Cyclone Chapala slammed into Yemen earlier this week, a rare occurrence in the region, displacing tens of thousands and creating an acute humanitarian problem in a region already steeped in civil war and pervasive poverty.
The storm, with its extremely heavy rains and flooding, affected as many as 1.1 million people, and displaced more than 36,000, according to the United Nations Office for the Coordination of Humanitarian Affairs (OCHA). The agency said that an airlift was being prepared for Socotra Island, where three people reportedly died from the storm.
Phil Klotzbach, a meteorologist at the Tropical Meteorology Project at Colorado State University, said that Cyclone Chapala was the first hurricane-strength storm to hit Yemen since the 1940s, and the first tropical storm-strength tempest to hit the country since 1960 — with the caveat that historical records from the region are not good. (Hurricanes in the Indian Ocean are called cyclones.)
It's more common for cyclones to impact neighboring Oman than Yemen, Klotzbach said.
"This Chapala storm is the second-strongest storm in the Arabian Sea, on record," he said. Records date to roughly 1990. He estimated that it was a category-one storm when it made landfall. The strongest storm was Cyclone Gonu, which hit Oman in 2007.
(Video via NASA)
While the water in the Arabian Sea is typically warm, which is good for hurricane formation, usually this region has high wind shear, which is bad for cyclone formation. Lower shear levels permitted this storm to form, he said.
Klotzbach referred to Cyclone Chapala as "unusual but not unprecedented."
The storm is unusual for another reason, said Amato Evan, an assistant professor at Scripps Institution of Oceanography at the University of California, San Diego. He said that when there is a cyclone in this region, it is more common for it to occur before the monsoon in India. This storm occurred post-monsoon.
"We've never had a really strong storm in this post-monsoon period," Evan said. "We've never had this kind of just really perfect conditions for a big storm to form, in the Arabian Sea, in this post-monsoon period."
Evan is the coauthor of a 2011 study in the journal Nature that argued that there was a connection between cyclones in the region and air pollution from the Indian subcontinent. That study posits that the air pollution can actually inhibit wind shear, allowing for more powerful storms to form.
Gabriel Vecchi, an oceanographer at NOAA's Geophysical Fluid Dynamics Laboratory, said that when thinking about a specific hurricane or cyclone, it's important to consider that storm's "narrative," or the literal factors that caused it, such as a lack of wind shear, or the winds that affected its certain path. He also noted that waters off Yemen were "anomalously warm" in recent days.
Then there are bigger factors that can affect the probability of a storm occurring, which are more difficult to nail down right away.
"If we find out that Chapala was more likely this year than other years, then we can start asking, well why was it more likely this year than other years," Vecchi told VICE News. One of those factors is the ongoing El Nino, he said, in which the tropical Pacific warms — but the western Indian Ocean warms up as well.
Then there's global warming as a potential factor to affect the probability of storms, he said.
"One of the things that there tends to be some agreement on [across different studies], is that the Arabian Sea, the northwest Indian Ocean— we expect it to be somewhat more favorable to tropical cyclones as you add greenhouse gases to the planet, and as the planet adjusts to those greenhouse gases," Vecchi said.
He added that the research about the effect of global warming on storms in this region is focused on end of this century. "So we need to be careful about over-interpreting these results about the end of the 21st-century when sitting in 2015," he said.
He compared the possible interplay of global warming and hurricanes to a professional athlete boosting his exercise regime. Imagine that athlete wins a game after training more. Did he win that game because of the extra training, or was it unrelated, and due to more specific game-time factors?
"I don't think I'm comfortable saying that global warming had a measurable impact [on] this storm at this moment," Vecchi said. "I'm not comfortable saying that I know it didn't… I am entirely open to either answer."
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