News•January 24, 2013

Risks of Hurricane Sandy-like Surge Events Rising

The track of Hurricane Sandy was unprecedented in the historical record of North Atlantic Ocean Basin hurricanes, and its deadly storm surge — while exceedingly rare — is likely to become a more frequent event as the climate continues to warm due in large part to manmade greenhouse gas emissions. Those are the conclusions of a forthcoming study from researchers at NASA and Columbia University's Lamont-Doherty Observatory.

Historical tracks of tropical storms and hurricanes in the Mid-Atlantic and Northeast, with Hurricane Sandy's track indicated.
Click to enlarge the image. Credit: NOAA via Bob Henson, UCAR.

Timothy M. Hall, a climate scientist at NASA’s Goddard Institute for Space Studies in New York, and Adam Sobel, a researcher at Columbia University’s Lamont-Doherty Observatory, set out to investigate how common Hurricane Sandy’s impact angle was under static climate conditions. Given that global average temperatures are rising, their findings may support the idea that global warming helped influence the jet stream in ways that made the storm’s left turn more likely to occur.

“... Either Sandy was an exceedingly rare storm, or our assumption of long-term average climate conditions is erroneous, and Sandy’s track was made more likely by climate change in a way that is yet to be fully determined,” the study said. The study is undergoing peer review for publication in the journal Geophysical Research Letters and was first reported by WNYC News.

Other research has found that rapid Arctic warming, and the related loss of Arctic sea ice, is increasing the odds of such unusual weather patterns, and the study’s finding that Sandy’s track is so rare under long-term average climate conditions would support the argument that there may have been a climate change influence. In an interview with Climate Central, Hall said that much more research needs to be done in this area before reaching more definitive conclusions.

“There are a lot of steps and chains of reasoning in that argument, each of which needs to be understood more fully,” he said.

Hurricane Sandy, which was officially re-classified as a post-tropical storm shortly before landfall, caused heavy damage along the New Jersey shore, and caused the most extensive coastal flooding event in modern-day New York City. Much of the subway system in lower Manhattan and parts of Brooklyn flooded, along with transit stations in adjacent areas of New Jersey. The storm was responsible for killing at least 131 people, 48 in New York City alone, and it was the second-most expensive hurricane in U.S. history, behind Hurricane Katrina in 2005.

The impact angle of Hurricane Sandy was its most unusual feature, ensuring the storm surge would case maximum damage, Hall said. The storm's left-hand turn put the most dangerous right-front quadrant on top of New Jersey and southeastern New York, pummeling these areas with an historic storm surge and record high waves. That, combined with astronomical high tides, led to record storm tide levels.

Water rushes into the Hoboken, N.J. PATH station during the height of Hurricane Sandy on Oct. 29, 2012.
Credit: The Port Authority of New York and New Jersey.

The researchers used statistical techniques and computer modeling to simulate millions of “synthetic” tropical cyclones in the Atlantic in order to determine the likelihood of another storm making a Sandy-esque dramatic left hook toward the coast, striking the most heavily populated region of the U.S. at a nearly perpendicular angle.

The study found that Sandy’s track stands alone in the historical record dating back to 1851, and that modeling simulations showed such a track is an event that would occur about once every 714 years. However, that does not mean that a storm like Sandy won’t affect New Jersey and New York for another 714 years, but rather that the average annual probability of another Hurricane Sandy occurring is .14 percent.

That may seem low, but according to Hall and separate research published in Nature Climate Change in February of 2012, global warming-related sea level rise is likely to make destructive storm surges like Hurricane Sandy’s much more common in the next few decades, regardless of whether storms follow a path similar to Sandy.

“We’re virtually guaranteed to get a foot or 2 feet of sea level rise within the next century,” Hall said, adding that this will have the effect of dramatically increasing the frequency of such rare and significant weather events, such as a Sandy-like storm surge. “[We are] Highly likely to get more frequent flooding like Sandy just due to sea level rise alone,” Hall said.

The 2012 study by researchers at MIT and Princeton found that the combined effects of changes in storm intensity and tracks that may occur as a result of climate change could cause the present-day 100-year surge event in New York to occur once every 3 to 20 years by 2100, and the present day 500-year surge event to occur once every 25 to 240 years. That represents a significant escalation in storm surge risk, the study said.

This graphic shows factors that contributed to the top 10 high-water events measured at New York’s Battery Park from 1900 to present. The water height for each event is shown here against the benchmark of mean lower low water averaged between 1983 and 2001. Sea level rise (about a foot since 1900) is depicted here as a component of storm surge. Although Sandy’s surge peaked close to high tide, other events had even higher tide levels.
Click to enlarge the image. Credit: Carlye Calvin and Bob Henson, UCAR; data courtesy Chris Zervas, NOAA National Ocean Service.

Hurricane Sandy brought impacts that were actually worse than the 500-year surge event calculated by the 2012 study, since the combination of the storm surge and astronomical high tides caused the peak storm tide during Hurricane Sandy at The Battery in Lower Manhattan to reach about 4.23 meters above mean sea level. That was about 1 meter higher than the 500-year storm tide calculated for the 2012 study.

By hitting the coast at a nearly perpendicular angle, Sandy brought its strongest winds and maximum storm surge to the New Jersey and New York coastlines. The storm surge was aided by the timing of high tide and the geographical features of the coastline, which tends to maximize the potential surge in certain areas depending on the wind direction, including New York Harbor.

Typically, the tropical storms and hurricanes that strike the Northeast are pushed in a northeasterly direction by the prevailing upper level winds. That was the case, for example, with Hurricanes Gloria in 1985 and Bob in 1991. Those storms hit the coast at a grazing angle, and only areas of land that stick out into the ocean, such as Cape Hatteras, N.C., and Cape Cod, Mass., have a long history of experiencing storms that hit at a more perilous perpendicular angle.

But with Sandy, the combination of a storm diving southeast from Canada, into the Midwest, and a large high pressure area in northeastern Canada and southern Greenland pinned the storm and forced it westward into the East Coast.