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A weakening La Nina adds a lot of uncertainty to climate predictions

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Thomas Barwick | Stone | Getty Images

A weakening La Nina climate pattern is going to make weather and climate forecasts a bit trickier this fall and winter.

The National Oceanic and Atmospheric Administration has called off its La Nina watch, and evidence suggests conditions are weakening and returning to neutral.

"It means we have a less confident forecast," said Tony Barnston, chief forecaster for Columbia University's International Research Institute on Climate and Society. Barnston is one of the 10 climate scientists from NOAA and institute who decide whether a watch should be put in place.

Climate prediction is always a matter of probabilities, and there is always a fair share of uncertainty. But a big change like an El Nino or a La Nina bears certain kinds of effects with enough reliability that forecasters have more confidence in their predictions at those times.

If the ocean's temperature rises a half degree above normal, it is a strong sign of an El Nino. A half degree below normal suggests a La Nina, and anything in between is neutral.

La Nina patterns tend to bring heavier rain and snow to the northern Rocky Mountain region and drought to the South. El Nino generally brings cool and wet weather to the South and warmer weather to the Northwest.

Without such a strong force like a La Nina acting on the climate system, there is a higher chance of more random and unpredictable events influenced by other variables.

"So not having a La Nina is good news for people who don't want the particular impact it would cause, like drought in the Southeastern U.S. in the winter," Barnston said. Wheat growers in the South, for example "are probably happy to see neutral, because it means anything could happen. It could even be a wet winter."

But, he added, skiers in the Pacific Northwest and northern Rockies might be disappointed that they have a lower chance of the heavy snow a La Nina could bring. Some disappointed people have already emailed Barnston and made comments on the institute's El Nino Southern Oscillation blog.

Not every meteorological agency seems to agree over whether there is a La Nina now, or whether we should be on the lookout for one.

The Japan Meteorological Agency, for example, says La Nina is in effect and will continue through fall, and the Australian Bureau of Meteorology, while agreeing with much of NOAA's assessment, still has a watch in effect.

Barnston told CNBC this is due in part to the fact that the agencies study different regions and give different weight to variables.

Mostly, he said, it shows that if there is a La Nina at all, it only meets the criteria by the slimmest of margins. The sea surface temperatures in the equatorial Pacific Ocean are low enough to be considered La Nina — 0.5 degrees below normal. But the trade winds across the ocean are not as strong as NOAA expects them to be during a La Nina, for example.

"When there is a stronger event, everyone agrees about it," Barnston said. "It is when we have very weak or borderline ones like this that we have these differences in diagnostic procedures."

Barnston also noted that removing the La Nina watch doesn't mean there will not be one at all. It simply means the probability of one fell below the NOAA's threshold for keeping the watch in effect.

Typically, companies plan for the season by looking at the prior year's weather, according to Evan Gold, executive vice president of Global Services for Planalytics, a firm that advises businesses on planning around weather. This is because of the uncertainty around long-term climate predictions.

Last year brought one of the warmest fall seasons on record in the Lower 48 states — something that is unlikely to repeat this year, Gold said.

Weather, especially in the Eastern U.S. was near record warm temperatures through December — New York City saw temperatures in the 70s in November and 60s in December.

"The likelihood of that repeating again is extremely low," Gold said. "I certainly wouldn't want to plan my business or my inventory off of it."

Just based on the high records set last year, 2016 is highly likely to be cooler overall, noting that that is typically what businesses such as restaurants and retailers are often paying attention to.

"Most businesses are likely looking for the cold, and it will be coming," he said.