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The science behind the U.S.’s strange hurricane ‘drought’ — and its sudden end

A satellite image of Hurricane Irma on Wednesday. (NOAA/RAMMB/AFP)
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With the destruction from Hurricane Harvey, now closely followed by the extraordinary intensity of Hurricane Irma as it wreaks havoc in the Caribbean and heads toward Florida, Americans can be forgiven for feeling like it’s 2005 all over again.

That was the year of Katrina, Rita and Wilma — and it was preceded by 2004, when four strong hurricanes slammed into Florida. Most alarming was the enormous Ivan, which produced 90 foot high waves offshore before striking the Gulf Coast.

Since 2005, though, we’ve experienced no major U.S. landfalls until Harvey this year. All of a sudden, it feels like we’re living in an Atlantic bowling alley once again. Or as NOAA hurricane scientist Eric Blake bluntly put it on Twitter: “2017 has that 2004/2005 feel ugh.”

But why is that the case?

The answer is anything but simple. There are some major oddities of hurricane behavior in the North Atlantic basin — the region that includes the Caribbean and Gulf of Mexico — that continue to puzzle scientists and spark debate. Surveying them helps explain where we now find ourselves — and also, how dangerous complacency about hurricane risks, triggered by long periods of calmer activity or fewer storm strikes, can too easily set in.

Atlantic ups and downs

Hurricane patterns in the Atlantic present a long-standing puzzle.

After causing considerable destruction in the 1940s through 1960s, Atlantic hurricanes largely calmed down in the 1970s and 1980s. That’s not to say there weren’t several intense storms — but average activity was lower.

Then, hurricanes bounced back again around 1995, and we’ve had mostly busy years ever since. This “active” period was capped by 2005, when a staggering 28 storms formed, and forecasters had to turn to the Greek alphabet (“Hurricane Epsilon”) once they ran out of assigned names.

But since 2005, while there have been ample number of hurricanes overall in the Atlantic, the United States itself has been lucky. Before Hurricane Harvey, the continental United States had not been hit by a Category 3 or higher “major hurricane” for 12 years — dating all the way back to 2005’s Hurricane Wilma.

This pattern of activity raises two questions. First, why does Atlantic hurricane activity seemingly switch on and off over decades? And second: Why, during a period when that activity was still presumably “on,” did the United States avoid being hit by strong storms for so long?

Is there a natural hurricane cycle in the Atlantic Ocean?

Atlantic hurricane seasons over the years have been shaped by many complex factors, explained Jim Kossin, a hurricane scientist with NOAA and the University of Wisconsin in Madison. Those include large scale ocean currents, air pollution — which tends to cool the ocean down — and climate change, which does the opposite.

“Pollution over the Atlantic increased for a few decades and then decreased after the Clean Air Acts and Amendments in the 1970s,” said Kossin. “Finally, added to all of this up-and-down behavior is the slow trend in ocean temperatures from increasing greenhouse gases.”

On top of all that is the notion of a so-called Atlantic Multi-Decadal Oscillation — a swing toward and away from more hurricane conducive conditions in the Atlantic, thanks to shifts in large scale ocean currents and their effect on Atlantic Ocean temperatures. The idea is that we shifted away from those hurricane-friendly conditions in the 1970s and 1980s, back to them in 1995, and then, just maybe, were poised to swing back to negative conditions again as of 2015.

But this idea is contested.

“A debate continues about why the 1970s and 80s were relatively quiet in the Atlantic,” said MIT hurricane researcher Kerry Emanuel. “Some believe that it was the consequence of a natural climate oscillation called the Atlantic Multi-decadal Oscillation (AMO), while others, including me, think is was mostly a consequence of sulfate aerosol pollution.”

Sulfate air pollution would have blocked some sunlight that might otherwise have reached the Atlantic Ocean — and thus, reduced its temperature. Reducing such air pollution through regulations would have the opposite effect — warming the Atlantic.

Gabriel Vecchi, a hurricane researcher at NOAA and Princeton University, thinks there’s something to the idea that ocean current changes in the Atlantic also set up hurricane phases — but he also agreed that the decrease in pollution mattered.

“At the same time you could have the Atlantic currents vary,” he said. “And so that to me seems the most likely explanation, that both things happened.”

Why did the U.S. escape major hurricane damage for so long?

Whatever you think about the idea of an Atlantic oscillation, storms have been generally active, overall, since 1995. This hurricane season, in particular, was widely forecast to be above average, featuring (per NOAA) some 14-19 named storms, 5-9 hurricanes, and 2-5 major hurricanes (we are at 11, six and two so far).

Yet another hurricane with strong potential, Jose, is following Irma in the open Atlantic. Seas this year are quite warm, and the crosscutting winds that can tear hurricanes apart have been light.

Yet while Harvey struck the United States at Category 4 strength, and there’s a similarly worrying forecast for Irma, the mainland United States had avoided a Category 3 or higher landfall since 2005. (Analyses of the so-called U.S. hurricane “drought” have been confined to the continental United States, but it’s important not to forget about U.S. territories, like Puerto Rico and the U.S. Virgin Islands, which have just sustained severe damage from Irma.)

Why the ‘major-hurricane-drought’ is the most overblown statistic in meteorology

The odds of this major hurricane drought are certainly low. Two years back, a study in Geophysical Research Letters found that even a nine-year drought would likely take 177 years to return — but that study only ran through 2014. The drought then continued for two more years after that.

This lack of recent strong U.S. hurricane strikes has been much remarked upon — and a number of ideas have been adduced to explain it. Hurricane scientists Phil Klotzbach and Brian McNoldy, writing at Capital Weather Gang, have noted in the last decade a kind of hurricane goalkeeper has set up over the eastern United States in the form of a low pressure area that steers storms away from us.

Kossin has also conducted research that combines the concept of the “drought” with the overall active era for U.S. hurricanes, finding that while storms are more numerous in active periods, they’re also more likely to weaken near the U.S. coast.

But there’s also the view that what we’re seeing is a mixture of luck and particular definitions of what counts as a “major” hurricane.

After all, in 2012, we experienced Superstorm Sandy which, while not technically classified as a major hurricane at landfall, caused damage highly consistent with what they’re capable of.

“The drought: I guess it has in fact come to an end with Harvey,” said Kossin by email. “If Irma makes landfall as a major hurricane, then all the more so. Is this something systematic? Maybe, but there has always been a large random component to this so much of what has been happening is luck, good or bad.”

For Vecchi, a lot of it is definitional. One plausible explanation for the hurricane “drought,” he said, could be that “we’re so focused on the Saffir-Simpson scale and this arbitrary difference between a Category 2 and a Category 3 at landfall, that we’re missing the bigger picture, and nothing has changed.”

Will global warming make it all worse?

Then there’s the climate change question. Some scientists, like Penn State’s Michael Mann, think too much has been pinned on Atlantic Multi-Decadal Oscillation. That implies things won’t swing back again — rather, busy hurricane seasons will stay put.

“It is really human-caused climate change that is causing the long-term trends we are seeing in intensity,” said Mann.

If so, that would be bad news, since climate change would tend to shift the odds toward stronger storms.

“The overall finding that is in several previous publications that climate change has already produced a substantial increase in the proportion of intense (Cat 4-5) hurricanes continues to hold,” said Greg Holland, a hurricane expert with the National Center for Atmospheric Research, by email. “So an active season now will tend to have a higher number of intense hurricanes than the same season 20 years ago.”

Intense hurricanes like Irma, which has just set a record for intensity combined with longevity, with over 30 hours with wind speeds of 185 miles per hour.

Yet the question of whether storms are measurably stronger at present remains contested, with NOAA’s Geophysical Fluid Dynamics Laboratory asserting that “it is premature to conclude that human activities — and particularly greenhouse gas emissions that cause global warming — have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity.” The effect could be there, NOAA said, but not yet clearly detectable in the statistics.

In the meantime, one thing is clear. Lulls in hurricane landfalls, during the 1970s and 1980s or during the last decade, can make the United States temporarily forget its vulnerability. And from coastal development to rising seas, to the vanishing of coral reefs in Florida and wetlands in Louisiana, that in turn could distract from the problems that make that vulnerability worse.