Back in April of this year NOAA added the 2014/15 El Niño to their Oceanic NINO Index (a.k.a. ONI). See the former version of ONI here. Last week, with NOAA’s switch to their new Extended Reconstructed Sea Surface Temperature dataset version 4 (ERSST.v4), the 2014/15 El Niño has now disappeared from their list of “official” El Niño and La Niña events. See the present (ERSST.v4) version of ONI here.
But the 2014/15 El Niño isn’t the only ENSO event to have disappeared from ONI with the change to the new ERSST.v4 dataset. The 2005/06 La Niña also dropped off ONI, and so did the 1983/84 La Niña. On the other hand, the 1967/68 La Niña and the 1979/80 El Niño became official ENSO events with the new ERSST.v4 data, where they weren’t qualified with the ERSST.v3b data.
The weaker, short-term El Niño and La Niña events (based on NINO3.4 region surface temperatures) appear and disappear from the NOAA Oceanic NINO Index with each revision and with changes in how anomalies are calculated for ONI. Tables 1 and 2 include the 5 versions of NOAA’s Oceanic NINO Index that have existed over the past decade or so. (Click on them for larger versions.) The ERSST.v3b data with the base years of 1971-2000 for anomalies are included on both tables. The older versions of the Oceanic NINO Index are available through the Wayback Machine archives.
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On Table 2, you’ll note that there are two Ocean NINO Indices using the ERSST.v3b data. The left-hand ONI uses the base years of 1971-2000 for sea surface temperature anomalies, while the middle (and right-hand, ERSST.v4-based) ONI uses multiple climatologies that shift every 5 years. Yes, that means that NOAA shifts the base years for anomalies every five years for ONI, meaning the anomalies are not referenced to a common base period.
NOAA discusses the reasoning behind this in their Description of Changes to Oceanic NINO Index (ONI) webpage, which was revised for the new ERSST.v4 data. The first two paragraphs there read (my boldface):
Due to a significant warming trend in the Niño-3.4 region since 1950, El Niño and La Niña episodes that are defined by a single fixed 30-year base period (e.g. 1971-2000) are increasingly incorporating longer-term trends that do not reflect interannual ENSO variability. In order to remove this warming trend, CPC is adopting a new strategy to update the base period.
There will be multiple centered 30-year base periods that will be used to define the Oceanic Niño index (as a departure from average or “anomaly”). These 30-year base periods will be used to calculate the anomalies for successive 5-year periods in the historical record…
We discussed the flaw in their assumption that there had been a “significant warming trend in the Niño-3.4 region since 1950” in the June 2012 post Comments on NOAA’s Recent Changes to the Oceanic NINO Index (ONI). In the real world, the “significant warming trend in the Niño-3.4 region since 1950” was a response to the 1976/77 Pacific Climate Shift, not some anthropogenic warming signal. So NOAA deleted the effect of the 1976/77 Pacific shift with the change in how they calculate anomalies for ONI.
The Oceanic NINO Index has been referenced in numerous climate studies. How and if these multiple revisions have impacted those studies is for the authors to determine. Sometimes, even after June 2012 when NOAA revised their method for calculating ONI, authors relied on the older version of the Oceanic NINO Index with the fixed base years of 1971-2000…like Trenberth et al. (2014) Earth’s Energy Imbalance. In other words, even the climate science community appears not to have bought into using multiple base periods for an ENSO index.
And just in case you’re wondering, the 2014/15 El Niño would appear on ONI if NOAA had continued to use 1971-2000 for the base years with the new ERSST.v4 data. Then again, NOAA has switched to the base years of 1981-2010 for many indices, so the 2014/15 El Niño would not exist in ONI with those base years using the new ERSST.v4 data.
The NOAA Oceanic NINO Index is used by some persons when they attempt to make claims about the causes of record-warm surface temperatures. So expect there to be some alarmist nonsense about the absence of an El Niño in the 2014/15 season from the standard sources, including NOAA.
We discussed the reasons for the reported record warm global sea surfaces in the post Did ENSO and the “Monster” Kelvin Wave Contribute to the Record High Global Sea Surface Temperatures in 2014? And as I’ll show in an upcoming post, the 2014/15 El Niño was stronger than most El Niño events during the satellite era if we look at the tropical Pacific as a whole.