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The active hurricane era that brought us Katrina and Sandy may be over

Hurricane Wilma of 2005 was the last major hurricane to make landfall on the U.S. coast. It came ashore in Florida as a category 3, after weakening from its maximum intensity of category 5. (NASA)

Floyd, Katrina, Wilma, Ike and Sandy — just a few of the devastating hurricanes we’ve seen in the years since 1995. It’s been an astonishingly active hurricane period of the Atlantic Ocean, costing the U.S. over $500 billion in damages. But there’s evidence to suggest that the painfully memorable, two-decade era that brought some of the most intense hurricanes on record — and some the most active hurricane seasons — is coming to a close.

Over the years from 1995-2012, the Atlantic Ocean averaged 3.7 major hurricanes (Category 3 or higher) per year. In the period before that, 1970-1994, the Atlantic produced an average of just 1.5 major hurricanes each year. And that pattern of active versus quiet extends back through the first half of the 20th century, as well.

However, over the past two years we’ve seen incredibly low hurricane activity, and the 2015 season is running at just 50 percent of normal to-date. Which leads us to ask the obvious question: Has the active Atlantic hurricane era come to an end?

The variations between active and inactive periods appear to be caused by fluctuations in sea surface temperature patterns across the Atlantic Ocean, known as the Atlantic Multidecadal Oscillation (AMO). When the AMO is in its positive phase, there is typically a horseshoe-shaped pattern in the ocean surface temperatures, with warmer than normal water in the far northern Atlantic and the tropical Atlantic, and normal or even cooler than normal temperatures off the East Coast.

During this positive phase, when ocean temperatures are warm and ripe in the tropical Atlantic, pressure and wind patterns are also altered in such a way that favor Atlantic hurricane formation and intensification. This pattern was also seen in earlier historical hurricane periods, and is likely driven by fluctuations in the Atlantic’s warm water conveyor belt, the thermohaline circulation, which is driven by the ocean temperature and its salinity.

But in 2013, this pattern changed considerably, with colder than average water emerging in the northern and tropical Atlantic, while warm water developed off of the East Coast. This shift leads us to ask if the AMO is driving active and inactive hurricane regimes, and if so, does this mean we have indeed entered an inactive period.

Another way that our research group monitors the AMO is through a measurement that combines ocean surface temperatures in the far northern Atlantic and pressure in the tropical Atlantic. When pressure is higher than normal, it implies a more stable atmosphere, which is not conducive for hurricane formation. Currently, this measurement of the AMO index is at the lowest level we’ve seen since 1994, in line with the Atlantic’s drop in hurricane activity.

What do these changes mean for landfalls? Utilizing historical data extending back to 1878 (and treating the 1878-1899 as an additional active period), the odds of major hurricane landfall do not change much for the Gulf Coast regardless of AMO phase, while the odds drop by about a factor of two for the Florida Peninsula and East Coast during a negative AMO period.

This makes sense from a physical perspective, as hurricanes that wind up along the East Coast tend to originate from waves of low pressure that track into the Atlantic off the coast of Africa, which are significantly affected by the large-scale environment embedded in the AMO. Gulf Coast storms often develop from areas of low pressure that are lingering in the region, typically in the wake of a strong cold front. These types of hurricanes are not significantly impacted by the AMO.

A caveat in the relationship between the AMO and hurricane activity is that there can be short-term periods that defy the long-term regime. For example, during the overall inactive period from 1970-1994, both 1988 and 1989 had above-average hurricane activity.

In addition, the activity of a given hurricane season also depends on what else is going on across the globe. El Niño, which is defined by warmer than normal waters in the eastern and central tropical Pacific, inherently decreases Atlantic hurricane activity because it increases wind shear in the region, making the environment unfavorable for hurricane formation. El Niño has been primarily responsible for the below-average season we’ve seen so far this year. So, given the likelihood that El Nino will be decreasing in intensity next summer, leading to either neutral or even La Niña conditions, it will serve as a good test of the hypothesis that we’ve entered a quiet Atlantic hurricane era.

Phil Klotzbach is an atmospheric scientist who specializes in hurricane research at Colorado State University. His most recent study, “Active Atlantic hurricane era at its end?” can be found in the journal Nature Geoscience.