A large and growing brown cloud of persistent air pollution hovering over northern India and surrounding regions has doubled — and occasionally tripled — the intensity of late spring cyclones in the Arabian Sea during the past three decades.
Within the past decade, several notable early-season tropical cyclones have ripped through the region. Gonu, the strongest, smashed through the Middle East in 2007, killing dozens and causing more than $4 billion worth of damage. “This supercyclonic storm was Katrina-like in size and intensity,” says climate scientist Amato Evan of the University of Virginia in Charlottesville.
These big storms, which invariably make landfall, represent a new environmental impact that can wreak havoc on people from northern India through the Middle East, Evan and his colleagues propose in the Nov. 3 Nature.
Warming sea-surface temperatures can boost the intensity of hurricanes, known in the Indian Ocean as cyclones. And for many decades, Evan says, water in the Arabian Sea has been “really toasty.” But winds in the upper and lower atmosphere there tend to blow briskly in opposing directions, in a phenomenon known as vertical wind shear. “This is the most hostile environment one could imagine for a hurricane,” Evan explains. “It literally tears a storm apart.” That appears to explain why major cyclones here were rare — until recently.
Curious about the recent emergence of big storms, Evan and his team analyzed every regional tropical cyclone going back 30 years. None had occurred during the monsoon season, and before 1998, cyclones in the months before and after that rainy season seldom reached wind speeds exceeding about 80 kilometers per hour. But since then, five monster storms reached wind speeds double the norm — sometimes exceeding 185 kilometers per hour. “’We wanted to know why,” Evan says
After poring over weather data for the period, the researchers found that in premonsoon months there had been a slow relaxation in the average vertical wind shear, from 11 meters per second to 8 meters per second. That lower speed appears to represent some type of threshold, Evan says: Once vertical wind shears reach it, “Boom! You get this explosion in storm intensities,” he says. “It’s almost like some switch has been flipped.”
The researchers ultimately linked the steady fall in vertical wind shear in cyclone seasons with a 3-kilometer-thick brown cloud of soot and other pollutant particles, known as aerosols, in the region’s lower atmosphere. This pollution has increased sixfold in 80 years, the scientists report, and now filters roughly 10 percent of sunlight, preventing it from reaching the sea surface.
Computer analyses indicate that the pollution cloud’s cooling effect on the ocean below has slowed wind speeds in the atmosphere — and allowed storm intensities to mushroom. “This link to the brown cloud is quite robust,” Evan says.
Evan and his team don’t have many storms on which to base this claim of a regional shift in climate signals, says Ryan Sriver of Pennsylvania State University in University Park. “But then, I’m skeptical of everything,” he says. “They do appear to have pretty strong evidence.”