News•December 11, 2012

Warming May Bring More ‘Black Swan’ Storm Surges

Hurricane Sandy offered a grim reminder of how vulnerable coastal cities, such as New York, are in the face of rare, powerful storms. As sea levels rise in response to global warming, this vulnerability is likely to increase, leaving officials across the U.S. and around the world to grapple with increasingly urgent questions regarding how to adapt in a rapidly changing world.

But while Sandy was a devastating event, with high winds and, more important, its massive storm surge causing at least $100 billion in damage and killing 125 people in the U.S., its destructive power did not greatly exceed other storms in New York City’s history. In fact, New York’s meteorological history is replete with costly encounters with hurricanes and nor’easters.

The Verrazano Bridge in Brooklyn as Hurricane Sandy approached on Oct. 29, 2012. Credit: Carlos Ayala.

In other words, Sandy was not a true “black swan” event — one that cannot reasonably be anticipated based solely on historical records. Such is the conclusion of meteorologist Kerry Emanuel of MIT and civil engineering professor Ning Lin of Princeton University, whose work on “black swan tropical cyclones” was presented at the annual meeting of the American Geophysical Union in San Francisco on Dec. 4.

In a warming world, however, the researchers found that black swan storm surges associated with tropical cyclones may become more likely. They’d still be very low probability events, but they would have very high impacts. This finding suggests that coastal residents and public officials may wish to expand the scope of their planning for how to cope with storm surge flooding, which is already being amplified by global warming-related sea level rise.

Lin and Emanuel focused their work on three cities, one of which — Tampa, Fla. — is widely considered to be extraordinarily vulnerable to storm surge flooding, but has been spared by a direct hit for many decades. The other two — Dubai, in the United Arab Emirates, and Darwin, Australia — have little historical precedent of tropical cyclone landfalls. For each city, they utilized advanced computer models and created thousands of synthetic storm scenarios to get an idea of both their probabilities and potential impacts.

The result is a more scientific version of a “Worst-Case Scenario Survival Handbook” – except without the survival tips.

NASA visualization of the wind field associated with Hurricane Sandy as it approached the Mid-Atlantic coast on Oct. 28, 2012. This map was produced with data from a radar scatterometer on the Indian Space Research Organization’s (ISRO) Oceansat-2. Wind speeds above 40 mph are yellow; above 50 mph are orange; and above 60 mph are dark red.
Click to enlarge the image.
Credit: NASA.

The research suggests that cities like Tampa, where tropical storms and hurricanes are common threats, may need to prepare for far greater storm surge impacts than they may have considered to date, while locations that aren’t used to such storms may need to consider themselves increasingly at risk in coming decades.

“In all three cases we are talking about possible but highly improbable events,” Emanuel said in an email conversation. “Planners usually think in terms of 100-year or perhaps 250-year return period events, whereas we are thinking about events of much smaller probability. Nevertheless, some interests like to be very conservative. For example, here in the U.S. the Nuclear Regulatory Commission wants to build nuclear power plants for the 1-million-year event; in this case, they might be interested in our work,” he said.

In the Tampa Bay area, a metro region of 4.2 million people, Emanuel and Lin found that geophysical features put the city at risk of a worst-case storm surge of up to 13 meters, or about 42 feet. This is far higher than the surge generated by a damaging hurricane in 1921, which was only 3 meters, or 9 feet, high. According to Climate Central research, about 67,000 people live below 9 feet in elevation in the Bay Area.

The 1921 storm tracked just north of the city, making landfall along Florida’s west coast at a perpendicular angle, placing Tampa in its most powerful quadrant. But the real worst-case scenario, according to Emanuel and Lin, would involve a storm that moves north-northwestward, parallel to the shore, striking Tampa Bay from the Southwest. This would allow the storm’s forward motion and winds to resonate with an oceanic wave phenomenon, known as a “coastally trapped Kelvin Wave,” that can create extraordinarily high waves.

For Dubai, Emanuel and Lin simulated 3,100 potential storms passing within 100 kilometers of the highly developed Persian Gulf port city. This area doesn’t have a history of tropical cyclones, largely because there tends to be low atmospheric humidity, which limits the formation of tropical storms and hurricanes. However, computer models show the potential for tropical storms to develop near the area and move into the Gulf, which would be unexpected if you just went by historical records, Lin said. One storm, Tropical Cyclone Gonu, moved relatively close to Dubai in 2007, making landfall in Iran as a weakening tropical storm.

Coastal flooding from Hurricane Sandy in Crisfield, MD.
Credit: Maryland National Guard.

Emanuel and Lin show that Dubai could be subjected to a 7-meter storm surge at some point, although the probability of that is very low.

“In the case of the Persian Gulf, there is no planning for tropical cyclones (to the best of my knowledge), simply because there are no such storms in the (somewhat limited) historical records of the region,” Emanuel said. “Perhaps planners in that region should consider events that may not be as strong as the one we showed, but could nevertheless do a great deal of damage.”

In Darwin, which lies on the north coast of the continent, a black swan tropical cyclone could generate a storm surge of 11.5 meters, or more than 37 feet. But the odds of this are extremely low, on the order of 1 in 7,000, according to Lin. Cyclone Tracy, which was a small but intense storm that hit the city in 1974, brought a surge of 1.6 meters, which is a little more than 5 feet.

Lin said Darwin was designed to withstand very strong winds from tropical weather systems, but was not designed with storm surge in mind. “These results may cause the city to reconsider its tropical cyclone risk,” she said.