- NASA Earth Observatory images by Joshua Stevens and Jesse Allen
- The 2017 Atlantic hurricane season is “extremely active,” according to the definitions used by the National Hurricane Center. That’s largely because there have been excellent conditions for hurricanes to form: low wind shear and high ocean temperatures. This year will set some records but won’t necessarily be as bad as 2005.
Tropical Storm Nate is picking up strength as it churns through the Caribbean and is likely to become a hurricane before hitting the Gulf Coast, according to the National Hurricane Center. The storm has already killed 22 and is forecast to make landfall near New Orleans this weekend.
It’s easy to wonder how this could be happening yet again – especially for those who have already suffered through one or more major storms this Atlantic hurricane season.
This season has indeed been exceptionally intense. Harvey, which dumped unprecedented amounts of rain on Houston and the Gulf Coast. Irma rolled over the Virgin Islands, St. Martin, and other parts of the Caribbean before slamming into Florida. And Maria stripped the roofs off of building after building in Dominica and wiped out the power infrastructure in Puerto Rico. Two of those storms first made landfall as Category 5 hurricanes, the other a Category 4.
And those are just the major hurricanes (defined as Category 3 or above) of 2017 that have made landfall. Another major hurricane, Jose, threatened some already ravaged Caribbean islands before taking a turn to to the north. Lee, another major hurricane, also stayed at sea. So far this year, there have been eight hurricanes in total, and with Nate, 14 named storms.
The National Oceanic and Atmospheric Organization has a formal definition for an “extremely active” Atlantic hurricane season – and this year certainly meets the requirements. In an “extremely active” year, storms have to generate at least 152.5 units of accumulated cyclone energy – a measure of storm intensity, duration, and frequency. As of October 6, the 2017 season was up to 204.9 units. The season also has to meet two out of these three conditions: 13 or more named storms, 7 or more hurricanes, and 3 or more major hurricanes. We’ve hit all three.
In fact, according to Phil Klotzbach, a meteorologist at Colorado State University specializing in Atlantic hurricane forecasts, we’d met the definition by September 20.
And hurricane season isn’t over until November 30, so there’s almost certainly more to come. So why do storms keep coming one after another?
In many ways, 2017 has had ideal conditions for a lot of big, powerful storms, several experts told Business Insider.
Two main factors have allowed these massive storms to form: the lack of an El Niño system, and the fact that the Atlantic is unusually warm.
El Niño systems generate particularly warm temperatures in the Pacific, which tends to create high wind shear in the Atlantic. James Belanger, a senior meteorological scientist with The Weather Company (the group behind the Weather Channel and Weather Underground), told Business Insider that wind shear “rips storms apart” before they develop into massive systems.
But this year, we’ve had what are referred to as “ENSO-neutral” conditions so far, meaning that there hasn’t been any El Niño or La Niña system whose wind shear could prevent hurricanes.
At the same time, the North Atlantic has been “quite warm,” Belanger said. Warm water helps storms intensify since the weather systems absorb heat energy from the water. As NASA puts it, “the more heat energy that goes in, the more vigorously a weather system can churn.”
Warm ocean temperatures in the Caribbean allowed this year’s storms to rapidly gain power. Maria is likely to set a record as the most rapidly intensifying hurricane ever measured. Caribbean and Gulf of Mexico temperatures are now helping Nate pick up steam, too.
Two combined factors affect Atlantic temperatures: ocean heat content (a measure of heat stored by the ocean), and sea surface temperatures (measured at the top layer of the ocean). There’s no simple explanation for this year’s high surface temperatures and ocean heat content, according to Belanger. One possibility is that weaker trade winds and wind speeds in the Atlantic have led to less evaporation, which would normally cool the ocean more.
Another factor is climate change, since data indicates that a warmer climate is likely to result in more intense hurricanes and more rapid storm intensification. Oceans absorb much of the heat that’s emitted into the atmosphere, which leads to warmer water. That’s one of the reasons researchers say that climate change will lead to more intense storms like the ones we’ve seen this year.
Along with raising sea temperatures, climate change also causes sea-level rise – which makes cities more vulnerable to the storm surge that comes with hurricanes. Plus, global warming is expected to lead to a higher concentration of atmospheric water vapor and heavier rainfall. Intense rainfall can be devastating, as Harvey showed in Houston and Maria has shown in Puerto Rico.
Climate change didn’t cause any of the storms that we’ve seen this year, and we don’t yet know how the changing climate affected these specific hurricanes. But climate scientists have warned that the world could see more storms as temperatures rise. If nothing else, this season could serve as an example of what we’ll see in the future.
“There is evidence that we are emerging from an era of messy meteorological data, where we were blind to warming seas strengthening hurricanes because the really damaging ones were rare,” meteorologist Eric Holthaus wrote for Grist. “If that’s true, weather historians may look to this year as the beginning of a frightening new phase of superstorms.”
Yet another explanation for the warmer Atlantic Ocean could be changes in high and low pressure systems that caused surface temperatures to fluctuate, according to Michael Ventrice, a meteorological scientist at The Weather Company. Ventrice said a current called the Atlantic Multi-decadal Oscillation (AMO) may also have played a role – that current is slow moving, and changes conditions on a 20 to 50 year scale. At the start of the 2017 season, meteorologists were unsure whether the AMO was still in a warm phase, according to NOAA. But if so, that could contribute to this season’s activity.
A record breaking year
Whatever the reason for 2017’s seemingly endless bombardment of hurricanes, we have certainly seen records broken this year.
This is the first time in known history that the Atlantic has had two storms with 150+ mph winds raging at the same time: Irma and Jose.
Maria went from a tropical depression to a Category 5 storm in two-and-a-half days, a speed that Mark DeMaria, acting deputy director of the National Hurricane Center, told the Washington Post is most likely the record for fastest intensification in the Atlantic.
This year we also set a record for cyclone energy generated in September.
But this season isn’t the worst we’ve ever seen, as much as it might seem that way. That distinction belongs to 2005.
That year, there were so many named storms that we ran through the whole alphabetical list of names (the letters Q, U, X, Y, and Z are not used) and then had to run through the Greek alphabet, using Alpha, Beta, Gamma, Delta, Epsilon, and Zeta. Fifteen storms in 2005 were hurricanes, with seven being major hurricanes.
Five storm names were retired that year because of the devastation the hurricanes caused: Dennis, Katrina, Rita, Stan, and Wilma. We will most likely have at least three names retired after 2017: Harvey, Irma, and Maria.
Based on the Atlantic hurricane activity so far this year, we’re not quite on pace to surpass 2005. But as Molly Rubin recently wrote in Quartz, if you look at both the Atlantic and Pacific simultaneously, the world is on pace for a new record number of named storms overall, outpacing 2012 and 2005.
After Maria dissipated, we hit a brief period of quiet in the Atlantic. High wind shear tends to appear around the start of fall, making storms less likely to form in the Atlantic. But as meteorologists predicted, the Caribbean – where waters remain warm – started to pick up as a source of storm activity, which often happens in October.
Right now, we’re watching to see what happens with Nate, and hoping there’s no more loss of life.
Hurricane season peaked on September 10, but the period of peak activity lasts though the middle of October. The season isn’t over until November 30.