August Climate Briefing: Fate of La Niña Up in the Air

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

Tony Barnston provides an overview of the briefing

Conditions 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. While ocean temperatures are indicating a weak La Niña event could be imminent, atmospheric conditions consistent with La Niña (i.e. stronger-than-average trade winds) are not yet present or are only weak. Weekly sea-surface temperature anomalies (see first image below) have been around the -0.5º threshold used to define La Niña for the last month, but without the requisite trade wind and tropical rainfall conditions there will not be an upgrade made in the ENSO alert system.  The La Niña watch issued by the National Oceanic and Atmospheric Administration’s Climate Prediction Center in June is still in effect. 

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-August 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 for the entire forecast period. Most 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 similar to last month’s, with odds topping out at just under 60% for the November-December-January period (see third graph in gallery above).

The probabilistic forecast issued by CPC and IRI in early August shows similar odds for La Niña conditions. This early-August forecast uses human judgement in addition to model output, while the mid-August 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 September-November period, the forecast shows some likelihood of drier-than-normal conditions over areas of the southeastern and interior western United States, as well as some coastal areas of West Africa (first image in gallery above, click to enlarge). The forecast also shows a slightly elevated chance of above-average precipitation in Indonesia. 

El Niño in context: Resource page on climate variability

The impacts listed above are specifically for the September-November season. In later seasons, other impacts associated with La Niña appear; 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.