'King Tides' Flood Florida Streets (Here's Why)

Sodden lawns and streets-turned-lakes plagued low-lying parts of Florida's coast such as Miami and Tampa Bay over the last week, but the culprit wasn't the latest howling storm. In fact, the skies were often sunny and blue.

The rash of flooding was instead the work of so-called king tides, the highest high tides of the year, brought on by a combination of seasonal factors and the alignment of the gravitational pull of the sun and the moon.

The floods from this annual event effectively allow locals to "step into a time machine" and glimpse their future as global warming drives up sea levels at an ever-faster pace, said Brian McNoldy, a tropical meteorologist at the University of Miami. The level of today's highest tides could become the average level for high tides of the future, scientists say. [6 Unexpected Effects of Climate Change]

Along the Atlantic coast of Florida, the tides are naturally highest in the fall, McNoldy said. This happens, he said, because of several factors: Ocean waters are at their warmest, causing the water to expand; the Gulf Stream — a warm Atlantic current that moves from the Gulf of Mexico to the tip of Florida and then up the state's eastern coast — slows down, allowing water to pile up along the coast; atmospheric pressure is low; and onshore winds are strong. When the sun and moon line up at the new and full moons, their combined gravitational pull acts on top of those already high-tide levels to create the biggest non-storm tides of the year.

Miami experienced even higher-than-expected king tides last week thanks to persistent easterly winds that helped push more water ashore. On Oct. 5, a tide gauge at Virginia Key measured water levels 2.1 feet (0.6 meters) higher than the average annual high tide (or the average of the high tides from that year), McNoldy told Live Science.

In the Tampa Bay area, southerly winds from Hurricane Nate also boosted sea levels above the expected high tide and caused flooding, which normally isn't as prevalent there during king tides, said Andrew McKaughan, a meteorologist with the National Weather Service's Tampa Bay office.

The high and low tides along the Gulf Coast don’t vary as widely as those on the eastern shore, as conditions there tend to be calmer, said William Sweet, an oceanographer with the National Ocean Service.

The king tide flooding provides a glimpse of the future for these areas, Sweet said, as rising sea levels make flooding events more common and flood levels higher. Nuisance flooding, as it is called, has already been on the rise in recent decades because of the sea-level rise that has occurred so far, he said, and both flooding and sea level rise are progressing at ever faster rates.

Along with more frequent floods in the future, in "places with the exact same king tide as this year, you'll have many more areas that flood because of it," McNoldy said.

Such floods hamper the everyday lives of people in affected areas; in Miami, the flooding caused major traffic slowdowns.

"There are areas along my drive that I know flood during the highest tide days, and I don't drive through those," McNoldy said. He said he had co-workers whose normal half-hour commutes took 2 to 3 hours, as cars plodded their way along inundated streets.

Flooding can also affect infrastructure, like wastewater plants, and coastal city planners are increasingly realizing they need to take action to protect the residents.

"Sea levels are expected just to keep increasing," Sweet said. "It doesn't bode well for a future that isn't ready."

Florida has two more king tide events coming up, with the new moon later this month and the full moon at the beginning of November, usually the highest tide of the year. The strong onshore winds may mean that the recent tides during October's full moon take the crown this year, though.

One thing is for certain, though: During the November full moon, the tide "will be high, and it will flood streets," McNoldy said.

Original article on Live Science.

Andrea Thompson
Live Science Contributor

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.