By late spring, the consensus among experts was unsettlingly clear: 2020 would be an abnormally active hurricane season. What the experts didn’t anticipate was just how wild things would get.

As of Sept. 23, with more than two months left in hurricane season, the Atlantic had already spit out 23 named storms — roughly double its long-term average for an entire season. For only the second time in its history, the National Hurricane Center exhausted its regular list of 21 names last week and began using the Greek alphabet.

Few coastal zones in the Gulf of Mexico and along the East Coast have remained untouched. Nearly 90 percent of these U.S. shores have been under a tropical storm or hurricane advisory in 2020, with a record nine storms making landfall (tied with 1916).

The pace has been frenzied even by the standards of the busiest year on record, 2005. That season didn’t make it to its 23rd tropical or subtropical storm until Oct. 22. This year, that happened more than a month sooner, on Sept. 18. That was also the only day in history on which the Hurricane Center has named three storms (Wilfred, Alpha and Beta).

While forecasters scramble to keep up, researchers are puzzling over what’s made 2020 such a banner year.

One of the most obvious culprits is La Niña, whose arrival was confirmed by the National Oceanic and Atmospheric Administration on Sept. 10. La Niña, a semiregular cooling of the eastern tropical Pacific, tends to reduce the wind shear that can impede Atlantic hurricane formation. A La Niña is only present about every third hurricane season, though — so by itself, it doesn’t fully explain why 2020 is so extraordinarily active.

Sizzling oceans, supercharged by climate change, may be an even bigger factor. Most of the tropical and subtropical Atlantic, including the Gulf of Mexico, has run warmer than average through the season, with sea surface temperatures (SSTs) at or near record values in some areas.

Unusually warm waters extended up the U.S. East Coast as Hurricane Isaias plowed northward, leaving in its wake an estimated $5 billion in damage and 15 deaths. Likewise, Hurricane Laura traversed warmer-than-usual SSTs en route to a destructive landfall in southwest Louisiana, where at least 33 deaths and some $10 billion in damage were tallied.

“Certainly, the SST pattern we’ve had over the past 30 days screams ‘active season!’ ” said hurricane forecaster Philip Klotzbach of Colorado State University in an email.

How 2020 stacks up against 2005

Thus far in 2020, two named storms, Laura and Teddy, have hit major-hurricane status, both Category 4. Two others, Paulette and Sally, came close to Category 3 strength.

By comparison, the Atlantic spawned five major hurricanes by the end of September 2005. Two of those, Katrina and Rita, hit Category 5, and both of them struck the U.S. Gulf Coast. Katrina alone caused as many as 1,800 U.S. deaths and $125 billion in damage. Even the less destructive Rita led to more U.S. fatalities and damage than the entire 2020 Atlantic season thus far.

Why has this season so far been more prolific yet less catastrophic than 2005? One reason is the randomness of landfall geography. For example, the storm surge from Hurricane Laura could have been much more devastating if it had hit 100 miles to the east or west, striking New Orleans or Houston, rather than just missing Lake Charles.

Another factor: Tropical waves coming off Africa, though plentiful, have struggled to intensify in the tropical Atlantic’s Main Development Region (MDR), where many of the most dangerous U.S. hurricanes get their start. All but one of this year’s hurricanes became hurricanes outside the MDR.

Some of this year’s plentiful tropical waves got caught up in large monsoon-type circulations in the MDR that impeded development. Others struggled when they reached a persistent zone of low pressure at high altitudes just east of the Lesser Antilles, which generated hostile winds.

Meanwhile, from May through July, a string of weaker named storms popped up at higher latitudes. According to hurricane researcher Falko Judt at the National Center for Atmospheric Research, most of these originated from upper-level low-pressure systems or weakening cold fronts rather than tropical waves, and the conditions weren’t ideal for growth.

“These kinds of storms develop in marginally favorable environments, so there’s usually quite a bit of shear and dry air. The waters aren’t as warm as farther south, either,” said Judt in an email.

Even these weaker, higher-latitude storms have been attention grabbers — in part because several have struck U.S. shores.

Climate change and the 2020 hurricane season

There’s ample evidence that a warming planet is tending to make the strongest hurricanes even stronger. The authors of a 2019 review paper published in the Bulletin of the American Meteorological Society agreed there is a detectable increase in the global proportion of tropical cyclones reaching Category 4 or 5 intensity. Most also concurred that human-produced greenhouse gases were at least partially to blame.

An analysis released by Climate Central on Wednesday showed that since 1979, the likelihood of a hurricane reaching Category 3 or higher in the Atlantic has increased from 20 to 40 percent.

Yet most global climate models project a drop in the total number of tropical cyclones globally as the planet warms. There’s active debate on whether this will hold true, particularly in the Atlantic. The region saw an unusually quiet period from the 1960s to the mid-1990s, followed by a sustained jump in activity that hasn’t let up.

Based largely on higher-resolution modeling, a subgroup of experts led by Kerry Emanuel of MIT holds that the planet, including the Atlantic, may see a long-term increase in both the number and power of its tropical storms and hurricanes.

Michael Mann, a climate researcher at Pennsylvania State University, agrees. He points out that the Atlantic seasons 2017 through 2019 saw a total of 50 named storms, compared with the long-term average of 12 per year. This year, Mann said in an email, “I’m guessing we’ll end up in the low 30s, something that would previously have been considered unimaginable.”

It was once assumed that Atlantic hurricane activity waxed and waned with the Atlantic Multidecadal Oscillation (AMO), a sequence of warming and cooling over the North Atlantic interpreted as a long-term cycle based on historical data. Mann, who coined the term, now thinks it’s largely an illusion. The late-century drop-off in Atlantic activity, and the rise thereafter, could simply be the result of Atlantic waters having cooled from sun-blocking aerosol pollution during the postwar era, then warmed for two reasons: increasing controls on aerosol pollution by late century, together with increasing amounts of human-produced greenhouse gases.

Studies are piling up on other ways in which climate change affects hurricanes, too:

  • Heavier rains: As evaporation increases from warmer oceans, there’s more water vapor for storms to ingest. Several studies found that the record-smashing rains in Southeast Texas from Hurricane Harvey were fueled by record-high ocean heat content and likely boosted by climate change.
  • Slower movement: An apparent slowing of average tropical cyclone speed globally since the 1950s includes a trend toward slower-moving systems near the U.S. coast (such as this year’s Hurricane Sally, which crawled to its landfall in Alabama). Regional variations may allow for a speedup of storm movement in some areas, especially if more tropical cyclones develop at higher latitudes, where steering flows tend to be faster.
  • Rapid intensification: Observers were stunned by the rapid strengthening of Harvey (2017) and Michael (2018) just before those hurricanes slammed into the Gulf Coast. A 2019 study found that the strongest storms appear to be undergoing the biggest boost in 24-hour intensification rates. This year, Hurricane Hanna’s sustained winds ramped up from 40 to 90 mph, and Laura’s from 75 to 150 mph, in the 36 hours before each hurricane made landfall. Every hurricane to make landfall in 2020 has seen its winds intensify by at least 15 mph in the 24 hours before coming ashore.

As another possible consequence of the abnormally warm SSTs in the Atlantic, intensified by climate change, several of the storms that formed early in the season formed much farther north than usual. For example, Dolly, in late June, flared up farther north than any tropical storm on record during that month, tweeted Sam Lillo, a postdoctoral researcher at NOAA.

Tropical Storm Alpha, which made landfall in Portugal on Friday, also formed in an unusual location in warmer-than-normal waters. A 2014 study showed the maximum intensity of tropical storms and hurricanes is migrating north, which the authors linked to warming waters.

As for the rest of 2020, the Atlantic is expected to close out September on a relatively quiet note. For the first time in more than two weeks, not a single tropical storm or hurricane roamed the Atlantic on Wednesday.

This hiatus probably won’t last. “Given the extremely warm Caribbean and the push towards La Niña conditions, I do expect the rest of the season to be quite active,” Klotzbach said.

Longer-range models suggest a classic La Niña mode by early October, Klotzbach added, with suppressed activity over the tropical Pacific and rising motion over Africa and India.

“That should reinvigorate the Atlantic for another spate of activity.”

Bob Henson is a meteorologist and journalist based in Boulder, Colo. His books include “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”

Jason Samenow contributed to this report.