@ClimateSociety: Weak La Niña Replaced With Neutral ENSO, and Uncertainty

Here’s the release from the International Research Institute for Climate and Society (Elisabeth Gawthrop):

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.

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.

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.
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.
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.
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.

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.

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.

From NOAA (Emily Becker):

Well, that was quick! The ocean surface in the tropical Pacific is close to average for this time of year, putting an end to La Niña, and forecasters expect that it will hover around average for a few months. Let’s dig in to what happened during January, and what the forecast looks like.

Not with a bang

This La Niña wasn’t exactly one for the record books. Our primary index, the three-month-average sea surface temperatures in the central Pacific Niño3.4 region, only dipped to about 0.8°C cooler than the long-term average during the fall of 2016. However, these cooler-than-average temperatures persisted for several months, and the atmosphere over the tropical Pacific responded as expected to the cooler waters. Namely, during the fall and winter to date, the Walker Circulation was strengthened: stronger near-surface east-to-west trade winds, stronger upper-level west-to-east winds, more rain than usual over Indonesia, and less rain over the central Pacific.

Monthly sea surface temperature in the central tropical Pacific Niño 3.4 region, from OISST.v2 temperature data. Data shown is the difference from the 1981-2010 average. Climate.gov graph from CPC data.
Monthly sea surface temperature in the central tropical Pacific Niño 3.4 region, from OISST.v2 temperature data. Data shown is the difference from the 1981-2010 average. Climate.gov graph from CPC data.

During January, the sea surface temperature edged close to normal, and the average temperature in the Niño3.4 region was just about 0.3°C below normal by the end of the month. (Note, this is using the weekly OISST data. There are some differences between our sea surface temperature data sets, which Tom described in detail here.)

Another factor that we watch is the temperature of the tropical Pacific Ocean below the surface. Over the past few months, the amount of cooler-than-average water at depth has been decreasing, and by the end of January it had disappeared. These deeper waters often give an idea of what we can expect at the surface in following months. Meaning, the lack of cooler water at depth makes it unlikely that the surface will cool off again substantially in the next few months

Monthly average heat content in the upper 300m of the equatorial Pacific. Data shown is the difference from the 1981-2010 average between 180°W and 100°W. Climate.gov graphic from CPC data.
Monthly average heat content in the upper 300m of the equatorial Pacific. Data shown is the difference from the 1981-2010 average between 180°W and 100°W. Climate.gov graphic from CPC data.

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