El Niño, the periodic warming of the water in the eastern Pacific Ocean, has been known for centuries. The name – the Spanish word for the Christ child – was long used by Peruvian sailors who noticed it often occurs around Christmas.
But it’s been only in the past few decades that scientists have been able to use the phenomenon to forecast trends in weather in North America and elsewhere. And they say this year’s El Niño will be a doozy.
That means the Triangle will likely have a soggy winter and possibly a cooler one, said Ryan Boyles, a professor at N.C. State University who serves as state climatologist and director of the State Climate Office.
“We have pretty high confidence it’s going to be a wet winter,” Boyles said. “Temperature wise, it’s a little tougher. The statistics aren’t as strong when we try to link temperature and El Niño.”
In terms of water temperature in the eastern Pacific, this year’s El Niño could rival or exceed the strongest on record, in the winter of 1997-98, scientists say. That coincided with the wettest January and fourth wettest February in the 120 years of record-keeping in North Carolina, said Corey Davis, a climatologist in the State Climate Office who has studied the effects of El Niño on the state.
About 7.5 inches of rain fell at Raleigh-Durham International Airport that January, more than twice the normal amount. The next strongest El Niño year, 1982-83, resulted in the eighth wettest February on record in North Carolina and the fourth wettest March.
Boyles says the warmer ocean waters of an El Niño produce more intense thunderstorms, which in turn act to shift the track of the sub-tropical jet stream – one of the air currents in the upper atmosphere that determine our weather. El Niño creates more pronounced north-south dips in the jet stream, drawing more moisture off the Gulf of Mexico and the Atlantic and causing more storms from Texas into the Carolinas.
A wetter winter doesn’t necessarily mean a snowy one, though. In fact, during the previous strongest El Niño of 1997-98, only about 2 inches of snow fell at RDU all winter, about 5 inches less than normal. All the rest fell as rain.
That’s in part because El Niño isn’t the only factor that affects our weather, Davis notes. There’s also the arctic jet stream, which determines when we get the blasts of cold air needed for snow and ice. One reason there wasn’t much snow in that big El Niño year of 1997-98 was an unfavorable Arctic jet stream, he said.
“That’s the fickle nature of winter weather here in North Carolina,” he said.
Even if El Niño cools us down a bit, it may not mean we’ll be sledding and shoveling this winter, said Shawna Cokley, a meteorologist with the National Weather Service in Raleigh. With the average wintertime highs in the low 50s and average lows right around freezing, Cokley said, “Cooler than average doesn’t necessarily bring you to snow, though it is certainly possible.”
The effects of El Niño on North Carolina’s weather are largely limited to the Piedmont and the coast, Boyles said. The phenomenon tends to reduce storm activity in the Ohio Valley, and the Appalachian Mountains mark the transition zone where nothing unusual happens, he said.