flooded benches.JPG (copy) (copy) (copy)

High tide floods park benches near Lockwood Boulevard on Wednesday, Feb. 20, 2019, in Charleston. The National Weather Service office in Charleston reports that, from Nov. 13 to Nov. 19, at least one high tide each day crested above 7 feet. File/Andrew J. Whitaker/Staff

In an already record-breaking year, Charleston recently came out of a full week of tidal flooding. 

The flooding isn't just affecting the Holy City. Up and down the East Coast, scientists say, tides are coming in at higher levels than predicted.

It's been happening for months, and nobody's sure exactly why. 

The National Weather Service office in Charleston reports that, from Nov. 13 to Nov. 19, at least one high tide each day crested above 7 feet. That's the point at which minor "sunny-day" flooding begins. 

The weather service monitors the tidal gauge that has floated in Charleston Harbor for more than a century, and which has shown more than a foot of global-warming driven sea-level rise already. But 2019 has proved an outlier in that record. 

While Charleston's usual average is around 40 days of tidal flooding every year, this year has already seen 73, said Blair Holloway, of the Weather Service. The previous annual record, from 2015, was first surpassed in the beginning of October.

Tides are pushed and pulled by a bevy of factors, including winds that can pile water onshore, the phase of the moon and swells from storms. 

The widespread nature of elevated tides this summer and fall point to a different culprit, said Brian McNoldy, a researcher at the University of Miami's Rosenstiel School of Marine & Atmospheric Science. 

"It’s not just a fluke," he said. "There’s some big-picture thing going on."

South Florida, where McNoldy is based, has faced its own jaw-dropping flooding this year. One Florida Keys neighborhood was inundated with salt water for more than 40 days, the Miami Herald reported

It's likely the Gulf Stream is playing a part, he said. The powerful, miles-wide jetway of water shoots from the southern tip of Florida along the Southeast coast and eventually into open ocean near Greenland. Its magnetism is so powerful that it pulls water away from the shore, but if the stream slows down, seas rise near land. 

When hurricanes or other storms cross the stream, they can cause it to slow down. That's happened a couple times this year, including with Hurricane Dorian. But those effects are only temporary. 

Current readings from one of the few measures of the Gulf Stream's power, a metal cable strung between West Palm Beach and the Bahamas at the beginning of the current, show a only slight slowdown, McNoldy said.

That's probably not enough to explain the five months of tidal records in Florida, or Charleston's outlier year. 

Ocean temperatures are also slightly higher than normal, another minor factor, McNoldy said. Warmer water takes up more space, because its molecules sit further apart, a process known as "thermal expansion." That makes water rise, too. 

But it will probably take the benefit of hindsight to piece together what's pushed the water so high this year. 

"It must be a few of these ingredients that are all conspiring at the same time," McNoldy said.

While a flood-causing high tide wasn't expected in Charleston on Friday, the harbor was still about a foot above where the Weather Service expected it to be, Holloway said. 

Local officials have gotten better at managing what's become semi-regular flooding, mainly by blocking off inundated streets. But the city is watching closely to see if this year's flooding will be a blip along a slower trend of rising seas.

"We do know the trend is up (for tidal floods)," said Mark Wilbert, Charleston's chief resilience officer. "We need another year or two to see if it’s going to be at this degree."

Sign up for our daily newsletter

Get the best of The Post and Courier, handpicked and delivered to your inbox every morning.

Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.