, examined the effect global warming might actually have on the number of tropical cyclones in the coming decades. “The most surprising result of our study is the reduction in the number of very intense tropical cyclones in simulations of future climate,” says study lead author Kohei Yoshida, an atmospheric scientist at the Meteorological Research Institute in Tsukuba, Japan. He adds that other studies have suggested similar trends.
The researchers used 60-kilometer-resolution global climate models—higher-resolution than previous—to investigate future tropical cyclone activity. Assuming a 4 degree Celsius rise in surface temperatures by 2090, the simulations suggested an overall global drop in the number of very intense tropical cyclones by 13 percent. However, the numbers of such powerful storms did rise in specific regions, including the south of Japan and south of Madagascar. The variations the researchers saw were due largely to differences in the degree of sea surface temperature change in the ocean basins of each region.
“These two studies clearly provide some different views of human impacts on tropical cyclones,” says Yuan Wang, an atmospheric scientist at the California Institute of Technology in Pasadena, who did not take part in either study. Conventional thinking, Wang notes, suggests that the major source of energy for tropical cyclones, sea surface temperature, will increase along with global warming. “These two studies show how the story is more complicated,” he says, noting that Takahashi’s aerosol results demonstrate why atmospheric conditions must also be taken into account.