IN 2015, a bit over two years after Hurricane Sandy hit his city, Bill de Blasio, New York’s mayor, announced the creation of a $3 billion restoration fund. Part of the money is intended to pay for sea walls that will help protect the place from future storms.
Building such walls may be an even more timely move than Mr de Blasio thought when he made his announcement. As a paper just published in Nature explains, for the past two decades a natural form of protection may have been shielding America’s Atlantic coast, stopping big storms arriving. Such protection, though, is unlikely to last forever. Mr de Blasio is thus taking the prudent course of mending the roof while the sun is shining.
The study in question was conducted by James Kossin of America’s National Oceanic and Atmospheric Administration, using wind and ocean-temperature data collected since 1947. In it, Dr Kossin shows that the intensity of hurricanes which make landfall in the United States tends to be lowest when the Atlantic’s storm-generation system is at its most active.
In Dr Kossin’s view, the cause of this apparent paradox is that, when conditions in the deep Atlantic conspire to produce the most hurricanes, precisely the opposite conditions obtain along the American coast. That creates a buffer zone which lowers the intensity of incoming storms before they make landfall. The agent responsible for this lowering of intensity is vertical wind shear—in other words, wind speeds and directions that vary greatly with altitude. Vertical wind shear removes energy from hurricanes by pulling heat and moisture out of a storm’s centre. When the Atlantic is in its hurricane-producing phase, with low wind shear and high surface temperatures in its central region, the part along the American coast behaves in the opposite manner, with high wind shear and low surface temperatures that sap storms’ energy.
The obverse is also true. When wind shear and sea-surface temperatures keep the Atlantic’s hurricane-generating region quiet, as they did between 1970 and 1992, those storms which do appear are two to three times more likely to intensify rapidly (defined as gaining 15 knots of wind speed in six hours) when they are near the coast than is the case during active periods.
Not everyone agrees with Dr Kossin’s proposed mechanism. James Elsner, a geographer at Florida State University, suggests that the correlations between storm generation and storm strength at landfall which Dr Kossin observes could be explained another way. The biggest storms tend to start out far from land rather than near it, and during periods of high activity hurricanes are generated farther out in the Atlantic than happens during lulls. These distant storms thus have more time to veer north—pushed that way by the interaction between Earth’s spin and their own, a phenomenon called the Coriolis effect—and therefore avoid landfall altogether.
Whatever its physical explanation, though, the correlation looks secure. And, with the current period of active hurricane formation now 24 years old, a lull, with accompanying superstorms, may not be long in coming. Time, perhaps, for other mayors along America’s Atlantic coast to follow Mr de Blasio’s example.