May Climate Briefing: Signs Point to La Niña

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

Tony Barnston provides an overview of the briefing

The El Niño event declared over a year ago is in its last weeks, with odds for at least a weak La Niña to develop by late summer are pegged at more than 50%. Sea-surface temperatures in the equatorial Pacific Ocean are continuing their cooling trend of the last several months. SSTs in the region of the Pacific Ocean that define El Niño and La Niña events, called the Nino3.4 region, are still just above the threshold of El Niño, but are expected to be in the ENSO neutral category soon. The El Niño advisory and La Niña watch issued by the National Oceanic and Atmospheric Administration’s Climate Prediction Center are both still in effect. More on that, as well as past data of La Niña events following El Niño events, in last month’s climate briefing summary.

Changes from last month’s briefing

In April, the monthly average sea-surface temperatures in the Nino3.4 region dipped below the 1.5ºC mark that scientists consider the threshold of a “strong” El Niño  for the first time since July 2015. At 1.09ºC above average, the temperature was still well above the +0.5ºC threshold used to define El Niño conditions. In the first two weeks of May, the weekly-averaged sea-surface temperatures have dropped further and now hover just above the 0.5º threshold. See the latest SST measurements in the first image below.  [Note: For many reasons outlined in this NOAA blog post, comparison of SSTs between datasets (and sometimes even within the same dataset!) should be cautious, largely due to variation in data collection methods and resolution of the datasets. Further, the dataset used for the above statistics (called OISSTv2) is not the one used in NOAA’s records for official peak strength (called ERSSTv4), though OISSTv2 is the one used in initializing models.] 

The sea-surface temperatures in the Nino3.4 region (approximated here) serve as a primary metric of El Niño 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-May 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 and the spread of the model outputs are similar to that of last month’s forecast. Both the dynamical and statistical models call for the El Niño event to continue to weaken and for ENSO neutral conditions to prevail in the May-June-July season. The dynamical models then call for a La Niña state by the following season of June-July-August, while the statistical models are still in the warm-neutral category during the JJA season. For the later months of 2016, dynamical models predict a weak-to-moderate La Niña state; the statistical models don’t reach the La Niña threshold, but stay in the cool-neutral category through the end of the year. 

Chances of an El Niño event for the upcoming May-July season have plummeted to less than 5% (see third image), down from 75% for last month’s April-June forecast. Neutral conditions are most likely in the immediate future, but La Niña takes over as the most likely scenario (just over 50% chance) for the July-September season. For the final months of 2016, the odds of La Niña conditions are around 60%.

The probabilistic forecast issued by CPC and IRI in early May shows somewhat higher odds for La Niña conditions. This early-May forecast uses human judgement in addition to model output, while the mid-May forecast relies solely on model output. Forecasters think the statistical models may be underestimating the likelihood for La Niña conditions and thus increased the odds for La Niña in the early-May forecast.

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.

June July August Forecast 2016

July August September Forecast 2016

August September October Forecast 2016

September October November Forecast 2016

For the upcoming June-August period, the forecast shows some likelihood of drier-than-normal conditions over areas of central South America (first image in gallery above, click to enlarge). For the first time since 2013, the forecast shows an elevated chance of above-average precipitation in  Indonesia. Eastern Australia as well as parts of North America and the Caribbean show a slightly elevated chance of above-average precipitation. 

El Niño in context: Resource page on climate variability

The impacts listed above are specifically for the June-August season. For the later seasons, 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.