Wednesday, April 25, 2018

Flooding Hot Spots:  Why Seas Are Rising Faster on the U.S. East Coast

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.

The average number of days per year in which U.S. coastal waters rose above the local threshold for minor flooding. (Credit: NOAA) Click to Enlarge.
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 NiƱo 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.

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.

Read more at Flooding Hot Spots:  Why Seas Are Rising Faster on the U.S. East Coast

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