Why are hurricanes weakening? Possible reasons and the long term pattern

Hurricane Matthew, a category 4 storm, hit Haiti on October 4, 2016 [Image: Hurricane Matthew Hits Haiti/NASA Goddard Space Flight Center under CC-BY licence]

In 2012, Hurricane Sandy, the second costliest hurricane in United States history, caused the loss of 233 lives and assessed damage of 75 billion dollars. With Sandy still on the minds of many, it may surprise you that America’s Atlantic coast may actually be in the middle of a decades long lull in hurricane activity. Researchers at the National Oceanic and Atmospheric Administration (NOAA) published this surprising finding in a recent study, which also notes the seemingly paradoxical increase in active hurricane formation in deep Atlantic.

To explain this apparent paradox, the NOAA team proposed that vertical wind smear (VWS), which occurs when different altitudes have differing directions and intensities of wind, was acting as a buffer zone near America’s Atlantic coast to reduce the intensity of hurricanes landfalls. Using wind and ocean temperature data collected from 1947, researchers observed that during hurricane-generating phases, the deep Atlantic region exhibited low wind smear and high surface temperature, while the coastal region exhibited the opposite. Their recently published study claimed on that basis that the strong VWS shielded America’s coast from intense hurricanes.

Far from being purely academic, if correct, their study has huge implications for predicting the severity of hurricanes. There are, however, several caveats. Most importantly, the correlation between strong coastal VWS and less intense hurricane landfalls does not directly translate into causation – especially due to the complexity of hurricane formation. Furthermore, the study did not account for the variation in distance traveled by each hurricane, potentially rendering their observation moot.

That said, if both the correlation and causation between VWS and weakening of hurricane are robust, the NOAA team’s observations may help shed light on the complexities of hurricanes in the deep Atlantic and aid in improving predictions of hurricane severity.

Acknowledgments: Many thanks to Yang Tian, a graduate student from the Earth and Planetary Sciences Program at Harvard University, for providing expertise and opinions on this subject matter.

Managing Correspondent: Bing Shui

Original article: Hurricane intensification along United States coast suppressed during active hurricane periods – Nature

Media Coverage: Why big hurricanes weaken before they hit America’s coast – The Economist