In the early morning on 20 September 2009, the waters lapping the coast of Charleston, S.C., began to rise. High tide was on its way. But this time, the water did something unusual—it kept rising.
By 10:00 a.m., the higher-than-normal tide had left its mark: 7 to 10 centimeters of water blocked streets near the shore. Beachfront parking lots filled with salt water up to the bumpers of cars. Lifeguards reported waves inundating the beaches, and salt water that splashed over sea walls doused city walkways. The flooding didn’t just affect Charleston either; it extended halfway down the Georgia coastline.
What was special about this flood? According to a new study, the rising waters may have been partly due to a massive type of slow-moving ocean wave, called Rossby waves. The study, published last month in Nature Communications, found that the waves can lift sea levels along coasts by 10 centimeters or more, increasing communities’ risk of coastal flooding.
Although the connection between Rossby waves and sea level is not new, the study is the first of its kind to link the waves to high sea levels on the U.S. East and Gulf coasts. And this link directly influences flooding, according to Thomas Wahl, an assistant professor at the University of Central Florida and an author of the study.
For example, “Charleston had periods of more nuisance flooding in some years because of the Rossby waves,” Wahl told Eos.
What’s in a Nuisance Flood?
Nuisance floods occur when sea levels peak above normal levels, even in the absence of heavy rain or storms. The floods happen frequently up and down the U.S. East and Gulf coasts. Although the floods are not usually life threatening and they recede quickly with the tidal cycle, repeated flooding erodes infrastructure and interrupts daily life. Officials have called for better ability to predict the floods before they strike.
Part of what makes forecasting difficult is that some years have more floods than others, and scientists aren’t sure why. For example, in 2009, Charleston had more than 30 nuisance floods, but the year after saw only 15–20 floods, and the year after that saw only about 10. Although rising globally averaged sea levels are partly to blame for more frequent floods, scientists know that sea level rise alone can’t explain what causes such dramatic swings in nuisance flooding from one year to the next.
Other factors, such as local subsidence, city management, and the robustness of infrastructure, also contribute to flooding. But Wahl and coauthors wondered if there was another hazard to consider: Rossby waves.
To learn more, the study’s authors turned their sights to open waters.
Rossby waves, also known as planetary waves, form in the open ocean and can stretch for hundreds of kilometers from crest to crest. They travel slowly, taking months or even years to cross the oceans.
They’re so big that you can’t see them when you’re standing on Earth: Only satellites can truly capture the slow-moving behemoths. “They look like a huge horizontal undulation extending hundreds of kilometers at any given time,” said lead author Francisco Calafat, a researcher at the National Oceanography Centre in Liverpool, United Kingdom.
Although they are visible at the surface through satellite images, Rossby waves actually travel 50 to 100 meters below the surface, along a thin layer called the thermocline. This layer, defined by a rapid temperature drop from top to bottom, separates wind-whipped upper layers from calmer and deeper waters below.
When Rossby waves reach the coastline, their impact creates a bulge at the sea surface. Because the waves are so large, sea level rises not just in one location but along a stretch spanning hundreds of kilometers.
Watching a Rossby wave reach the coast, Calafat explained, is like seeing sea level move “in perfect synchrony over entire sections of coastline.” The bump in sea level can last “a few months,” said Calafat.
Modulating the Sea Level, One Giant Wave at a Time
The researchers were keen to explore a fundamental question: Could tide gauge signals independently find when a Rossby wave hit the shore? They combed through tide gauge data, using statistical methods to pinpoint elevated sea level values seen over huge swaths of the coast.
The researchers found several such widespread high sea levels; for example, a tide gauge in Key West was buffeted by roughly 15 such instances over the last century. These events stretched up and down the coast, spanning, in some cases, all the way from Charleston, S.C., to Pensacola, Fla. One of these episodes even reached extremes, with levels nearly doubling the consecutive 2008 and 2009 fall seasons, raising waters from 8 to 15 centimeters.
The evidence showed that average sea levels at the coasts fluctuated from year to year, but the researchers still needed evidence of the actual waves. To find it, they searched for sea level changes in satellite data, in locations where models deemed it likely for the waves to occur. Using observational data, computer modeling, and statistical methods, the researchers found what they were looking for: Rossby waves thousands of kilometers offshore correlated with annual sea level changes along the southeast and Gulf coasts some months later.
Many of the waves were spawned in the open ocean, a result of naturally occurring disturbances in the ocean and atmosphere. In fact, Charleston’s 20 September nuisance flood occurred during one of the seasons of high sea levels that the researchers were able to link to Rossby waves offshore.
“For me, as a coastal engineer, the most exciting result was that we were able to link high sea levels that occur along the coast to something—Rossby waves—that happens months earlier in the open ocean,” Wahl told Eos.
The Silver Lining of Slow-Moving Waves
Christopher Piecuch, an assistant scientist at Woods Hole Oceanographic Institution in Woods Hole, Mass., not involved with the study, said that the research takes “an important step forward.” He added, “These insights could aid efforts to predict future changes in the coastal sea level.”
For residents on the coast, such predictive efforts can’t come soon enough. Events like the flooding in Charleston on 20 September 2009 can lead to long-term infrastructure damage. The city of Charleston recently spent more than $400,000 to upgrade storm drains in chronically flooded areas.
Just how would such predictions work? Scientists could try to take advantage of one of the hallmarks of Rossby waves: their leisurely gait. The waves travel under the surface so slowly that they can take “months or even years before their effects are felt along the U.S. coasts,” said Calafat.
Thus, the information could help scientists “develop a tool that can help coastal managers to anticipate high sea level events,” Wahl said. “Thanks to satellites which are orbiting the Earth, we can track Rossby waves, and this gives us the opportunity to assist coastal communities to be better prepared and more resilient.”
Duncombe, J. (2018), Massive ocean waves may play a role in nuisance flooding, Eos, 99, https://doi.org/10.1029/2018EO105433. Published on 04 September 2018.
Text © 2018. The authors. CC BY-NC-ND 3.0
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