September Climate Briefing: More Confidence than Ever

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

Tony Barnston provides an overview of the briefing

Changes from last month’s briefing

An El Niño event is now in full swing, with probabilities of its continuance similar to those forecasted last month and remaining at close to 100% through the first few months of 2016 (bottom image).

While these probability calculations are based only on the chance of at least weak El Niño conditions persisting, the El Niño is squarely in the “strong” category, and forecasters expect it to remain strong for the next several months. Barnston says it will be competitive for the three strongest events since 1950, but he is uncertain whether it will top the strongest (1997-1998) event.

The IRI/CPC probabilistic ENSO forecast issued mid-September 2015. 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.

Sea-surface temperatures in the region that defines El Niño, called Nino3.4 (see image), have continued to warm over the last month. Last week, sea surface temperatures (SSTs) in the Nino3.4 region were 2.3ºC above average (see image, top left). The SST anomaly for the month of August was 2.06ºC (July was 1.6ºC).

To predict El Niño, computers model the SSTs in the Nino3.4 region over the next several months. The graph in the top right image 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 used statistics, based on the long record of historical observations.

The mean of both statistical and dynamical models call for an event peaking above the +1.5ºC “strong” El Niño threshold. The average outlook of the dynamical models is similar to last month’s model run, with its maximum strength nearly reaching the 2.5ºC mark in the October-December season. The average of the statistical models has increased by a few tenths of a degree since last month, with strength peaking at ~2.2ºC in the November-January season. While the variation among the model outputs is significant, all of the models call for at least a weak El Niño through the end of the year.

The El Niño advisory issued in March by the National Oceanic and Atmospheric Administration’s Climate Prediction Center and IRI is still in effect.

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 ENSO SST projections and indicate which areas are more likely to see above or below normal temperatures and rainfall.

“The seasonal forecasts we’re issuing this month have the strongest probabilities we’ve seen in some areas since we began issuing these forecasts,” said Barnston.

For the October-December period, the forecast shows a strong likelihood of drier-than-normal conditions over broad areas of northern South America, the Caribbean, Indonesia and the Philippines (image above). There are also increased odds for drier-than-normal conditions in western Africa, parts of the northern US and southern Canda, southeast Asia, west Asia, eastern Russia, central South America, Australia and New Zealand. Some of these regions are adjacent to areas very likely to have wetter-than-average conditions, such as southern South America and central-southwest Asia. Eastern-equatorial Africa is also very likely to see above-average rainfall. The southern US, central India, parts of eastern Asia and southern Alaska have moderately increased chances of above-average precipitation.

Barnston noted that the teleconnection in central-southwest Asia was originally discovered by scientists at IRI in the early 2000s because of a relationship between La Niña and drought. Many of the maps currently in circulation that show typical El Niño climate impacts are based on earlier studies, and they don’t include this teleconnection. Access the most recent IRI teleconnection map here.

The impacts listed above are specifically for the October-December season. Some of these impacts are predicted to persist in the following seasons, but some areas, including southern Africa and Peru, see different impacts in the other seasonal windows. See forecast maps in the image gallery and on our seasonal forecast page.

Learn more about El Niño 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 El Niño questions.

Forecast in Brief

Download a PDF summary of the September IRI Climate Briefing