The Weekend Wonk: Is Florence a View of the Future?

September 15, 2018

Above, video from 2012. Most relevant passage starts at 3:15.

Hurricane Sandy’s sharp left turn. Harvey’s slow walking thru Texas. Florence lingering over the East Coast.
All one-offs?
Or evidence of climate dynamics affecting storms in an unexpected way?


We know that global warming makes hurricanes more destructive in several ways — stronger winds, higher storm surge, much more precipitation.

But can global warming actually steer a storm into a more deadly path? The growing evidence is that it can — and already has — by creating more powerful high-pressure systems that remain almost stationary and divert a cyclone from what would have been its normal path.

“It’s possible that the Arctic [warming] may be having a double-whammy effect that is favoring an increased frequency of blocking highs like the one that’s steering Florence in a highly unusual path, and like the one that did the same to Sandy,” as Dr. Jennifer Francis explained in an email to Think Progress.

The high-pressure system in the atmosphere, in effect, appears to be blocking the path of Hurricane Florence, and instead leading it directly towards landfall rather than allowing it to veer northwards into the Atlantic.


The path of Florence has been extremely unusual. As Philip Klotzbach, an Atlantic hurricane expert, tweeted on Friday, “33 named storms (since 1851) have been within 100 miles of Florence’s current position. None of these storms made US landfall. The closest approach was Hurricane George (1950) — the highlighted track [in white].”

Florence, tragically, has made a beeline toward the Carolinas. And it clearly was steered away from the historical (or “climatological”) path by a major high-pressure system blocking its typical path — north and away from land.

But why has this happened now and not before?

ThinkProgress asked Dr. Francis that question. She explained that “the science of blocking highs is a hot research topic. Climate models tend to underestimate their frequency compared with observations, which suggest they’re occurring more frequently in the North Atlantic during summer months.”

Back in 2016 Francis published a study on the link between blocking highs and global warming. At the time, she told ThinkProgress: “Our new study does indeed add to the growing pile of evidence that amplified Arctic warming and sea-ice loss favor the formation of blocking high pressure features in the North Atlantic. These blocks can cause all sorts of trouble…”

In her previous work into how global warming worsened superstorm Sandy, Francis and co-author, Charles Greene, director of Cornell’s Ocean Resources and Ecosystems program, argued that warming may have been responsible for Sandy’s unusual storm track.

As Cornell University explained their findings:

Cornell and Rutgers researchers report in the March issue of Oceanography that the severe loss of summertime Arctic sea ice  —  attributed to greenhouse warming  —  appears to enhance Northern Hemisphere jet stream meandering, intensify Arctic air mass invasions toward middle latitudes, and increase the frequency of atmospheric blocking events like the one that steered Hurricane Sandy west into the densely populated New York City area.

Figuring out all of the causes is tricky, Francis told ThinkProgress Tuesday, but “certainly the uptick [in blocking highs] is consistent with expectations in a world with rapid Arctic warming, which tends to weaken jet-stream winds and favor a more meandering path. Large northward swings can break off in an eddy, forming a blocking high.”

Another factor with Florence, she explained, “is the massive area of much warmer-than-normal ocean temperatures off the eastern seaboard of North America, which also favors those big northward jet-stream swings, and thus blocking highs.”

Map of sea surface temperatures  shows huge pool of warm water off the East Coast (as well as a pocket of cool water south of Greenland). CREDIT: NOAA.

Francis notes that this large pool of warm water currently sitting in the Atlantic is similar to the one seen in the eastern Pacific that helped cause a blocking high directly linked to global warming that was also a major driver of the multi-year drought California suffered in recent years.

As NOAA’s map above shows, not only are sea surface temperatures off the East Coast very warm, but there is a pool of cool water just south of Greenland. Francis explained that this pool — created by the staggering amounts of melting ice from global warming —  may also be contributing to a blocking high:

Interesting new twist: some new research suggests that the “cold pool” in the North Atlantic is caused by an influx of freshwater from the Arctic Ocean and Greenland meltwater, and that freshwater cap is backing up the flow of warm Gulf Stream water. So it’s possible that the Arctic may be having a double-whammy effect that is favoring an increased frequency of blocking highs like the one that’s steering Florence in a highly unusual path, and like the one that did the same to Sandy.

Rob Meyer in the Atlantic:

All of this talk of a “stalled storm” may sound familiar. Last year, Hurricane Harvey stalled out as it approached Houston. It lingered over that city for more than two days, during which it both moved slowly and took a zigzagging track that kept it over the metro area. Ultimately, Harvey killed 88 people and displaced more than 30,000.

Neither Harvey nor Florence will be the last slow-moving cyclone to strike the United States. Stalled hurricanes appear to be getting more common, and human-caused climate change may be to blame.

According to a paper published earlier this year in the scientific journal Nature, hurricanes are now moving more slowly across the Earth than they once did. From 1949 to 2016, the speed of tropical cyclones worldwide over land decreased by 10 percent.

In the North Atlantic, where both Harvey and Florence originated, hurricanes have slowed some 20 percent in their track speed. (This study did not account for 2017’s slow storms, including Harvey.)

“Storms can get worse without getting more intense” if they’re slow moving, James Kossin, the author of that paper and an atmospheric research scientist at the National Oceanic and Atmospheric Administration, told me at the time.

“That’s just through very fundamental mathematics, because if you slow down how fast a storm is moving through a neighborhood, there’s probably more rain; storm surge would probably be greater; and while wind speeds won’t get any stronger, the amount of time that structures are hit by that wind would get longer,” he said.

The paper did not formally attribute this effect to human-caused climate change, but scientists have long hypothesized that tropical cyclones will move more slowly in a warmer world. That’s because a warmer world will have more stagnant, slow-moving air masses, which will stall out storms of all types around the world. And while tropical cyclones are very powerful systems, they are largely blown around the planet by the same global circulation that carries normal squalls. “Tropical cyclones are, for the most part, just carried passively along in the background wind,” Kossin told me.

Another study, published this year in the Journal of Climate, found that when computer models simulated 32 hurricanes in a warmed climate, those hurricanes were more likely both to be more intense and to stall out. And last year’s National Climate Assessment, a report produced by academic scientists, civilian agencies, and the military, similarly found that global warming will likely produce more frequent major hurricanes. The number of minor hurricanes may decrease.

Kossin warned that slower hurricanes could change how governments prepare for storms. “We get people out of harm’s way along the coast pretty well because historically, most mortality was associated with storm surge,” he said. “But we don’t typically evacuate people who are inland. And this slowing is going to impact inland flooding.”

According to the NWS, the Carolinas will face that inland flooding—and the “serious hazards” that come with it—this weekend, “regardless of where exactly the center moves.”


One Response to “The Weekend Wonk: Is Florence a View of the Future?”

  1. dumboldguy Says:

    A great post with lots of good info. Unfortunately it’s also full of too many wafflings like “could’s”, “maybe’s”, and “it’s possible’s” by the scientists. WHEN TF are they going to find the courage to tell it like it is? If we wait until we are 100% sure, we will all be dead. Stuff like: “The paper did not formally attribute this effect to human-caused climate change…”

    “So it’s possible that the Arctic may be having a double-whammy effect that is favoring an increased frequency of blocking highs like the one that’s steering Florence in a highly unusual path, and like the one that did the same to Sandy…”

    “The paper did not formally attribute this effect to human-caused climate change…”

    Look at Klotzbach’s tweet again—33 storms since 1851? And now Sandy and Florence?—-twice in just 6 years? Do the freaking math, folks!

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