Guest post by Bob Tisdale
As happens all too often, the press release for a paper has an incorrect title and begins with an unfounded claim. The JPL press release for Lee and McPhaden 2010 “Increasing intensity of El Niño in the central‐equatorial Pacific” is no exception. The title of the press release “NASA/NOAA Study Finds El Niños are Growing Stronger” is wrong. The paper discusses the increase in strength in Central Pacific El Niño events, but does not conclude that El Niño events in general have increased. In fact, as will be illustrated in this post, the strengths of NINO3 and NINO4 based El Niño events, when combined, have actually decreased over the period of the Lee and McPhaden study.
And the press release begins with, “A relatively new type of El Niño…”
New? Central Pacific El Niño events, also known as El Niño Modoki, are not new. Figure 1 is a long-term graph of the El Niño Modoki index, calculated using the method described in Ashok et al (2007) “El Nino Modoki and its Possible Teleconnection.” Link to Ashok et al:
As illustrated, El Niño Modoki events, the wiggles above the red line that marks the threshold of El Niño Modoki, appear throughout the record since 1900. This was discussed and illustrated in the post There Is Nothing New About The El Nino Modoki. The only thing new about Central Pacific El Niño events is the researchers’ new-found interest in them.
The science web pages of newspapers have repeated these bits of misinformation. The New York Times headline of Pacific Hot Spells Shifting as Predicted in Human-Heated World is actually contradicted by the study. Lee and McPhaden note about past studies, “This region has experienced a well‐documented warming tendency for at least a few decades [e.g., Cane et al., 1997; Cravatte et al., 2009], which appears to be consistent with theoretically predicted change of the background SST under global warming scenarios [Cane et al., 1997]. Cravatte et al.  also discussed the implications of warming trend in the warm pool to ocean‐atmosphere interactions and El Niño events. Here we use satellite observations of SST in the past three decades to examine SST in the CP region, distinguishing between the increases in El Niño intensity and changes in background SST.”
But the abstract of Lee and McPhaden reads, “Therefore, the well‐documented warming trend of the warm pool in the CP region is primarily a result of more intense El Niño events rather than a general rise of background SST.” This contradicts the earlier studies with “theoretically predicted change of the background SST under global warming scenarios.”
The Los Angeles Times headline, El Niño has grown more intense and shifted westward in last three decades, data show., is also misleading for the same reason noted above. And they include quotes from Bill Patzert of JPL. “Patzert said the paper was observational rather than conclusive. ‘What will happen if this new type of El Niño becomes permanent? Will it give us wetter or drier El Niños?’ he asked.” [My boldface.]
Again, El Niño Modoki events are not new.
Since there are a multitude of other papers carrying the press release in one form or another, let’s look at the paper itself.
The Los Angeles Times post includes a link to the paper, which was published in Geophysical Research Letters:
Lee and McPhaden use NINO3 and NINO4 SST anomaly data based on Reynolds OI.v2 SST data. The Reynolds OI.v2 dataset can be accessed through the NOAA NOMADS website:
Since Lee and McPhaden used base years other than the NCDC standard climatology, I’ve used the KNMI Climate Explorer for data. It allows users to select the base years. Link to the KNMI Climate Explorer:
Figure 2, below, is Figure 1 from the Lee and McPhaden. It includes the NINO3 and NINO4 SST anomalies plotted separately. Note the difference in the scales.
Figure 2 = Figure 1 from Lee and McPhaden
If plotted together, Figure 3, the magnitudes of the variations in the two NINO SST anomaly subsets are put in perspective. Any increase in NINO4 (Central Pacific) SST anomalies during El Niño events should be easily overcome by the decrease in NINO3 (Eastern Pacific) SST anomalies, as we shall see.
Lee and McPhaden base their “Intensity” analyses on peak readings of El Niño and La Niña events for the NINO4 (Central Pacific) and NINO3 (Eastern Pacific) regions. And the point of the paper is to illustrate that Central Pacific El Niño events are growing in strength. Refer to Figure 4, which is their Figure 3. Note again that the scales are different, and that they do acknowledge this in the text below the graph. But also note the magnitudes and signs of their “Intensity” trends. The NINO4 (Central Pacific) linear trend for El Niño events is said to be 0.20(+/-0.18) deg C/decade. That’s really the whole point of the paper. There has been an increase in the intensity of Central Pacific El Niño events.
Figure 4 = Figure 3 from Lee and McPhaden
The NINO3 (Eastern Pacific) linear trend is said to be 0.39(+/-0.71) deg C/decade, but that has to be a typographical error, since the trend is negative. They appear to be missing the (all-important) minus sign. If the trend value (0.39) is correct, but the minus sign is missing, then the decrease in the intensity of the Eastern Pacific El Niño events is twice that of the increase in Central Pacific intensity. Since the NINO3 region is larger than the NINO4 region, it means the overall intensity of El Niño events for both regions is decreasing.
The linear trend of the “Intensity” of the NINO4 (Central Pacific) La Niña events are shown to be decreasing, which means they’re increasing in strength, but the linear trend of the NINO3 (Eastern Pacific) La Niña event “Intensity” is of the opposite sign and again it’s twice the value. In summary, looking at the graphs presented in Lee and McPhaden, the overall “Intensities” of El Niño and La Niña events appear to be decreasing.
To confirm that, let’s look at NINO3 and NINO4 SST anomalies where the ENSO-neutral data have been deleted. That is, if the positive anomalies were less than 0.5 deg C, the data were deleted, and if they were greater than -0.5 deg C, the negative anomalies were deleted. It’s an awkward looking graph, Figure 5, with all of the mid-range data missing, but it does illustrate the point. The negative linear trend of the El Niño events based on the NINO3 (Eastern Pacific) data is more than twice the positive trend in the NINO4 (Central Pacific) SST anomalies. The opposite holds true for the La Niña events, indicating the Eastern Pacific La Niña events are becoming less intense at a rate that’s twice the rate that Central Pacific La Niña events are increasing in intensity.
And if we combine the two datasets, Figure 6, the linear trend of the El Niño events is decreasing, indicating they’re becoming less intense. And the La Niña trend is basically flat.
Lee and McPhaden include the following in their Concluding Remarks, “Why these changes are occurring and what accounts for them are important questions. Theories have suggested that the intensity of El Niño could be affected by changes in background conditions such as the depth of the thermocline [e.g., Fedorov and Philander, 2000]. More generally, it is important to know if the increasing intensity and frequency of CP‐El Niño events are related to changes associated with natural decadal-to-multi-decadal variability [e.g., McPhaden and Zhang, 2002; Lee and McPhaden, 2008] or whether the changes are due to anthropogenic greenhouse gas forcing [Yeh et al., 2009].”
But the press release and the press have missed this realistic conclusion.