After a spring that soaked parts of California and saw destructive tornadoes rip through towns stretching from Texas to Pennsylvania, scientists are paying close attention to how much of the country’s recent severe weather is due to the return of El Niño, a naturally occurring climate pattern that last drove a series of climate anomalies and extremes four years ago.

El Niño occurs when changes in the strength or direction of trade winds cause waters in the central and eastern tropical Pacific Ocean to become warmer than usual. El Niño events, which happen on irregular cycles that last two to seven years, have far-ranging effects on global temperatures, rainfall, hurricanes and other severe storm systems.

“El Niños are the biggest source of changes in weather patterns around the world,” said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado.

The last major El Niño occurred from 2015 to 2016, resulting in a record 16 tropical cyclones in the Pacific basin and a global temperature record set in 2015. Last October, climatologists once again detected warm waters in the equatorial Pacific, and in February the federal Climate Prediction Center officially declared El Niño conditions. Climatologists say the planet is currently experiencing a weak El Niño that will likely persist into the summer, but even weaker El Niños can alter weather around the globe in significant ways.

Typically, El Niño events increase rainfall across the country, particularly in the southern United States and parts of California.

“In the last 12 months, if you look at the U.S. as a whole, it has been the wettest 12-month period on record since 1895,” said David Zierden, the state climatologist of Florida, who is based at Florida State University in Tallahassee. “El Niño didn’t cause all that rainfall, but it certainly contributed.”

Elsewhere, drier climates tend to prevail during El Niño events.

“In general, it will be drier for a lot of the world, so we’ll likely see more drought,” Trenberth said. “In places like Indonesia and Australia, the risk of wildfire goes up. But in North America, it’s the other way around.”

Floodwater from the Mississippi River cuts off the roadway from Missouri into Illinois on May 30, 2019 in Saint Mary, Missouri. Scott Olson / Getty Images

The recurring climate phenomenon also affects hurricanes, which are usually suppressed in the Atlantic during El Niño. Warmer water in the Pacific changes the air temperature over the ocean, which skews how winds normally circulate around the planet. One of these effects is increased wind shear, which can tear hurricanes apart or stop them from forming in the first place. In the agency’s yearly outlook for the hurricane season, which began Saturday and lasts until Nov. 30, NOAA cited El Niño as one of several competing weather patterns that is expected to contribute to a “near-normal” season.

But while El Niño events can curb the number of strong Atlantic hurricanes, the opposite is true for storms that develop in the Pacific.

“El Niño increases Pacific hurricanes and tends to favor longer-lived storms in the western Pacific, like typhoons,” said Phil Klotzbach, an atmospheric scientist at Colorado State University in Fort Collins.

The relationship between El Niño and tornadoes is less well understood.

Within the planet’s subtropical zone, where many of these weather systems develop, “there are a lot of other things that play a role to create the necessary ingredients to get severe storms going,” Klotzbach said.

This spring has already seen its share of destructive tornadoes, with strong twisters pummeling western Ohio, Texas, Missouri and Nebraska. Over a 12-day stretch beginning on May 17, there were 225 confirmed tornadoes across the country.

A 2016 study published in the journal Environmental Research Letters found that even a weak El Niño could increase the risk of spring tornadoes in the upper Midwest. But at Florida State University, research done by Zierden and his colleagues found that El Niño conditions tend to decrease the risk of springtime tornadoes in Texas, Oklahoma, Nebraska, Kansas and South Dakota — a swath of the south-central United States that is known as “Tornado Alley” because severe storms strike this region with a disproportionately high frequency.

“If you ask 100 scientists about the impact of El Niño on tornadoes, you might get 100 different answers,” Klotzbach said. “With tornadoes, it’s a more nuanced process.”

Another thing that is not yet well understood is how El Niño events may be affected by climate change. According to a study published in December 2018 in the journal Nature, El Niños may become stronger and more frequent as carbon dioxide and other greenhouse gases trap heat in the atmosphere, raising global surface temperatures and warming the planet’s oceans.

But Klotzbach said more research is needed, especially to determine whether any observed changes are due to global warming or natural variability in El Niño cycles.

“Our models tend to indicate that we should see more central Pacific El Niño events, and ones that are more intense, but there’s quite a bit of uncertainty,” he said. “How much is El Niño going to change in the future? That’s a huge open question that will be very important in terms of how climate change impacts future weather.”

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