Photograph by NOAA/ZUMA/Corbis
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Hurricane Katrina approaches the coast of Louisiana and Mississippi in 2005.

Photograph by NOAA/ZUMA/Corbis

Why This Hurricane Season May Be Quiet for Atlantic Ocean

An El Niño and cooler waters in the Atlantic could be harbingers of a slow season for tropical storms.

Cooler waters and the meteorological phenomenon known as El Niño could mean a quiet summer in the Atlantic Ocean, with only eight named tropical storms expected to form during the 2015 hurricane season, which starts Monday.

The forecast, released Monday by meteorologists at Colorado State University, predicts that three tropical systems could intensify into hurricanes with winds of at least 74 miles per hour (119 kilometers per hour). And one of those hurricanes could become a major storm with winds exceeding 110 miles per hour (177 kilometers per hour) by the time the season officially ends on November 30.

The forecast is for the Atlantic Basin, which includes the Caribbean Sea, Gulf of Mexico, in addition to the Atlantic Ocean.

El Niño and Cooler Waters

An El Niño occurs when waters in the eastern Pacific Ocean off the coast of northern South America are unusually warm. When that happens, it generates upper-level winds—known as wind shear—over the Atlantic. The shear makes it more difficult for tropical storms to form and inhibits their strengthening when they do form.

WATCH: Find out how simple changes in ocean currents can lead to catastrophic weather conditions.

The cooler waters in the Atlantic also are a harbinger of a quieter season. Tropical storms draw their strength from surface waters that have been warmed to at least 80 degrees Fahrenheit.

Jeff Masters, director of meteorology for the website Weather Underground, says cooler temperatures could mean that a cycle of warmer water in the Atlantic that started in 1995 is ending. That warming trend was a major factor in some of the most active hurricane seasons on record, especially 1995 and 2005—the year of Hurricane Katrina. (Watch our video on the science behind Katrina.)

The warming cycles are thought to be caused by the salinity of ocean waters. Higher salt content produces warmer water temperatures. Variations in salinity are caused by ocean currents.

Still, Masters cautions that it’s too early to determine whether the Atlantic is beginning a cycle of cooler waters and less-active hurricane seasons.

“If the cycle were flipping, this is what you’d expect to see,” he says.

Strong Hurricane Still Possible

The waters in the Gulf of Mexico and around the Bahamas, meanwhile, are still warmer than average, and that could produce tropical storms there.

The formation of Tropical Storm Ana nearly a month ago does not indicate that the 2015 storm season will be more active than expected. “It didn’t form in the main development region for hurricanes,” Masters says. “Early season formation doesn’t give an idea of what the early part of the season will be like.”

Meteorologists warn that the forecast for a less-active season doesn’t mean that an extremely powerful hurricane won’t form. CSU forecasters noted that very powerful hurricanes have formed during seasons when an El Niño was active.

Powerful Hurricane Betsy was an example of an intense hurricane during an El Niño. The storm made landfall near New Orleans on September 9, 1965 with winds exceeding 140 miles per hour (225 kilometers per hour).

And while the Atlantic may be quiet, the Pacific Ocean is already active. Masters notes that three powerful hurricanes already have formed in the Pacific. The eastern Pacific and Hawaii could be in for a rough summer, he says.

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WATCH: Find out how hurricanes form and why they can be so destructive.