Historically, notes Kossin, researchers (including himself) have focused primarily on the tropical Atlantic — the main hurricane development region –without distinguishing how hurricane-producing conditions may vary outside of it.
They knew a combination of warm ocean temperatures in the tropics and low vertical wind shear (changes in wind speed relative to altitude) results in favorable conditions for hurricane formation, while cooler than average sea surface temperatures work in tandem with higher than average wind shears to produce quieter hurricane seasons.
Scientists also knew that environmental conditions, primarily ocean temperatures and wind shear, determine whether Atlantic hurricanes intensify or weaken as their natural track pushes them northwesterly toward the U.S. coast.
But Kossin, a National Oceanic and Atmospheric Administration National Centers for Environmental Information scientist working out of UW-Madison’s NOAA Cooperative Institute, wondered “what other patterns there might be.”
His study took a step back and looked for related patterns over the entire basin.
Kossin analyzed two datasets gathered over three 23-year periods from 1947 to 2015.
The first dataset, from the historical record of hurricane observations maintained by the U.S. National Hurricane Center, supplied observations taken every six hours and included information on location, maximum winds and central pressure.
The second, an environmental data set from the National Centers for Environmental Prediction and the National Center for Atmospheric Research, provided a benchmark for sea surface temperatures and wind shear for the period of interest.
Overall, when the tropics generate many hurricanes — during periods of low wind shear and high ocean temperatures in the tropical Atlantic — they also create a situation where those hurricanes lose energy if they approach the coast, as they encounter a harsh environment of higher wind shear and cooler ocean temperatures.