Rapid Intensification Turned Meranti Into a Monster Storm

Super Typhoon Meranti became the strongest tropical cyclone to form so far this year when the winds around its eye reached an estimated 190 mph on Tuesday after it rapidly intensified over the Northwest Pacific.

China is feeling the monster storm’s ferocious storm surge, wind and rain just a day after it clipped Taiwan. Though Meranti was weakened by Taiwan’s mountains, it still packs a major punch.

As global temperatures rise, the most intense tropical cyclones (as typhoons and hurricanes are more broadly known) are expected to become both more intense and more common. Recent studies have shown that typhoons, in particular those that strike land, have shown a rise in intensity since the 1970s. But the rapid intensification that these storms often undergo isn’t a well-understood process, with potential implications for our understanding of how tropical cyclones might change in a hotter future.

The Northwest Pacific sees more tropical cyclone activity each year than any other ocean basin because of the large, deep well of ocean heat that storms can tap and generally more favorable atmospheric conditions there.

The tropopause, or the top of the layer of the atmosphere that Earth’s weather occurs in, is also higher in the western Pacific than it is in the Atlantic. When the tropopause is higher, it is colder, which means a larger contrast with the warm ocean surface. That, in turn, means more instability in the atmosphere, which helps drive tropical cyclones.

“So in general, the western north Pacific is just ripe for these mega cyclones every year,” Brian McNoldy, a hurricane researcher at the University of Miami’s Rosenstiel School, said in an email.

Of the 15 tropical cyclones from the Northern Hemisphere whose winds have reached at least 160 knots (184 mph) since 1970, about two-thirds were in the Northwest Pacific, according to Philip Klotzbach, a hurricane expert at Colorado State University.

But while it’s more likely that storms in the Northwest Pacific will reach such a stunning strength than those elsewhere, the system still surprised forecasters. They expected it to intensify, but it ramped up considerably more, and more quickly, than initially forecast.

This runaway rapid intensification is a process that is not well understood and that constantly stymies forecasters when it happens.

“A forecaster will very rarely go out on a limb and expect extremely rapid intensification, even if they realize it’s a possibility,” McNoldy said.

Hurricane researchers don’t even know what aspects of rapid intensification they might be missing.

“There are processes at work that we don’t fully understand or even know about,” McNoldy said. “If we knew exactly what the key was, we’d be dumping huge resources into observing that specific thing. We’re still trying to find out what the key is.”

That hole in the understanding of hurricanes could have implications for anticipating how tropical cyclone activity might change in the future as the planet continues to heat up.

Right now, most studies suggest that while overall tropical cyclone numbers may not change much, the intensity of the strongest storms will rise, and those storms will make up a bigger proportion of the total.

One group of researchers, examining trends in the Northwest Pacific, specifically, have found that the intensity of typhoons has risen since the 1970s. In particular, those that make landfall have seen an increase in intensity, but not those that stay out at sea. It is not yet clear, though, whether those trends are linked to global warming, or to some natural feature of Earth’s climate.

But potentially complicating the picture is the finding of another recent paper that most of the strongest storms undergo rapid intensification, and that this subset of storms is the reason that such top-tier tropical cyclones happen more frequently than would be expected. Because the process isn’t fully understand, it could affect how models represent what will happen in the future.

“I don’t think we’ve proven anything about how well climate models can simulate tropical cyclones, good or bad, right or wrong or indifferent,” study co-author Adam Sobel, who studies extreme weather at Columbia University, said in a statement. “But it certainly raises a bunch of questions we want to think about now.”