Scientists are unraveling the reasons why some parts of the world are experiencing sea level increases far beyond the global average. A prime example is the U.S. Eastern Seaboard, which has been experiencing “sunny day flooding” that had not been expected for decades.

Seen from a pedestrian footbridge overlooking Myrtle Park — a sliver of land that Norfolk, Virginia is allowing to revert to wetlands — the panorama of surrounding homes illustrates the accelerating sea level rise that has beleaguered this neighborhood along the Lafayette River. A grey house, among the first raised in the area, is slightly elevated on cinderblocks, standing 2 feet off the ground. Nearby, owners of a white-sided house with black shutters have lifted their dwelling about 4 feet above ground level. And on the right, a brick house resting on cinderblocks rises incongruously 11 feet above the street. The roads circling Myrtle Park are cracked and disintegrating due to frequent flooding. Tidal grasses like Spartina are springing up. The boulevard a block away, which leads to the world’s largest naval base, floods several times a year and the frequency is increasing. “Things are getting worse,” says William “Skip” Stiles, executive director of Wetlands Watch, a local advocacy group. “We’re now requiring you to go even higher in Norfolk because we get it.”

From 2011 to 2015, sea levels rose up to 5 inches in some locales from North Carolina to Florida.

What Norfolk gets is that while sea level is rising globally at about a tenth of an inch per year, cities along the Eastern Seaboard of the United States — including Norfolk; Baltimore; Charleston, South Carolina; and Miami, among others — have suffered “sunny day” flooding from seas rising far faster than the global average. One study published last year shows that from 2011 to 2015, sea level rose up to 5 inches — an inch per year — in some locales from North Carolina to Florida. Given growing concerns over the flooding, scientists are now working to unravel the mystery of why some parts of the globe are experiencing so-called “sunny day” flooding that had not been expected for decades under conventional sea level rise projections. Along the southeastern coast of the U.S., researchers have zeroed in on three factors that have made this shoreline a regional hot spot of sea level rise. They include a slowing Gulf Stream, shifts in a major North Atlantic weather pattern, and the effects of El Niño climate cycles. “These coastal areas are more vulnerable than they realize to short-term rapid acceleration of sea level rise,” says Andrea Dutton, a University of Florida geologist who studies the history of sea level fluctuations. “If they’re hanging their hat on sea level rise projections looking at the potential over decades, they need to refocus and think about the potential for short-term variability in that rate.” Around the world, sea levels are not rising equally like water in a bathtub. The oceans are more akin to a rubber kiddie pool where the water sloshes around unevenly, often considerably higher on one side than another.

The average number of days per year in which U.S. coastal waters rose above the local threshold for minor flooding. NOAA

Scientists from the National Center for Atmospheric Research (NCAR), for example, have found that sea levels in the northern Indian Ocean are rising more rapidly than the global average and threatening densely populated shores, particularly along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, and Sumatra. Scientists say that shifting monsoon patterns have significantly warmed the north Indian Ocean, causing unusually rapid thermal expansion of the region’s seawater and thereby increasing sea levels. In a paper published earlier this year, those NCAR scientists modeled sea level rise for 20 cities worldwide. They found that cities like Boston and New York might experience twice the global mean increase, while San Francisco and Buenos Aires will likely be 15 to 25 percent below the mean. Sunny day flooding — what one researcher calls “high tide on steroids” — has increasingly disrupted coastal cities in the southeastern U.S. coast. In Charleston, tidal flooding increased to 50 days in 2016, up from four days annually 50 years ago, causing millions of dollars in damage and disrupting travel to the city’s hospital district. In Miami, flooding during unusually high tides, what local forecasters call “king tides,” is becoming an increasingly severe problem, with clear-weather flooding accelerating to nearly 20 days a year. But much worse is to come. A report earlier this year from the National Oceanic and Atmospheric Administration (NOAA) said that “by 2100, high tide flooding will occur every other day (182 days/year) or more often” under an “intermediate low” scenario along the Atlantic coast and the western Gulf of Mexico. Scientists have been steadily increasing their estimates of how much sea level overall will rise this century from melting glaciers and polar ice sheets. The current best estimates are in the range of 3 to 6 feet. The problem of variable sea level rise along the eastern U.S. seaboard gained widespread attention in the summer of 2009, when dozens of communities suffered from unexpected flooding during clear weather. City and state officials reached out to NOAA seeking answers. What, they asked, was going on?

The role of the slowing Gulf Stream has been the focus of much research by scientists.

William Sweet, a NOAA oceanographer, began to investigate. He examined tide gauge data, wind data from buoys, and the speed of the Gulf Stream, which has been tracked since 1982 using an abandoned telephone cable that snakes for 55 miles from West Palm Beach, Florida, to Grand Bahamas Island. He discovered that a prevailing northeasterly wind and a slowing Gulf Stream — coupled with a full moon tide, above-normal summer high tides, and decades of sea level rise — created the conditions for sunny day flooding that summer. “That really was the first time I was able to connect several different atmospheric and oceanic factors that contributed to impacts that really were concerning to folks,” Sweet says. “What I’ve set out to do since then is to explain the flooding folks are seeing and the underlying causes.” The role of the slowing Gulf Stream has been the focus of much research by scientists such as Sweet and Tal Ezer, an oceanographer at Norfolk’s Old Dominion University who has examined the effects of currents and weather patterns on sea levels. Ezer has been studying the Gulf Stream for 30 years. Though he’s never actually been out in the Gulf Stream, he creates computer models simulating the stream using information from tide gauges, the abandoned cable, satellite data recording sea-surface heights, and coastal radar that measures surface currents. WATCH: A visualization of the Gulf Stream as it moves up the U.S. East Coast (in gray, upper left). Credit: NASA

Beginning in 2012, he published a series of papers matching long-term slowing of the Gulf Stream with increased sea level rise. The Gulf Stream — about 60 miles wide, a half-mile deep, and generally flowing 100 to 200 miles off the U.S. East Coast — transports warm water from the Gulf of Mexico into the North Atlantic, all the way to Western Europe. A rapidly flowing Gulf Stream in effect whisks water away from the eastern U.S. seaboard. Using satellite altimetry, Ezer has found that the sea-surface elevation across the width of the Gulf Stream has a slope. On the coastal side, sea level can be 3, 4, or 5 feet lower than on the east side. When the current is stronger, the slope is steeper, aided by the Earth’s rotation. But when the Gulf Stream flow slows, that slope decreases, pushing more water up against the land, causing flooding during high tides.

Oceanographers say that the Gulf Stream could well slow even more in coming decades as the melting of Arctic Ocean ice and the Greenland ice sheet dump huge quantities of fresh water into the North Atlantic and disrupt longstanding ocean circulation patterns. Other evidence underscoring the role of the Gulf Stream in sea level rise anomalies along the East Coast has surfaced in recent years. In 2015, Hurricane Joaquin hovered over the Bahamas for days, more than 800 miles from Norfolk, yet tides as much as 3 feet higher than expected flooded the city. A year later, Hurricane Matthew missed the Norfolk area but caused massive flooding, power outages, and more than $35 million of damage throughout the state thanks to relentless rainfall and a high sea level that blocked drainage. In both cases, Ezer says, the storms slowed the Gulf Stream. That’s because strong winds near the surface weaken the Gulf Stream flow, contributing to high sea levels farther north in places like Norfolk.

Scientists hope to help localities prepare for years when sea levels and high tide flooding increase dramatically.

“This is a relatively new phenomenon,” says Ezer. “Sea level rise is changing all the time and the Gulf Stream is changing all the time. But only in the last few years have we found this nice correlation between the measurement of the Gulf Stream offshore and coastal sea level.” Ezer and other researchers have been examining additional factors that create sea level anomalies. Building upon Sweet’s earlier work, Dutton and Arnoldo Valle-Levinson, her University of Florida colleague, noticed unexpected changes in tide gauge data near Cape Canaveral, Florida that prompted them to look at the effects of the North Atlantic Oscillation and the El Niño Southern Oscillation. They discovered rapidly rising sea levels from 2011 to 2015 along the coast from Cape Hatteras, North Carolina south to Miami. In some cases, sea levels rose nearly 5 inches in five years. What was happening? El Niño occurs in the Pacific Ocean but spreads its effects across North America, changing wind patterns and causing water to build up along the Eastern Seaboard. Meanwhile, changes in the North Atlantic Oscillation, a major weather pattern, alter the jet stream, prevailing winds, and storm tracks, which affects the distribution of water in the North Atlantic and can increase sea levels along the U.S. East Coast. The study by Dutton and her colleagues determined that El Niño controls the timing of increases — and decreases — in waters along the coast. The North Atlantic Oscillation, Dutton says, acts as a seesaw that serves as a steering mechanism, focusing increased sea level rise either north or south of Cape Hatteras. She is now looking at potential hot spots for sea level rise along the Gulf of Mexico.

High-tide flooding overflowed streets in Norfolk, Virginia in July 2017. Skyler Ballard/Chesapeake Bay Program

Dutton says that variability makes sea level rise like global warming. “It pulses up and down as it’s going up,” she says. “It means we’re going to hit 2 feet (in sea level rise) before 2060 during extreme events or periods of high water where we get these hot spots.” With sea levels changing dramatically in such short time spans, Sweet has turned to providing coastal communities with actionable science, creating annual reports that recap flooding from the year before and make projections for the year ahead. NOAA also issues two- to four-day flooding forecasts based on tides and computer models. The goal is to help localities prepare and budget for years when sea levels — and high tide flooding — increase dramatically.