Image: Science cover
In a Science cover story, researchers look at the possibility that climate change affects the rise and fall of weather patterns such as El Nino.

Large amounts of money and even human lives might be saved if we could brace ourselves well in advance for future El Niño episodes, but here’s the hitch: these events don’t occur at regular intervals. To improve El Niño forecasts, scientists must know whether global warming pumps up the intensity or changes the frequency of such cycles. In a new report in Science magazine, two researchers say that El Niño does follow a measurable pattern, which may well be under the influence of climate change.

El Niño and its counterpart, La Niña, happen when the surface of the eastern tropical Pacific bounces between warm and cool temperatures. Both phases can profoundly affect weather all over the world.

The two most intense El Niño episodes of the 20th century occurred in 1982 and 1997. Researchers must now decide whether these extra-strong El Niños are part of a larger pattern or just flukes caused by random variation. If a trend is underway, the next step is to learn just what role global warming might be playing.

In the 16 June issue of Science, Princeton scientists Alexey Fedorov and George Philander propose that the frequency and intensity of the El Niño Southern Oscillation (ENSO) cycle does indeed follow a pattern. To understand how it works, they say, you have to consider what drives its El Niño and La Niña phases in the first place.

Parts of the whole
The ocean and atmosphere are intimately connected, exchanging moisture, heat, and momentum in a dizzying number of ways. During an El Niño event, for example, the westward-blowing trade winds ease up. As a result, the warm surface water of the western Pacific shifts eastward, preventing cold bottom waters in the east from welling up along the South American coast. During La Niña, the trade winds intensify and more cold water wells up.

What Fedorov and Philander find most important, however, is that the relative importance of the various parts of this process may differ from one El Niño event to the next.

In fact, a variety of exchanges between the ocean and atmosphere work together to produce the ENSO cycle, much the way the forces of the universe seem to conspire against us during the morning commute. Make it through the road construction without losing too much time, and you’ll inevitably get stuck behind that slow-moving bus.

Two sets of exchanges in particular influence sea surface temperatures in the eastern tropical Pacific.

One set raises and lowers the transition zone between the warm surface waters and the colder deep waters. If the layer of warm layer at the surface is relatively thick, it takes longer for the system to switch from the El Niño to the La Niña phase.

When the other set of interactions is at its forte, strong winds in the east help draw cold water up past the transition zone to the surface, bringing a hastier end to El Niño.

From decade to decade, the primary influence on sea surface temperatures in the eastern tropical Pacific comes first from one set of interactions and then the other, Fedorov and Philander believe.

“Scientists were relatively happy with the ENSO predictions from their climate models in the late 1980s. But, the same models didn’t do so well during the 1990s, probably because the background conditions had changed,” Fedorov says.

Hope for long-range forecasts
If Fedorov and Philander are right, their research holds promise for improving El Niño forecasting, which is currently limited to detecting the earliest signs of the next event. Not everyone thinks this is possible.

“Quite a few scientists would say we have absolutely the same El Niño from one decade to another, but because of noise in the climate system, it’s simply irregular,” Fedorov says.

It’s probably true that the ENSO cycle is strongly affected by random events such as tropical cyclones and other weather disturbances (atmospheric “noise”). But that’s only part of the story, Fedorov and Philander say.

When the two scientists look at the century-long record of sea surface temperature in the eastern tropical Pacific, they see signs of a subtle pattern. Beneath the seemingly erratic swings from year to year, as El Niño gives way to La Niña and vice versa, there appears to be a gradual fluctuation that occurs over periods of decades.

In the last twenty years, this gradual change in sea surface temperatures has bumped up a few degrees, making the high temperatures of El Niño years even higher. Although it’s too early to say for certain that global warming is behind this trend, Fedorov and Philander have an idea for how that might work.

At the heart of all the activity that influences sea surface temperature in the eastern tropical Pacific are at least two basic factors: wind strength and the depth of the transition zone between the warm and cold layers of the ocean. Both are supremely vulnerable to shifts in global temperature.

The most likely way for global warming to affect El Niño, Fedorov and Philander believe, would be to change these two conditions, shifting the relative importance of the various parts of the ENSO cycle.

The two scientists have set up a computer model that lets them plug in possible values for the wind strength and depth of the transition zone. The computer then calculates how much this balance will shift, and how the frequency and intensity of the ENSO cycle should respond. The pair found that, in the last two decades, the background conditions that favor less frequent but more intense El Niños have become dominant.

The model results seem to match with reality. Although not everyone agrees, some evidence indicates that the frequency of El Niño may be decreasing, from about once every three years in the 1960s and 1970s, to once every five years in the 1980s and 1990s, according to Fedorov.

Charting the fluctuation in background conditions from one “mode of operation” to the other will be the key to planning long-term precautionary measures for El Niño. But the process of understanding how El Niño can change over time is just getting started. To make further strides, researchers will need far more sophisticated climate models that can incorporate the many interactions between the ocean and atmosphere.

© 2013 American Association for the Advancement of Science


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