February Climate Briefing: Weak La Niña Replaced With Neutral ENSO, and Uncertainty

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

Tony Barnston provides an overview of the briefing

Since last month’s briefing, sea-surface temperatures have warmed in the area of the central equatorial Pacific Ocean that define El Niño and La Niña events, called the Nino3.4 region. Last week, the weekly anomaly for Nino3.4 was +0.1ºC — the first time it’s been above 0.0ºC since June. The first image below shows the latest week’s anomalies.

While the sea-surface temperatures (SSTs) point to a neutral ENSO state, the convection patterns in the equatorial Pacific (i.e. at what longitudes along the equator clouds and thunderstorms form) continue to show a La Niña-like pattern. Although this pattern is what is most likely to in turn influence precipitation patterns around the world, it is expected to weaken or disappear during the remainder of February and early March.

The upcoming seasonal forecasts are not showing much in the way of La Niña influence. “The models are expecting the convection patterns to return to neutral very soon, such as within the coming few weeks,” said Barnston. “The March-May climate is not expected to be materially influenced by the current cloudiness conditions.”  The National Oceanic and Atmospheric Administration’s Climate Prediction Center issued a Final La Niña Advisory last week.

The sea-surface temperatures in the Nino3.4 region (approximated here) serve as a primary metric of El Niño and La Niña conditions. Data from the IRI Data Library. Image: IRI/Elisabeth Gawthrop

Outgoing longwave radiation (OLR) anomalies averaged over 5ºS-5ºN are shown in blue (indicating above-average convection) and orange (indicating below-normal convection) shades. On the x-axis is longitude and on the y-axis is time. A pattern of above-average convection activity (i.e. more thunderstorms than normal) along the equator in the western Pacific and below-average convection activity along the equator in the region of the International Dateline (180º) is indicative of La Niña conditions and is the primary initiator of rainfall effects from La Niña globally. That pattern can be seen here since around July/August 2016. Image: NOAA/CPC.

ENSO Forecasts

To predict ENSO conditions, computers model the SSTs in the Nino3.4 region over the next several months. The graph in the first image of the gallery below 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 mean of the statistical models’ forecast is similar to that of last month, with Nino3.4 SSTs staying around 0ºC or just above through the end of the year. The mean of the dynamical models’ forecast, especially later in the year, has increased from last month’s forecast. These dynamical models now call for anomalies around +0.8ºC as the northern hemisphere’s summer comes to a close. Last month’s forecast from the dynamical models didn’t quite reach +0.5ºC. 

These forecasts, however, extend past what’s known as the spring predictability barrier — a function of ocean dynamics that makes it hard to predict ENSO past June of each year, so uncertainty is high.

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-February 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, 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.

Based on these model outputs, odds for La Niña are close to zero for the next several seasons, with neutral conditions dominating (see second graph in gallery above). The warmer SSTs shown in the plume graph, especially in the dynamical models, are reflected in the increasing likelihood for El Niño conditions later in the year.

ENSO in context: Resource page on climate variability

The official probabilistic forecast issued by CPC and IRI in early February shows a similar overall pattern. This early-February forecast uses human judgement in addition to model output, while the mid-February forecast relies solely on model output.

Effects of La Niña 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 seasons, the forecast shows little signal from ENSO, as would be expected given the neutral ENSO forecast. The forecast maps are available on our seasonal forecast page.

Learn more about El Niño and 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.