For people in the southeastern United States, and especially in Florida, who feel that annoying tidal flooding has sneaked up on them in recent years, it turns out to be true. And scientists have a new explanation.
In a paper published online Wednesday, University of Florida researchers calculated that from 2011 to 2015, the sea level along the American coastline south of Cape Hatteras, N.C., rose six times faster than the long-term rate of global increase.
"I said, 'That's crazy!' " Andrea Dutton, one of the researchers, recalled saying when a colleague first showed her the figures. " 'You must have done something wrong!' "
But it was correct. During that period of rapid increase, many people in Miami Beach, Fort Lauderdale and other coastal communities started to notice unusual "sunny-day flooding," a foot or two of salt water inundating their streets at high tide for no apparent reason.
In the paper, published in the journal Geophysical Research Letters, the scientists proposed a mechanism to explain the rapid increase: Two large-scale atmospheric patterns had intersected to push up the water off the Southeast coast, causing a "hot spot" of sea-level rise.
This new mechanism, if it holds up to scientific scrutiny, might ultimately give researchers the ability to predict tidal flooding more accurately and warn communities what to expect months in advance.
William V. Sweet, a sea-level researcher at the National Oceanic and Atmospheric Administration who was not involved in the new work, pointed out that the long-term trend in sea level was a relentless increase, but that much is unknown about the variations that can occur over short periods. "The more we can understand what's causing those, the more we can be prepared for the next influx of tidal flooding events," Sweet said.
Many people think the ocean works something like a bathtub, with sea level being the same all the way around. In reality, the ocean is lumpy, with winds, currents and other factors pushing water around to produce substantial variations in sea level from place to place.
Worldwide, the average level of the ocean is rising at a rate of about a foot per century, a consequence of the warming of the planet caused by the human release of greenhouse gases like carbon dioxide.
The excess heat trapped by those gases accumulates primarily in the ocean, and the seawater expands as it warms. Land ice is also melting into the sea because of the planetary warming, contributing to the rise, which appears to be accelerating over time.
But within that long-term trend, sea level in particular regions can sometimes rise more rapidly or more slowly than the global average. It can even fall for a few months or years.
In previous research, scientists had noticed big jumps that tended to occur either north or south of Cape Hatteras, on the North Carolina coast. For instance, a notable jump occurred along hundreds of miles of shoreline north of Cape Hatteras in 2009 and 2010, followed by a sharp increase south of the cape from 2011 to 2015.
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The increase in the Southeast was the largest sudden jump there since the late 1940s, the scientists found. It amounted to about three-quarters of an inch of sea-level rise, which may not sound like much but equates to billions of extra gallons of water just off the coast. That water inundates streets and lawns when the tides and winds conspire to push it inland.
Cape Hatteras is geographically significant. The Gulf Stream, a swift current carrying especially warm water from the Gulf of Mexico toward the North Atlantic, runs close to the coast for hundreds of miles. But when it passes Cape Hatteras, it veers off into the deeper ocean. That had led scientists to suggest that changes in the Gulf Stream might account for some of the rapid variations in sea level.
But now, three University of Florida scientists — Dutton, Arnoldo Valle-Levinson, and Jonathan B. Martin — suggest that the Gulf Stream was not the primary culprit in the 2011 to 2015 rise.
Instead, they found that two large atmospheric patterns most likely accounted for the hot spot off the Southeast coast: the el niño cycle and the North Atlantic Oscillation, which is a shift in atmospheric pressure over the ocean that can have large effects on the winds blowing toward the American coast.
The paper suggests that the two sometimes interact in a way that causes water to pile up. The work confirms and extends two earlier papers, including one published in 2015 by a group led by Gerard D. McCarthy of Britain's National Oceanography Center in Liverpool.
The new work is based on strong correlations, going back decades, between particular atmospheric patterns and the high sea levels.
Sweet, critiquing the paper, said he felt that the correlations were indeed suggestive, but he found the paper somewhat weak in explaining the exact mechanisms by which the atmospheric shifts may be causing water to bunch up. "It's a little bit short, I think, in terms of physical understanding," he said.
Valle-Levinson, one of the authors, acknowledged this point. "How the system is working is not crystal clear to us yet," he said.
Still, the paper is likely to open up new research about why sea-level hot spots seem to wander up and down the U.S. coastline. The paper indicates the Southeast may now see some relief — even if sea level does not fall, which several of the scientists described as unlikely, the pace of the increase may slow for a while.
But communities that have already started to experience severe tidal flooding, like Miami Beach, should not relax their guard, the scientists warned. These towns can expect continued rising seas over the long term, even if the rise occurs in a stepwise fashion.
"Even if it does get a little better for a while," Dutton said, "that should be a period that people use to their advantage, to prepare for the next hot spot."