IRI scientists weigh-in on the potential for El Niño’s return

By Elisabeth Gawthrop

This Thursday, IRI will issue its latest climate forecast update, which includes our global seasonal forecasts as well as the probability for an El Niño developing later this year. Based on last week’s official NOAA forecast, issued in collaboration with IRI, the chance of an El Niño event occurring later this year is around 50%. That forecast takes into account both what models are predicting as well as human judgement. In the monthly forecast released by IRI the week following the official NOAA forecast, the models are presented without human interpretation. For the last few months, the forecasts using only models have predicted a higher chance of El Niño than the forecasts that incorporate human judgement.

The IRI/CPC probabilistic ENSO forecast issued mid-March 2017. Note that bars indicate likelihood of El Niño occurring, not its potential strength. Unlike the official ENSO forecast issued at the beginning of each month (first image), IRI and CPC issue this updated forecast based solely on model outputs. The official forecast also incorporates human judgement.

Part of the reason for the difference between the two kinds of forecasts lies in scientists’ confidence, or lack thereof, that models are able to fully capture the entirety of the ocean-atmosphere system. But moreover, during the April to June period the models are particularly notorious for being less accurate in predicting what will happen in the rest of the year — scientists call this the spring predictability barrier.

IRI’s Tony Barnston says the models are not as “cautious” as they should be for forecasts traversing the spring barrier. “Their lack of caution, which should cause them to give weaker or more tentative forecasts, is a bias known to humans. So we think the models may be too bullish (strong) in their El Niño forecasts. Right now we’re particularly hesitant to buy into the models’ probabilities, and magnitudes of the predicted event.”

At this time, conflicting signals from the tropical Pacific are increasing the uncertainty of the ENSO forecasts. In the eastern Pacific, warmer-than-average waters have contributed to a coastal El Niño, with heavy rains leading to flooding in Colombia, Peru, Ecuador and Bolivia. In the central and western Pacific, however, indicators point more towards a neutral or La Niña outlook. In a recent article for NOAA’s Climate.gov, Barnston explains these mixed signals in more depth.

If another El Niño does develop this year, it will be only the second time for a back-to-back El Niño-La Niña-El Niño sequence in our best recorded history (i.e. since 1950). This could mean that some areas that experienced climate stress from the recent event could be more vulnerable to extremes from the upcoming event. But, as expressed by IRI’s Daniel Edward Osgood in the New York Times last week, the memory and experience from the recent event could help inform and encourage decisions based on the forecasts for this El Niño. “It’s about getting organized so you’re not caught unprepared,” he said.

For the latest on ENSO and our seasonal forecasts, tune in to #IRIforecast on Twitter at 2PM* Eastern Time on the third Thursday of each month, including this Thursday, April 20.

More of IRI in El Niño news:

Government forecaster sees chance of El Niño later this year (The Weather Network)
El Niño may develop in late 2017 and could impact hurricane season (The Weather Channel)

*Not a typo! Out briefings have previously been at 11AM, but they will now be at 2PM.