It’s back — that familiar disruption of the ocean-atmosphere system in the tropical Pacific that we call El Niño.

For meteorologists, El Niño as a physical occurrence is a proven and accepted fact, but the way it works remains a theory. Still to be understood about El Niño and its counterpart, La Niña, are the way they arrive and depart in cycles, but cycles that are not regular. Why they begin at all is also a mystery; predicting their onset has proved difficult for forecasters. El Niño and La Niña will appear about 30 times over a century, usually alternating every three to seven years, but the time between can vary anywhere from one to 10 years. Sometimes they follow each other, and sometimes they are separated by intervals of normal conditions.

When an El Niño does start, it starts like this: the westerly trade winds in the Pacific slacken. A big pool of seawater, warmed by the sun, half again as large as the United States, builds and flows backward toward South America. It moves through the ocean like an iceberg, most of it submerged, a small part protruding above the sea’s surface. That is possible partly because its warm water is less dense than the cool water that surrounds it. In addition, the El Niño’s waters are less salty than normal seawater; it’s always raining over an El Niño, and the rain dilutes the seawater.

The energy contained in that warm water is immense: it would take 1 million large power plants running full on for a year to create it. Because El Niño represents such a vast quantity of energy, it is second only to the seasons as a shaper of global weather. Even though the El Niño pattern directly involves only about one-fifth of the circumference of the earth, it is able to transform the weather around the globe.

This time around, though, meteorologists agree that El Niño’s transformative power is minimal. “It’s a weak El Niño,” says John Nielsen-Gammon, Texas state climatologist and professor of meteorology at Texas A&M. “What’s strange is that normally there are enhanced thunderstorms over the warmer temperatures, and that hasn’t happened. That’s normally the connection mechanism between what’s happening in the ocean and the mid-latitudes. It’s just a matter of it being fairly weak and not being able to dominate the atmospheric response. Other factors are stronger than El Niño.” So this is one of those years when the weather is not being dictated by the equatorial Pacific, which might explain some of the weather we’ve seen that is unusual; for example, El Niño doesn’t usually lead to cold weather in the Northeast, but we’ve had that.

Michael Halpert, a meteorologist at the Climate Prediction Center of the National Oceanic and Atmospheric Administration, agrees this weak El Niño will last for another couple of months. “We haven’t seen a strong link to atmospheric circulation, so even though ocean temperatures are above normal, the precipitation in the tropics we usually see hasn’t happened,” he says. “The temperature gradient across the Pacific isn’t what we’d see even with a weak El Niño. Towards summer, we’re not expecting any link to El Niño at all. It hasn’t played much of a role this winter, so expectations that it will play much of a role on summer are pretty low.” Most current predictions are for the current El Niño to continue to be weak and to dissipate in the spring.

In fact, its influence may already have peaked several months ago. “There’s a delay in the cause and effect. Six months ago, it was looking as if we’d see more of a moderate El Niño. It hasn’t developed as anticipated,” says Leon Osborne, professor of atmospheric science and director of the University of North Dakota’s Regional Weather Information Center.

Though El Niño is primarily an ocean-influenced event, a direct response to solar radiation and oceanic circulation, it changes our weather by causing a change in the atmosphere. That atmospheric variation is called the El Niño Southern Oscillation  —  and it’s really the Southern Oscillation part of the equation that is meteorological in nature. Scientists are still figuring out just how that oscillation interacts with other kinds of oscillations, particularly those at the higher latitudes; while El Niño is a big driver of global weather, it’s not the only driver. “El Niño seems to have become the catchall,” Mr. Osborne says. “Is there bad weather? Then people say it must be El Niño. But many other influences and factors go into regional climates.”

Other influences on the weather
One of those is a longer time-scale variation of Pacific decadal oscillation, Dr. Nielsen-Gammon says. “It’s a pattern that looks similar to El Niño, but it is stronger in the mid-latitudes than at the equator. That’s been in the negative for most of the last decade.” It tends to minimize the effects of El Niño and maximize the effects of La Niña.

“We’re only beginning to understand how you can have different types of events with warm temperatures in different places and how that affects atmospheric response,” Dr. Nielsen-Gammon says. “We have only half a century’s worth of atmospheric and oceanic measurements, so it’s difficult to make statistical predictions. Global models are only somewhat good at predicting atmospheric response to temperature patterns. The details are still murky.”

There is at least one other tropical disturbance that may also have been figuring into the weather in this country recently. “The Madden-Julian Oscillations are disturbances that last for 30 to 60 days, whereas El Niño lasts six months to a year. Those can work to initiate or strengthen an El Niño,” Mr. Halpert says. “The very heavy rain and snow in California in January was at least partly linked to these. It has played a bigger role in this winter’s weather than El Niño.”

Typically, even a stronger El Niño wouldn’t play much of a role in the United States in the summer months. As for the coming spring, Mr. Halpert explains a phenomenon called the spring barrier that makes its effects difficult to predict. “Forecasting El Niño is poor during the spring and increases in the summer,” he says. Because of that, expectations for the fall and winter will be clearer once the summer arrives.

Drought update
There is mixed news on the drought front. “On average, there’s been an improvement in drought, particularly in the southwestern United States  —  California, Arizona, southern Nevada, southern Utah,” Dr. Nielsen-Gammon says. “There’s been close to record rain and snowfall. But things have gotten worse in some areas of the Northern Plains. The biggest area of concern is the Pacific Northwest, where there is limited snow pack. If that were to continue for the rest of the winter, there would be serious problems come summertime.” Of course, a lot can still happen in the spring.

But despite some improvements, the drought across the intermountain West, which has gone on for about 10 years, is not over. “The large reservoirs in Colorado and New Mexico were so far down it will take awhile for them to come back,” says Mr. Halpert. “It takes years to get into a drought and years to get out.”

There has been an erosion of the area affected by drought, Mr. Osborne says. “Generally speaking, there’s an improving trend in drought areas toward more moisture.”  That trend may continue. “We’re looking ahead at a fairly benign weather pattern. As El Niño weakens, there will be a trend back to longer-term averages, not biased in one direction or another. It could be one of those unique years when we see what the average is.” A complex set of factors must come together to create a drought, Mr. Osborne adds; it matters what the soil moisture content is going into the growing season. Throughout the Midwest, it’s been fairly moist.

Because the local environment has a significant influence on the weather, broad predictions are always tricky, but there are no specific predictions for drought  —  or lack thereof  —  for the eastern two-thirds of the country, says Dr. Nielsen-Gammon. “Predictions are for continued moisture in the Southwest, warm temperatures on the West Coast and cool temperatures in the middle of the country.”

In Texas, Oklahoma and the Northern Plains, there may be a colder-than-normal spring, Mr. Halpert adds, and in the Southwest, above-normal temperatures. “Of course,” he continues, “there’s always a lot of variability.”