Guest Post by Bob Tisdale
Weekly NINO3.4 Sea Surface Temperature Anomalies from NOAA Are Approaching the Threshold of a Moderately Strong La Niña. Australia’s Southern Oscillation Index from BOM is in La Niña values. And NOAA’s Multivariate ENSO Index is Still in ENSO Neutral Territory.
NOAA’S WEEKLY NINO3.4 REGION SEA SURFACE TEMPERATURE ANOMALIES
NOAA’s weekly sea surface temperature anomalies for the NINO3.4 regions (based on the original Reynolds OI.v2 data) are furnished on Mondays. The most recent update for the week centered on October 6, 2016 shows the sea surface temperature anomalies of the NINO3.4 region (5S-5N, 170W-1250W), which NOAA uses to define a La Niña and its strength, is at -0.9 deg C, well below the -0.5 deg C threshold of La Niña conditions and approaching the -1.0 deg C threshold of a moderately strong La Nina.
The weekly NINO region sea surface temperature anomaly data for Figure 1 are from the NOAA/CPC Monthly Atmospheric & SST Indices webpage, specifically the data here. The anomalies for the NOAA/CPC data are referenced to the WMO-preferred base years of 1981-2010.
The top graph in Figure 1 includes the weekly sea surface temperature anomalies of the NINO3.4 region of the equatorial Pacific, which is bordered by the coordinates of 5S-5N, 170W-120W. The sea surface temperature anomalies for the NINO3.4 region are commonly used (especially by NOAA) as an indicator of the strength, timing and duration of El Niño and La Niña events. And for the bottom graph, the evolutions of the sea surface temperature anomalies in 2015 and 2016 are compared to 1997/98, for comparison to the transition from another very strong El Niño.
BOM’S SOUTHERN OSCILLATION INDEX
The Southern Oscillation Index (SOI) from Australia’s Bureau of Meteorology is another widely used reference for the strength, frequency and duration of El Niño and La Niña events. We discussed the Southern Oscillation Index in Part 8 of the 2014/15 El Niño series. It is derived from the sea level pressures of Tahiti and Darwin, Australia, and as such it reflects the wind patterns off the equator in the southern tropical Pacific. With the Southern Oscillation Index, El Niño events are strong negative values and La Niñas are strong positive values, which is the reverse of what we see with sea surface temperature-based indices. The September Southern Oscillation Index value is +13.5, which well above the threshold of La Niña conditions. (The BOM threshold for La Niña conditions is an SOI value of +7.0. See the BOM webpage here.)
The top graph in Figure 2 presents a time-series graph of the SOI data. Note that the horizontal
red blue line is the present monthly value, not a trend line. The bottom graph in Figure 2 compares the evolution of the SOI values in 2015/16 to those in 1997/98.
Also see the BOM Recent (preliminary) Southern Oscillation Index (SOI) values webpage. The current 90-day average is cycling just above the +7.0 threshold of La Niña conditions.
Regardless of the SOI being in La Niña conditions, the BOM is still in a La Niña Watch. This may be caused by the BOM using the outdated base years of 1961-1990 for sea surface temperature anomaly-based indices.
THE MULTIVARIATE ENSO INDEX
The Multivariate ENSO Index (MEI) is another ENSO index published by NOAA. It was created and is maintained by NOAA’s Klaus Wolter. The Multivariate ENSO Index uses the sea surface temperatures of the NINO3 region of the equatorial Pacific, along with a plethora of atmospheric variables…thus “multivariate”.
According to the most recent Multivariate ENSO Index update discussion, the tropical Pacific is still in ENSO Neutral conditions:
Compared to last month, the updated (August-September) MEI has dropped further to -.10 (down by 1.1 in last three months), which translates into ENSO-neutral rankings.
There’s something else to consider about the MEI. El Niño and La Niña rankings according to the MEI aren’t based on fixed threshold values such as +0.5 for El Niño and -0.5 for La Niña. The MEI El Niño and La Niña rankings are based on percentiles, top 30% for the weak to strong El Niños and the bottom 30% for the weak to strong La Niñas. This is difficult to track, because, when using the percentile method, the thresholds of El Niño and La Niña conditions vary from one bimonthly period to the next, and they can change from year to year.
The Multivariate ENSO Index update discussion and data for August/September were posted on October 8th. The top graph in Figure 3 presents a graph of the MEI time series starting in Dec/Jan 1979. And the bottom graph of Figure 3 compares the evolution in 2015/16 to the reference El Niño of 1997/98.
WANT TO LEARN MORE ABOUT EL NIÑO AND LA NIÑA EVENTS AND THEIR AFTEREFFECTS?
I published On Global Warming and the Illusion of Control (25MB .pdf) back in November. The introductory post is here. It’s free. Chapter 3.7 includes detailed discussions of El Niño events and their aftereffects…though not as detailed as in Who Turned on the Heat?
My ebook Who Turned on the Heat? – The Unsuspected Global Warming Culprit: El Niño-Southern Oscillation (23MB .pdf) goes into a tremendous amount of detail to explain El Niño and La Niña processes and the long-term aftereffects of strong El Niño events. It too is free. See the introductory post here. Who Turned on the Heat? weighs in at a whopping 550+ pages, about 110,000+ words. It contains somewhere in the neighborhood of 380 color illustrations. In pdf form, it’s about 23MB. It includes links to more than a dozen animations, which allow the reader to view ENSO processes and the interactions between variables.