September Climate Briefing: No Niña, But Some Impacts Expected

Read our ENSO Essentials & Impacts pages for more about El Niño.

Tony Barnston provides an overview of the briefing

Sea-surface temperatures in the area of the central equatorial Pacific Ocean that define El Niño and La Niña events, called the Nino3.4 region, remain similar to those from last month’s briefing. Since July, weekly sea-surface temperature anomalies (see first image below) have been around the -0.5º threshold used to define La Niña.

Due to the lack of stronger-than-average trade winds, however, it’s less likely that the atmospheric component needed to sustain La Niña conditions will materialize. Models are also reflecting this, with forecasts indicating less likelihood for La Niña conditions through the remainder of the year and into 2017.  The National Oceanic and Atmospheric Administration’s Climate Prediction Center has therefore canceled the La Niña watch it issued in June. 

This does not mean that a La Niña event is out of the question, nor does it mean that the borderline La Niña conditions will have no effect on global climate patterns. “The system of ENSO and its impacts works on a continuum,” said Tony Barnston, IRI’s Chief Forecaster. “A weak La Niña is expected to bring weaker impacts, on average, than a moderate one. And not all regions that normally get impacts are guaranteed to get them in every moderate La Nina, so this is even more the case for a weak event.”

Barnston also said that the difference between a weak event (averaging around a -0.75ºC SST anomaly) and a borderline one (averaging -0.5ºC anomaly) is fairly small. Still, a borderline event is expected to have, on average, weaker impacts, since -0.5 is weaker than -0.75. “Our forecasts are based on SSTs that are not quite borderline La Niña in strength, but the hints of weak impacts are still seen in our rainfall forecasts,” Barnston said.

The sea-surface temperatures in the Nino3.4 region (approximated here) serve as a primary metric of El Niño and La Niña conditions.

This graph shows forecasts made by dynamical and statistical models for SST in the Nino 3.4 region for nine overlapping 3-month periods. Note that the expected skills of the models, based on historical performance, are not equal to one another.

The IRI/CPC probabilistic ENSO forecast issued mid-September 2016. 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, IRI and CPC issue this updated forecast based solely on model outputs. The official forecast, available at http://1.usa.gov/1j9gA8b, also incorporates human judgement.

To predict El Niño, computers model the SSTs in the Nino3.4 region over the next several months. The graph in the second image of the gallery shows the outputs of these models, some of which use equations based on our physical understanding of the system (called dynamical models), and some of which use statistics, based on the long record of historical observations.

The means of both the dynamical and statistical models mean call for SST conditions to continue in the -0.3ºC to -0.7ºC range through the end of 2016. Unlike last month’s forecast, however, the models then show a weakening of the borderline La Niña conditions and a return to neutral conditions. All of the models are within 1.5ºC of each other. 

Based on these model outputs, chances for La Niña development in the remainder of 2016 are down slightly from last month’s, with odds topping out at 55%. That figure is for the current September-October-November season, with odds for La Niña declining in the following seasons (see third graph in gallery above).

The probabilistic forecast issued by CPC and IRI in early September shows lower odds for La Niña conditions, with neutral conditions as the most likely for the entire forecast period. This early-September forecast uses human judgement in addition to model output, while the mid-September forecast relies solely on model output.

Effects of El Niño on global seasonal forecasts

Each month, IRI issues seasonal climate forecasts for the entire globe. These forecasts take into account the latest sea-surface temperature projections and indicate which areas are more likely to see above- or below-normal temperatures and rainfall.

For the upcoming October-December period, the forecast shows moderate likelihood of drier-than-normal conditions over the southeastern United States (first image in gallery above, click to enlarge). There is also some likelihood of drier-than-normal conditions in southeast South America, eastern equatorial Africa and central southwest Asia. The forecast also shows an elevated chance of above-average precipitation in southeast Asia, especially in Indonesia and the Philippines, as well as parts of northern Australia. 

El Niño in context: Resource page on climate variability

The impacts listed above are specifically for the October-December season. For later seasons, see forecast maps in the image gallery and on our seasonal forecast page.

Learn more about La Niña on our ENSO resources page, and sign up here to get notified when the next forecast is issued. In the meantime, check out #IRIforecast or use #ENSOQandA on Twitter to ask your La Niña questions.