Why the 2020 Atlantic hurricane season has spun out of control
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.