It’s the storm surge that can be most bru­tal

The Washington Post - - FRONT PAGE - BY BEN GUARINO

The worst part of hur­ri­canes is likely to only get more de­struc­tive.

As Hur­ri­cane Irma bore down on Florida, the wa­ter be­tween the storm’s cen­ter and the shore­line bulged. Mighty winds whipped the At­lantic Ocean into a mound. Ex­traor­di­nar­ily low pres­sures al­lowed it to rise even higher. This bulge, the storm surge, took up so much wa­ter that long stretches of the Caribbean coast went dry. The hur­ri­cane had flailed the ocean into an un­fa­mil­iar shape.

A storm surge is “lit­er­ally just the brute force of the winds push­ing the wa­ter into the land,” said Karthik Balaguru, who stud­ies hur­ri­canes at the Pa­cific North­west Na­tional Lab­o­ra­tory.

And where a hur­ri­cane dumps its surge can be the site of catas­tro­phe. On Mon­day, the wa­ter flood­ing down­town Jack­sonville reached a record five feet, ac­cord­ing to the Na­tional Weather Ser­vice. That was more than a foot over the mark that had stood for more than half a cen­tury.

Lo­cals were told to flee. The Jack­sonville sher­iff ’s of­fice tweeted, “AT­TEN­TION: Evac­u­a­tion Zones A/B along the river. Get out NOW.”

The surge’s reach was long. More than 200 miles up the coast from Jack­sonville, of­fi­cials in Charleston, S.C., is­sued a flash­flood warn­ing as a swell of wa­ter spilled into that city.

Surges can reach in­cred­i­ble heights, too. Dur­ing a Mon­day news con­fer­ence, Florida Gov. Rick Scott (R) said Mon­roe County was hit by a 10-foot surge. In Mi­ami, spared the brunt of the storm, the surge reached four feet. (Given hur­ri­canes’ de­struc­tive, wet power, it may be easy to con­fuse a surge with other el­e­ments of an oceanic storm. A surge is not a wave or tide, though a ris­ing tide can lift a surge higher.)

Be­cause so much wa­ter is in­volved, surges are fre­quently the dead­li­est as­pect of a hur­ri­cane. Many of the es­ti­mated 1,500 deaths from Hur­ri­cane Ka­t­rina could be at­trib­uted to the storm surge along the Gulf Coast, ac­cord­ing to the Na­tional Hur­ri­cane Cen­ter. In Mis­sis­sippi, the surge achieved “his­tor­i­cal pro­por­tions,” with the high­est el­e­va­tion marked at more than 28 feet.

Two ma­jor fac­tors con­trol how a storm surges — the strength of a hur­ri­cane’s wind and the shape of the coast­line. The At­lantic con­ti­nen­tal shelf is rel­a­tively flat, af­ford­ing a hur­ri­cane there more shal­low wa­ter that it can push into a surge. What’s more, the coun­ter­clock­wise ro­ta­tion of a hur­ri­cane’s north­east quad­rant shoves the wa­ter to­ward land in­stead of away from it, Balaguru said.

Pre­dict­ing a storm surge is dif­fi­cult. Even slight changes in the hur­ri­cane’s cen­ter and strong­est winds can in­flu­ence its path, said Rebecca E. Morss, a sci­en­tist at the Na­tional Cen­ter for At­mo­spheric Re­search in Boul­der, Colo.

Plus, the way surges move can take com­mu­ni­ties by sur­prise. “If the wa­ter has a path to get to you and there’s not a hill in the way, it’s go­ing to get to you,” Morss said. “If you’re in the wrong place, you don’t want to be there.” Dur­ing Hur­ri­cane Ike in 2008, the surge did not go over the sea wall in Galve­ston, Tex., but in­stead came around from the other side, dev­as­tat­ing the city.

Sci­en­tists ex­pect storm surges to crest even higher in the fu­ture be­cause of global warm­ing.

Even if car­bon emis­sions peak in the next few decades — a very con­ser­va­tive sce­nario — surges at the end of the 21st cen­tury will be se­vere, Balaguru and his col­leagues re­ported in a 2016 ar­ti­cle in Cli­mac­tic Change. They com­pared surges at the end of the 20th cen­tury with those pre­dicted at the end of the 21st and cal­cu­lated that the av­er­age storm surge decades from now will have in­creased by 25 to 47 per­cent over those 100 years prior.

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