October 11, 2018
Michael introduced itself to North America with 155-mile-per-hour gusts of wind and a barometric pressure of 919 millibars, the third-strongest hurricane to ever make continental US landfall. It was a monster, and it stayed a monster as it rolled through Georgia and then on toward the Carolinas.
And monsters are made, not born. “The most striking thing about Michael is that it was barely a tropical storm a couple of days ago, and all of a sudden it almost touched cat-5 intensity,” says Karthik Balaguru, an oceanographer who studies hurricanes at the Pacific Northwest National Laboratory. Typically hurricanes ramp up more slowly (if at all), and landfall reduces their intensity. “The thing with Michael is, it just kept on intensifying,” Balaguru says.
That’s called—perhaps as you’d expect—rapid intensification, and it used to be a rare phenomenon among Atlantic hurricanes. That’s good, because rapidly intensifying hurricanes also tend to be the ones that are the most dangerous, the deadliest and the most expensive. Unfortunately, rapidly intensifying hurricanes seem to be a rarity no longer.
Of the record-breaking hurricane season of 2017—17 named storms, 6 major hurricanes—all four category-4 or 5 storms underwent rapid intensification. That’s Harvey, which drowned Houston, Irma, Jose, and Maria, which was so devastating to Puerto Rico. Now, to be fair, most Atlantic cat-4 and cat-5 storms go through rapid intensification; the dangerous ones do it near the coastline, just before landfall.
More intense hurricanes are one of the central predictions scientists have made about Earth’s changing climate. Human beings pump more greenhouse gases like carbon dioxide into the air, and the air changes. The planet’s overall temperature goes up, and that means, in part, more energy in the top of the oceans. That’s a hurricane’s engine.
Advances like satellites and radar have made high accuracy possible in predicting a hurricane’s eventual track, the path it takes across ocean and land. Intensity, though, is another story. It’s hard—as the hurricane researcher Kerry Emanuel wrote last year, a good intensity prediction would require better numerical models, better understanding of boundary layers between air and water, and a better model of how the upper ocean interacts with storms. Scientists don’t have any of those. Forecasting rapid intensification is even harder.
And again, perhaps as one might expect, a warmer climate makes rapid intensification more likely. Emanuel’s group calculated from observations of past storms that a hurricane with wind speeds that grew 68 miles per hour in the day before making landfall should only happen once in a century under the conditions of an unchanged climate—basically a hundred-year storm. And the vulnerable cities were just about where you’d expect, too: Houston, New Orleans, Tampa, Miami. But, the researchers then calculated, in the post climate-change climate of our Burning World, those storms could happen once every five years. And the new hundred-year storm? Pre-landfall intensifications of 114 mph, so massive as to be “essentially nonexistent in the late twentieth-century climate,” Emanuel writes. People will have never seen anything that bad.
Balaguru’s group found similarly grim news. Their calculations, based again on observed rapid intensification, say that climate change and variations in Atlantic conditions could drive up the magnitude of that intensification. Point is, climate change means more rapid intensification like what Michael did, and worse.
How did Michael pull it off? “If I had to guess, I’d say it experienced perfect conditions for a hurricane—a lot of heat available in the ocean and favorable atmospheric conditions,” Balaguru says. Wind shear, the changes in magnitude and direction of winds from the ocean’s surface all the way up to the top of the troposphere—about six miles up—must have also subsided. High enough wind shear can disrupt convection at the storm’s center, weakening it overall. That didn’t happen here.
The result: yet another perfect storm in yet another season of perfect storms, crashing into a heavily populated coastline full of environmental disasters waiting to happen. As of Thursday, Michael has ramped back down. It’s merely a powerful tropical storm now. And it’s tracking right over the towns where Hurricane Florence made landfall last month.
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