Thursday, October 27, 2016

Dude, Where's La Niña?

A quick note on terminology for normal people who don't obsess about this stuff:
  • ENSO = the El Niño/Southern Oscillation
  • ENSO Neutral = the ocean surface temperature anomaly in the ENSO 3.4 region is between +0.5°C and -0.5°C.
  • El Niño/La Niña conditions exist when the anomaly is greater than (Niño) or less than (Niña) the half-degree cut-off for neutral.
  • A full blown El Niño/La Niña is declared when the conditions persist for three overlapping three-month periods i.e. five consecutive months.
And that's the point of this post, that although we'll get to La Niña conditions, which will have some weather effects, and may even become a (weakening) La Niña, it is really starting to look as though the conditions won't stick around long enough to become the monster La Niña we were looking for back in March.

First up, from NOAA's Climate Prediction Center, Oct. 13:

Synopsis:  La Niña is favored to develop (~70% chance) during the Northern Hemisphere fall 2016 and slightly favored to persist (~55% chance) during winter 2016-17.

ENSO-Neutral conditions were observed during September, with negative sea surface temperatures (SSTs) anomalies expanding across the eastern equatorial Pacific Ocean by early October (Fig. 1). All of the Niño regions cooled considerably during late September and early October, with the latest weekly value of Niño-3.4 index at -0.9°C (Fig. 2). Subsurface temperature anomalies also decreased toward the end of the month (Fig. 3), reflecting the strengthening of below-average temperatures at depth in the east-central equatorial Pacific (Fig. 4). Atmospheric anomalies across the equatorial Pacific edged toward La Niña during September, with a stronger tendency toward La Niña late in the month. The traditional Southern Oscillation index and the equatorial Southern Oscillation index were positive. The lower-level winds were near average across most of the basin during the month, but enhanced easterlies were becoming more persistent west of the International Date Line. Upper-level winds were anomalously westerly near and just east of the International Date Line. Convection was weakly suppressed over the central tropical Pacific and was more enhanced over Indonesia compared to last month (Fig. 5). Overall, the combined ocean and atmosphere system reflects ENSO-Neutral during September, but are more clearly trending toward La Niña conditions. 
The multi-model averages favor borderline Neutral-La Niña conditions (3-month average Niño-3.4 index less than or equal to -0.5°C) persisting during the Northern Hemisphere fall and continuing into the winter (Figs. 6 and 7). Because of the recent cooling in the Niño-3.4 region and signs of renewed atmospheric coupling, the forecaster consensus now favors the formation of a weak La Niña in the near term, becoming less confident that La Niña will persist through the winter. In summary, La Niña is favored to develop (~70% chance) during the Northern Hemisphere fall 2016 and slightly favored to persist (~55%% chance) during winter 2016-17 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).period)....

And from Columbia University/IRI, the plume of model predictions, published Oct. 20: