December Climate Briefing: La Niña Lingers, Likely to Lapse

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

Tony Barnston provides an overview of the briefing

Since last month’s briefing, weekly sea-surface temperature anomalies in the area of the central equatorial Pacific Ocean that define El Niño and La Niña events, called the Nino3.4 region, have remained within a tenth of a degree of the -0.5ºC threshold indicative of La Niña (see first two images in gallery).

While these sea-surface temperatures (SSTs) point to a borderline La Niña event, the coupled nature of La Niña events means that both the ocean and atmosphere must be “participating.” The strongest indication that the atmosphere is also active in this event is the convection pattern that has lasted in the equatorial Pacific for the last several months (see third image in gallery). This is also the component that is most likely to influence precipitation patterns around the world. Other atmospheric indicators of La Niña, such as stronger-than-normal trade winds and positive values of the Southern Oscillation Index, have been less consistent.

As we have been reporting since September, some La Niña-like impacts are expected to occur regardless of whether a La Niña is officially declared; the -0.5ºC threshold is a guideline for impacts, but not an absolute. The La Niña advisory issued last month by the  National Oceanic and Atmospheric Administration’s Climate Prediction Center is still in effect.

Weekly sea-surface temperature anomalies in the Nino3.4 region of the central equatorial Pacific Ocean. For reasons outlined in this NOAA blog post, comparison of SSTs between datasets should be cautious, largely due to variation in data collection methods and resolution of the datasets. The dataset used for this figure (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. Image: IRI/Elisabeth Gawthrop. Data: NOAA

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.

To predict El Niño, 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 dynamical and statistical models calls for SST conditions to gradually warm. Similar to the forecasts of the last two months, these models call for SST anomalies to reach 0ºC in the March-April-May season.

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-January 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 quickly decline beginning with the current December-January-February season, with neutral conditions dominating in the new year (see second graph in gallery above).

La Niña in context: Resource page on climate variability

The official probabilistic forecast issued by CPC and IRI in early December shows comparatively higher odds for La Niña conditions to remain for the few months. This early-December forecast uses human judgement in addition to model output, while the mid-December 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.

Please refer to our licensing agreement for permission to use any IRI forecast material.

Please refer to our licensing agreement for permission to use any IRI forecast material.

Please refer to our licensing agreement for permission to use any IRI forecast material.

Please refer to our licensing agreement for permission to use any IRI forecast material.

For the upcoming January-March period, the forecast shows a moderate to strong likelihood of drier-than-normal conditions over the southern United States and northern Caribbean (first image in gallery above, click to enlarge).

The January-March forecast also shows an enhanced chance of above-average precipitation in some areas of Indonesia and the Philippines. Other regions with an elevated chance of above-average precipitation include northern South America, southeastern Asia and portions of the northern United States. 

The impacts listed above are specifically for the January-March 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.