Guest Post by Bob Tisdale
La Niña conditions are typically defined by NOAA as sea surface temperature anomalies less than or equal to -0.5 deg C for the NINO3.4 region of the east/central equatorial Pacific. The NINO3.4 sea surface temperature anomaly for the week of August 31, 2016 from NOAA’s Monthly Atmospheric and SST Indices webpage (data here) is -0.7 deg C, well into weak La Niña conditions.
Regardless of the existing (and strengthening) La Niña conditions, NOAA has canceled its La Niña Watch, which had been in effect since April. According to the NOAA ENSO Diagnostic Discussion, updated September 8, the “forecaster consensus” is now favoring ENSO-neutral conditions:
ENSO Alert System Status: Not Active
Synopsis: ENSO-Neutral conditions are slightly favored (between 55-60%) during the upcoming Northern Hemisphere fall and winter 2016-17.
ENSO-Neutral conditions were observed over the past month, although sea surface temperatures (SSTs) were below-average over the east-central equatorial Pacific Ocean (Fig. 1). While the Niño-3.4 and Niño-3 regions remained around -0.5°C for most of the month, Niño-4 and Niño 1+2 were -0.1°C and +0.3°C, respectively, by the end of the month (Fig. 2). Subsurface temperatures across the eastern and central Pacific remained below average (Fig. 3), and negative temperature anomalies remained weak across the western Pacific (Fig. 4). Atmospheric anomalies over the tropical Pacific Ocean largely indicated ENSO-Neutral conditions. The traditional Southern Oscillation index and the equatorial Southern Oscillation index were weakly positive during August. The lower-level winds were near average, while the upper-level winds were anomalously westerly in a small region to the east of the International Date Line. Convection was suppressed over the western and central tropical Pacific, although less suppressed compared to last month (Fig. 5). Overall, the combined ocean and atmosphere system continues to reflect ENSO-Neutral.
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) during the Northern Hemisphere fall, continuing into winter (Fig. 6). However, the more recently updated model runs from the North American Multi-Model Ensemble (NMME) more strongly favor ENSO-Neutral (Fig. 7). The forecaster consensus prefers this outcome, which is supported by the lack of significant anomalies in several indicators over the past month (winds, convection, subsurface temperatures). Overall, ENSO-Neutral conditions are slightly favored (between 55-60%) during the upcoming Northern Hemisphere fall and winter 2016-17 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).
We’ll keep an eye on the tropical Pacific in months to come regardless of NOAA’s “forecaster consensus”.
My full sea surface temperature update for August 2016 is here. The Blob (elevated sea surface temperature anomalies in the eastern extratropical North Pacific) appears to be returning (reemerging).