NOAA’s Ever-Changing Definition of La Niña Years

UPDATE: I’ve added two illustrations to the end of the post.  At the request of blogger “Kurt in Switzerland” on the cross post at WattsUpWithThat, I plotted a comparison of the new and old versions of the Oceanic NINO Index data and posted it in a comment. I also plotted the difference, which is much more interesting. Both graphs have been added at the end of the post as suggested by blogger Gary Pearse.


There have been two changes in NOAA’s Oceanic NINO Index over the past year—not one—in addition to their monthly updates.

NOAA redefined their Oceanic NINO Index (ONI) last year. Prior to the change, they had used their ERSST.v3b-based NINO3.4 sea surface temperature anomalies with their standard base years 1971-2000. For the Oceanic NINO Index, the monthly NINO3.4 anomaly data is then smoothed with a 3-month filter and rounded to the closest 0.1 deg C value. The last few years of old version of the Oceanic NINO index are shown in Figure 1.

01 ONI Old

Figure 1

When they changed the Oceanic NINO Index last year, NOAA used a series of sliding base years to determine the anomalies. Their logic for the change was that sea surface temperatures in the NINO3.4 region were being impacted by human-induced global warming. That’s nonsense. As discussed in the post Comments on NOAA’s Recent Changes to the Oceanic NINO Index (ONI), NOAA failed to recognize that the increase in their NINO3.4 sea surface temperatures occurred as a result of the 1976 Climate Shift, and that NINO3.4 sea surface temperature anomalies for the periods Jan 1950-December 1975 and January 1977 to present both had negative trends. That, of course, indicated that the 1976 Climate Shift was the cause of the long-term warming in NINO3.4 region sea surface temperatures—not manmade greenhouse gases.

The January 4, 2013 version of the Oceanic NINO Index is shown in Figure 2.

02 ONI New 1-4-13

Figure 2

I’ve highlighted 2006 to show that the first three months of 2006 are now considered La Niña months, where with the old version they were not. Why is that important? NOAA defines whether a given year is an El Niño year or La Niña year based on the first 3 months of their Ocean Nino Index. The logic is, it takes 5 to 6 months for global surface temperatures to respond to El Niños. Refer to the opening paragraph of the Introduction to Trenberth et al (2002). They write:

Following an El Niño the global surface air temperature typically warms up by perhaps 0.1 deg C with a lag of ~6 months [Newell and Weare, 1976; Pan and Oort, 1983; Jones, 1989; Wigley, 2000]. In an exceptional event such as the 1997–1998 El Niño the amount exceeds 0.2 deg C. Christy and McNider [1994] and Angell [2000] show that the entire troposphere warms up with an overall lag of 5–6 months, but the lag is slightly less in the tropics and is greater at higher latitudes. Consequently, the empirical evidence suggests a strong diabatic component to El Niño–Southern Oscillation (ENSO).

NOAA then uses the definition of El Niño and La Niña years to determine if the most recent year was the warmest El Niño or La Niña year—as if that’s meaningful, especially when one considers their ocean heat content data and satellite-era sea surface temperature data both indicate the oceans warmed naturally.

The recent changes in their definition of El Niño and La Niña years caused some confusion at NOAA. They mistakenly identified 2012 as the warmest La Niña year in their 2012 State of the Climate Report. Refer to the post NOAA’s Definition and Data Contradict Their Claim That 2012 Was The Warmest La Niña Year.

A month or so later, NOAA then corrected their mistake. Refer to the post NOAA Corrects Their 2012 State of the Climate Report – 2012 Was NOT the Warmest La Niña Year on Record. In their correction, NOAA stated (my boldface):

In the most recent version of the dataset, using the newer base period methodology, 2006 and 2009 are now classified as La Niña years.

As you’ll note in Figure 2, the first 3 months of 2009 are not identified as La Niña months, and, therefore, according to the NOAA definition, 2009 as a year would not be considered a La Niña year. How can they now be saying it’s a La Niña year?

Between January 21, 2013—when I noted the error—and February 2013—when NOAA corrected the State of the Climate Report—NOAA had once again changed their Oceanic NINO Index. See Figure 3. Now 2009 is a La Niña year, based on the first 3 months of 2009 in the latest version of the Oceanic NINO Index.

03 ONI New 3-4-13

Figure 3

The Oceanic NINO Index didn’t change during the first 3 months of 2009—it was a change in the November 2008 value that caused the reclassification to a La Niña for that season. Recall, according to the NOAA definition, the Oceanic NINO Index value has to be at –0.5 deg C or cooler for five consecutive months (or what they call seasons since they’re looking at 3-month averages) for there to be an “official” La Niña.


If you’d like to verify the recent changes in the Oceanic NINO index, use the following http address…

…in the Wayback Machine. You’ll find that the recent changes impacted Oceanic NINO Index values over the entire term of the data.


First NOAA redefined how sea surface temperature anomalies should be calculated to account for a fictitious global warming signal in the NINO3.4 region. Then they conveniently forgot about their redefinition of La Niña years in the 2012 State of the Climate Report—then corrected the mistake long after the mainstream media has moved on to other ways to misinform the public about human-induced global warming. Now NOAA can’t seem to figure out how to determine El Niño and La Niña events based on their new definition.

If NOAA wants to stop confusing themselves, maybe they should switch back to the old version and forget this nonsensical new version of their Oceanic NINO Index.


As noted above, NOAA’s ocean heat content data and their satellite-era sea surface temperature data indicate the oceans warmed naturally, primarily in response to El Niño and La Niña events. For further information, refer to my illustrated essay “The Manmade Global Warming Challenge” (42MB), and to the two-part YouTube video series “The Natural Warming of the Global Oceans” (Part 1 and Part 2), and to my ebook Who Turned on the Heat? which is available in pdf form here for US$8.00.


Figure 4 is a comparison of the old and new versions of the NOAA Oceanic NINO Index.

Fgure 4 ONI Comparison

Figure 4

The difference between the old and new versions, Figure 5, is more informative. In my comment at WUWT, I wrote, After a long-term decrease, there’s a sudden shift in 1990. Then the difference plateaus (increases slightly) over the last 2 decades.

Figure 5 ONI Difference

Figure 5

In reality, before 1980 and working back in time, a gradually increasing upward bias (toward El Niño conditions) was added. Then there was a sudden downward bias (toward La Niña conditions) in 1990. The bias has been returning very slowly toward zero since then.

NOAA went to all that trouble to eliminate the effect of the 1976 climate shift, as discussed and illustrated in Comments on NOAA’s Recent Changes to the Oceanic NINO Index (ONI). In the end, NOAA has created a meaningless index.


About Bob Tisdale

Research interest: the long-term aftereffects of El Niño and La Nina events on global sea surface temperature and ocean heat content. Author of the ebook Who Turned on the Heat? and regular contributor at WattsUpWithThat.
This entry was posted in El Nino-La Nina Processes. Bookmark the permalink.

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