El Nino: What's happening and what might it mean for Pennsylvania forests?

Posted: August 18, 2014

What is the El Nino phenomenon and what impacts might it have on our northeastern weather and forests?

For the first time in the ten years I've been in Pennsylvania, when I ask a colleague who focuses on summertime rainfall, he says, "we've caught up to our average and might be a little bit ahead!" It's not hard to believe when we're in the midst of summer that feels more like spring or fall. (I feel badly for those folks who operate pools!)

Way back in April, we heard a lot about how this was going to be an El Nino year and what that might mean for the U.S. and the Northeast. Considering the current summer weather, I got curious and thought I'd see if I could figure out what might be going on. What we've been experiencing in the Northeast does not seem to jive with the predictions made this past spring.

Have you wondered about El Nino? Why "the Little Boy" and what makes it happen? El Nino is a warming of the eastern Pacific Ocean off the equatorial coast of South America. Sometime in the past Peruvians and Chileans noticed warmer ocean temperatures occurring around Christmas and so connected the phenomenon with the arrival of Christ, hence, "El Nino." As the Pacific Ocean warms in the east, with average or slightly below average ocean temperatures in western and central Pacific Ocean, this sets up an east to west temperature gradient. Warmer oceans send warm, moist air into the atmosphere, causing low pressure to build. Cooler oceans are affiliated with dryer air and high pressure systems, creating a cyclic atmospheric effect that drives the southern oscillation of the jet stream. This resulting circulation system is often called the El Nino Southern Oscillation or ENSO.

An El Nino year doesn't mean that weather is going to play out in an easily predictable way. On average, El Nino years foretell a quieter hurricane season in the Atlantic. The ENSO creates wind shear that clips the tops off storms forming in the Atlantic, keeping them from developing fully into hurricanes. Yet, Hurricane Andrew occurred during an El Nino year. El Nino is usually responsible for bringing wet weather to places that don't normally get wet. California and Texas were counting on El Nino this year to bring an end to their horrific droughts. Sometimes it causes warmer winters and the northeast doesn't see much winter precipitation; sometimes there are freak patterns, like the ice storm of January 1998 that wiped out forests and power across New England and the Canadian Maritime provinces. It all depends on multiple systems coming into play, how and where they interact, and those things aren't predictable half a year in advance.

When El Nino is weak to moderate, it forces the southern jet stream further south, which allows the polar jet stream to come south too. This interaction of warm, moist air and the polar jet stream's cold, dry air can cause heavy winter precipitation. When El Nino is strong, the southern jet stream moves more northward which brings milder air to our neck of the woods. Many are hoping for a strong El Nino to moderate our winter temperatures after last year's cold.

What would El Nino mean for the forests of Pennsylvania? A warmer winter might have pest implications. Cold winters have been cited as helping to keep the hemlock woolly adelgid populations from spreading too far north. If the temperature barrier is lifted for a year or two, how quickly might they gain a foothold in the northern reaches of hemlock territory? Everyone had hoped this past winter's frigid air might have killed some of the emerald ash borer. Unfortunately that hope wasn't borne out, but warmer winters can keep pests active for longer periods.

A wetter winter, or precipitation in different forms, might be beneficial to ticks. Some researchers have postulated that the white-footed mouse (a necessary host for lyme disease carrying ticks) doesn't survive well in years of extreme cold or food scarcity. If we have a lush fall and moderate winter, the mouse population may increase giving us increased tick numbers in 12-15 months. A meta-analysis of tick research indicated significant correlations between lyme disease incidence and a warmer winter a year and a half prior to infections.

Invasive plants are some of the earliest to green up and the latest to drop their leaves. Imagine a warmer and/or wetter scenario that encourages them get started that much sooner. Warmer winters could also change maple sap flow timing. A few years ago, after a very warm winter, some maple producers here in central Pennsylvania missed the season entirely, it came so early.

So what happening with El Nino this year? When I first began researching this article, the scientists at the National Oceanic and Atmospheric Administration and their colleagues around the world were predicting an 80% chance of El Nino beginning this month and continuing through winter and early spring 2015. As they updated their research, the chance of El Nino dropped to only 65%. The Pacific Ocean is warming, but it is warming in the wrong places and the cycle that drives El Nino is not getting started. The engine is not turning over. Significant warming is occurring off the coasts of Australia and Indonesia, areas that need to be cooler or stay at average temperatures to start the process. Warming has begun off the coast of South America and that pattern is beginning to look like a normal El Nino event, but the necessary east to west gradient is not present. So after all that hoopla, we may not see El Nino develop as we expected back in April. That doesn't mean we won't see strange events happening with an active and changing climate. In the East, since late June we've been in an "atmospheric anomaly" that has contributed to our below average temperatures. At the same time the Pacific Northwest is seeing higher than average temperatures, and lower than average precipitation. And the drought in California keeps going on…

So even with a 65% chance of El Nino forming in the fall and early winter, they now predict it would peak at weak to moderate strength. For those hoping for a quieter winter, this may not fit your wishes. The take home message is that nature is unpredictable. Models can only give us so much ability to predict. The pattern that drives El Nino started back in the spring, but then things went wonky in other parts of the world and the engine didn't catch. Maybe we'll see its effects later this year, maybe not. As a forest landowner or stakeholder, what do you do? Be prepared. Get your firewood in (and you'll be ahead of the game if we end up with a milder winter). Prepare your forest. Keep it diverse, healthy, and working for you, the wildlife, our larger society, and the ecosystem writ large.

For more information about El Nino, seek out the National Oceanic and Atmospheric Administration's webpage. Simply search "El Nino" when you're on their site. The update their forecast and models every Monday, and their scientists are keeping up some very well-written blogs about El Nino and other climate phenomena.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of free publications, call 800 235 9473 (toll free), send an email to, or write to Forest Stewardship Program, Natural Resources Extension, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry and USDA Forest Service, in Partnership with Penn State's Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Forest Stewardship Program Associate
Phone: 814-865-3208