A winter-weary public might find the dueling headlines atop El Nino stories perplexing.
“Here comes El Nino; good news for US weather woes.” "Return of El Nino Could Bring Rain, Heat, Misery."
Certainly, rain, heat, misery, and even good news are likely somewhere on the planet during the next year, and they may or may not have anything to do with “El Nino,” an anomalous warming of surface waters in the tropical Pacific.
That’s if it actually does return. The government’s Climate Prediction Center has issued an “El Nino Watch,” saying the chances are 50-50 that it will sometime in the summer or fall.
For an El Nino to be declared, a continent-size pool of warm water in the Pacific has to brew for months. Since that region is an important driver of the climate system, the warming of the overlying atmosphere can have chain-reaction effects on the atmosphere all the way across North America.
On the plus side, El Nino can have a dampening effect on Atlantic Basin hurricanes by generating strong upper-air winds from the west that can shear off incipient storms before they can mature to hurricanes.
Florida tends to get a measure of payback for that good fortune with more rain in winter, since El Nino can energize the storm carrying southern jet-stream winds. That’s why California has been hammered with rain during El Nino.
What would El Nino would mean for us? If it developed and continued into winter, about the only thing we could count on would be something strange and persistent.
El Nino has coincided with a regular Whitman’s Sampler of conditions in Philadelphia, but generally – and only generally – we’ve found that what happens tends to keep happening, and the winters are characterized by extremes.
El Nino conditions were present in the winter of 2009-10, the snowiest on record in Philadelphia. By contrast, the El Nino of 1997-98 flooded North American with warm air, resulting in the mildest winters on record here, with grand total of 0.8 inches of snow.
The winter of 1972-73, during another strong El Nino, was the only one in Philadelphia history with no measureable snow. Yet, puzzlingly, temperatures and precipitation in January and February 1973 were close to normal.
Why have conditions been so variable?
Two obvious points: First, not all El Ninos are alike in intensity and coverage. Secondly, and perhaps this should be first, El Nino is not the only force governing the atmosphere.
We’ve learned again and again that nothing happens in a vacuum up there, and no human or groundhog has been able to figure it all out yet.