The warm tropical waters of the Atlantic clear the 2017 hurricane season

According to a new study, the very hot waters of the tropical Atlantic Ocean were the main cause of the many hurricanes in the region last year. And this model of ocean warming will likely become more common in the future, fueling stronger hurricanes, the researchers say.

Hiroyuki Murakami, a research scientist in climatology at University University for Atmospheric Research in Princeton, NJ, and his colleagues used climate simulations to determine whether several factors had influenced the hurricane season of 2017 3 or more. This is about double the average number of major hurricanes observed each year between 1979 and 2017. The simulations suggest that the relative warmth of the waters in the tropical Atlantic, rather than factors such as the onset of the La Niña climate, was the strongest storms, researchers report online September 27 Science.

La Niña is a cyclical phenomenon – the weather backlash of El Niño – which brings cooler waters to the tropical Pacific Ocean and causes a change in wind patterns over the Atlantic that can help strengthen hurricanes (SN Online: 6/9/16).

To determine what was the most influential factor, the team made several experimental predictions using a climatic model called HiFLOR. Each experiment began with different sea surface temperatures. In one scenario, the team used average sea surface temperatures for the tropical Pacific from 1982 to 2012, essentially eliminating the effects of La Niña. "The model was still simulating a very active season of strong hurricanes," said Murakami. In another scenario, the team removed warmer than average temperatures observed along the east coast of the United States last summer. This too did not seem to affect the number of major cyclones.

But when the team eliminated the sea surface temperature anomaly observed in the tropical Atlantic last year, "the main hurricanes were gone," Murakami said. This disappearance suggests that the temperature anomaly in the tropical Atlantic was mainly responsible for the overabundance of major hurricanes.

In addition, the simulations suggest a correlation not only between warm tropical waters and storm intensity, but also between the tropical waters of the Atlantic and tropical waters, such as those of the Pacific. According to Murakami, when the tropical waters are warm everywhere, the atmospheric layers tend to be more stable, which is not conducive to the formation of hurricanes. On the other hand, the abnormal warming in the tropical Atlantic Ocean compared to other regions of the world contributes to the formation of hurricanes.

The team's projections on climate change and global warming also suggest that this relative heat will increase in the future, resulting in more major hurricanes.

Climate change attribution studies seek to determine whether and how climate change of human origin affects weather phenomena. These studies have attracted more and more attention lately, with scientists declaring last December that three extreme events in 2016 would not have occurred without climate change (SN: 1/20/18, p. 6).

As Murakami's team began their investigation as the 2017 hurricane season was still underway, this work could be considered a real-time attribution study, he says. Real-time attribution studies could ultimately help emergency planners better anticipate threats to communities, he adds. Another team of scientists conducted a real-time attribution study on Hurricane Florence, which landed Sept. 14 in North Carolina. This study suggests that the size of the storm and the heavy rainfall were influenced by exceptionally warm ocean waters (SN Online: 9/13/18).

Some researchers do not agree that the relative warmth between the tropical Atlantic and the Pacific is more important than the degree of heat of the water. The new study "is a solid job" that demonstrates the importance of the tropical Atlantic relative to the natural weather patterns of the 2017 hurricanes, said Michael Mann, a Penn State climatologist. "But I have a different interpretation" of the data, he adds. He and others have already shown that the absolute heat of these waters can better predict the seasonal activity of storms, he says.

Mann notes that Penn State's own forecasts for the 2017 hurricane season have used absolute sea surface temperatures and predicts a total of 15 storms for the season – close to the final score of 17. This shows that these absolute temperatures are a better predictor. . "I prefer to go with what we know to be true in the real world."