February Climate Briefing: La Niña Pattern Holds

From IRI:

What’s New
Since last month’s briefing, sea-surface temperature (SST) anomalies have cooled slightly in the area of the central equatorial Pacific Ocean that helps define El Niño and La Niña events, called the Nino3.4 region (see first map below). The average of these weekly anomalies for the last month was nearly -0.9ºC, which falls in the category of a weak, bordering moderate, La Niña state. For the previous month, the Nino3.4 anomaly was -0.7ºC.

Sub-surface ocean temperatures were on a warming trend, although that has now been somewhat tempered (see second figure below). These sub-surface conditions are often a preview of what’s to come at the surface, so it’s likely that La Niña conditions at the surface will diminish in the next couple of months.

Making a quick end to La Niña even more likely is a recently burst of westerly winds in the western Pacific, likely related to a Madden-Julian Oscillation event. La Niña is sustained in part by stronger-than-normal trade winds that blow from east to west, so these bursts of winds from the west can help break down a La Niña event. The wind event can be seen in the bottom left corner of the leftmost panel in the third image below. Barnston said what sets apart this wind event from another one a few months ago is the eastern extent of it — a few months ago the event was confined to the western Pacific.

Because IRI and the National Oceanic and Atmospheric Administration’s Climate Prediction Center predict that La Niña will continue for now, the ENSO alert level remains at a La Niña Advisory, which was initially issued in November.

ENSO Forecasts
To predict ENSO conditions, computers model the SSTs in the Nino3.4 region over the next several months. The plume graph 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 average of the SST anomalies predicted by the models in this month’s forecast are similar to the predictions in last month’s forecast. The means of both the dynamical and statistical models show gradual warming, with both crossing into neutral ENSO territory by the March-May season. The dynamical models warm more quickly and eventually reach a mean anomaly of just over +0.5ºC in the October-December season. The statistical models’ mean anomaly only reaches about +0.2ºC, similar to last month’s prediction. This period at the end of the forecast, however, is past the spring predictability barrier and is highly uncertain.

Based on the model outputs, odds for La Niña conditions are only slightly above the odds for neutral ENSO in the the current February-April season, with both around 50%. Neutral conditions then take over as the most likely ENSO state through most of the northern hemisphere summer. By the August to October season, El Niño chances become (barely) the most likely, but, again, uncertainty is high.

The official probabilistic forecast issued by CPC and IRI in early February indicates similar overall trends in ENSO probabilities, but with less likelihood of El Niño in the later months. This early-February forecast uses human judgement in addition to model output, while the mid-month forecast relies solely on model output.

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