Guest Post by Bob Tisdale
This post provides an update of many of the ENSO-related variables we presented as part of last year’s 2014-15 El Niño Series. For the posts this year, we’ve used the evolution years of different El Niños as references to the goings-on this year. This month we’re including the 1997/98 El Niño because it was the strongest El Niño in our short instrument temperature record. For the other reference, we’re using 1987, which is the second year of the 1986/87/88 El Niño. Next month, we’ll compare this year to the El Niños of 1982/83 and 1997/98.
Figure 00 compares NINO3.4 sea surface temperature anomalies for the evolutions of the 1986/87/88 and the 2014/15/16 El Niño. Sea surface temperature anomalies for the NINO3.4 region did reach El Niño conditions in the latter part of 2014 so it’s reasonable to compare the two events.
We can see that 1987 (the second year of the 1986/87/88 El Niño) began with noticeably higher NINO3.4 temperature anomalies than in 2015. However, the 1986/87/88 El Niño started to decay before the usual end-of-year peak in 1987, while the El Niño this year continues to evolve.
Note: Depending on the sea surface temperature dataset, the 2014/15 El Niño registers on NOAA’s Oceanic NINO Index as an “official” El Niño. See the comparison in Table 1. With NOAA’s ERSST.v3b data, 2014/15 was an “official” El Niño, but with the new NOAA ERSST.v4 “pause buster” data, 2014/15 was not an “official” El Niño. It did, however, reach El Niño conditions even with the ERSST.v4 data.
We discussed the differences in the new and former versions of NOAA’s ONI index in the post here.
There are a couple of notable things this month. First, NINO3.4 region sea surface temperature anomalies, which NOAA uses as its primary metric for determining the strength of an El Niño, are still running neck-and-neck with the 1997/98 El Niño. See the weekly comparison here. Considering the volume of warmer-than-normal water below the surface of the central and eastern equatorial Pacific, we definitely should not expect the El Niño to decay anytime soon. See the NOAA animation of subsurface temperature anomalies through October 25 here. In fact, we should expect the NINO3.4 region anomalies to continue to rise.
Second, there appears to have been yet another westerly wind burst in the western tropical Pacific recently. As a result, the El Niño should continue to strengthen.
ENSO METRIC UPDATES
This post provides an update on the progress of the evolution of the 2015/16 El Niño with monthly data through the end of September 2015, and for the weekly data through late-October. The post is similar in layout to the updates that were part of the 2014/15 El Niño series of posts here. The remainder of the post includes 13 illustrations so it might take a few moments to load on your browser. Please click on the illustrations to enlarge them.
Included are updates of the weekly (and monthly) sea surface temperature anomalies for the four most-often-used NINO regions. Also included are a couple of graphs of the monthly BOM Southern-Oscillation Index (SOI) and the NOAA Multivariate ENSO Index (MEI).
For the comparison graphs we’re using the El Niño evolution years of 1997 and 1987 (a very strong El Niño and the second year of a multiyear El Niño) as references for 2015.
Also included in this post are evolution comparisons using warm water volume anomalies and depth-averaged temperature anomalies from the NOAA TOA project website.
Then, we’ll take a look at a number of Hovmoller diagrams comparing the progress so far this year to what happened in both 1997 and 1987.
I’ve excluded the comparisons of the maps and cross sections (2014 and 2015) from the GODAS website this month. We already know the 2015 El Niño conditions are stronger than those of 2014. Next month I’ll include the animations again so that we can watch the evolutions of conditions in both 2014 and 2015.
NINO REGION TIME-SERIES GRAPHS
Note: 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 base years for anomalies for the NOAA/CPC data are referenced to 1981-2010.
Figure 1 includes the weekly sea surface temperature anomalies of the 4 most-often-used NINO regions of the equatorial Pacific. From west to east they include:
- NINO4 (5S-5N, 160E-150W)
- NINO3.4 (5S-5N, 170W-120W)
- NINO3 (5S-5N, 150W-90W)
- NINO1+2 (10S-0, 90W-80W)
As of the week centered on October 21, 2015, the sea surface temperature anomalies for the often-referenced NINO3.4 region are closing in on the values reached at the peak of the 1997/98 El Niño. But they’re falling well behind the 1997/98 El Niño in the NINO3 and NINO1+2 regions.
Note that the horizontal red lines in the graphs are the present readings, not the trends.
EL NIÑO EVOLUTION COMPARISONS FOR NINO REGION SEA SURFACE TEMPERATURE ANOMALIES
Using monthly sea surface temperature anomalies for the four NINO regions, Figure 2 compares the goings on this year with the 1997/98 and the second year of the 1986/87/88 events. All of the NINO regions this year are warmer than during the same times in 1987…the second year of the 1986/87/88 El Niño. The NINO1+2 and NINO3 regions are now lagging well behind the 1997/98 El Niño.
In other words, the 1997/98 El Niño was a stronger East Pacific El Niño than the 2015/16 El Niño.
The monthly Reynolds OI.v2 sea surface temperature data are available from the KNMI Climate Explorer.
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 slew of atmospheric variables…thus “multivariate”.
According to the most recent Multivariate ENSO Index update discussion, strong El Niño conditions exist, but they haven’t yet reached “super” El Niño conditions:
Compared to last month, the updated (August-Spetember) MEI has increased by 0.16 standard deviations to +2.53, or the 2nd highest ranking, surpassed only in 1997 at this time of year. This new peak value of the current event ranks third highest overall at any time of year since 1950, closing in on 1982-83 and 1997-98 with ‘Super El Niño’ values around +3 standard deviations.
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 back on October 3rd. Figure 3 presents a graph of the MEI time series starting in Dec/Jan 1979. And Figure 4 compares the evolution this year to the reference El Niño-formation years of 1997 and 1987.
# # #
Like the sea surface temperature-only based ENSO indices, the MEI started out higher in 1987 than in 2015, but decayed early.
EL NIÑO EVOLUTION COMPARISONS WITH TAO PROJECT SUBSURFACE DATA
IMPORTANT NOTE: The 1987 values of the TAO Project subsurface data have to be taken with a grain of salt. The deployment of the TOA project buoys started in the late 1980s and was not compete until the early 1990s. Also keep in mind that these values are the output of a reanalysis, not observations-only-based data.
The NOAA Tropical Atmosphere-Ocean (TAO) Project website includes the outputs of a reanalysis for two temperature-related datasets for the waters below the equatorial Pacific. See their Upper Ocean Heat Content and ENSO webpage for descriptions of the datasets. The two datasets are Warm Water Volume (above the 20 deg C isotherm) and the Depth-Averaged Temperatures for the top 300 meters (aka T300). Both are available for the:
- Western Equatorial Pacific (5S-5N, 120E-155W)
- Eastern Equatorial Pacific (5S-5N, 155W-80W)
- Total Equatorial Pacific (5S-5N, 120E-80W)
Keep in mind that the longitudes of 120E-80W stretch 160 deg, almost halfway around the globe. For a reminder of width of the equatorial Pacific, see the protractor-based illustration here. Notice also that the eastern and western data are divided at 155W, which means the “western” data extend quite a ways past the dateline into the eastern equatorial Pacific.
In the following three illustrations, we’re comparing reanalysis outputs for the evolution of the 2015/16 “season” so far (through month-to-date October 2015) with the outputs for the evolutions of the 1997/98 and 2014/15 El Niños. The Warm Water Volume outputs are the top graphs and the depth-averaged temperature outputs are the bottom graphs. As you’ll see, the curves of two datasets are similar, but not necessarily the same.
Let’s start with the Western Equatorial Pacific (5S-5N, 120E-155W), Figure 5. The warm water volume and depth-averaged temperature anomalies show the Western Equatorial Pacific began 2015 with noticeably less warm water than during the opening months of 1997, but higher than in 1987. Both western equatorial datasets now, though, are higher than in both 1987 and 1997, but note that the values this year are now negative and have been for a number of months. They simply aren’t as “cool” as they were in 1987 and 1997.
Both warm water volume and depth-averaged temperature anomalies in the Eastern equatorial Pacific (5S-5N, 155W-80W) continue to lag behind the values of 1997, but are greater than the 1987 values. See Figure 6.
The total of the TAO project eastern and western equatorial subsurface temperature-related reanalysis outputs, Figure 7, are as one would expect looking at the subsets. Warm water volume and depth-averaged temperature anomalies in 2015 are higher than they were in 1987, but comparable to where they were in 1997.
SOUTHERN OSCILLATION INDEX (SOI)
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 temperatures. The September 2015 Southern Oscillation Index value is -17-7, which is a much greater negative value than the threshold of El Niño conditions. (The BOM threshold for El Niño conditions is an SOI value of -8.0.) Figure 8 presents a time-series graph of the SOI data. Note that the horizontal red line is the present monthly value, not a trend line.
The graphs in Figure 9 compare the evolution of the SOI values this year to those in 1997 and 1987…the development year of the 1997/98 El Niño and the second year of the 1986/87/88 El Niño. The top graph shows the raw data. Because the SOI data are so volatile, I’ve smoothed them with 3-month filters in the bottom graph. Referring to the smoothed data, the Southern Oscillation Index has recently surpassed the values in 1987 and 1997. We can also see the early decay of El Niño conditions in 1987.
Also see the BOM Recent (preliminary) Southern Oscillation Index (SOI) values webpage. The current 30-day running average is a very high negative value, as is the 90-day average.
COMPARISONS OF HOVMOLLER DIAGRAMS OF THIS YEAR (TO DATE) WITH 1997 AND 1987
NOTE: The NOAA GODAS website has not yet added 2015 to their drop-down menu for Hovmoller diagrams. For the following illustrations, I’ve used the Hovmolller diagrams available for the past 12 months, deleted the 2014 data and aligned the 2015 data with the other 2 years.
Hovmoller diagrams are a great way to display data. If they’re new to you, there’s no reason to be intimidated by them. Let’s take a look at Figure 10. It presents the Hovmoller diagrams of thermocline depth anomalies (the depth of the isotherm at 20 deg C. Water warmer than 20 deg C is above the 20 deg C isotherm and below it the water is cooler). 2015 is in the center, 1997 on the left and 1987 to the right. (Sorry about the different sizes of the Hovmollers, but somewhere along the line NOAA GODAS changed them, but they are scaled, color-coded, the same.)
The vertical (y) axis in all the Hovmollers shown in this post is time with the Januarys at the top and Decembers at the bottom. The horizontal (x) axis is longitude, so, moving from left to right in each of the three Hovmoller diagrams, we’re going from west to east…with the Indian Ocean in the left-hand portion, the Pacific in the center and the Atlantic in the right-hand portion. We’re interested in the Pacific. The data are color-coded according to the scales below the Hovmollers.
Figure 10 is presenting the depth of the 20 deg C isotherm along a band from 2S to 2N. The positive anomalies, working their way eastward early in 1997 and 2015, were caused by downwelling Kelvin waves, which push down on the thermocline (the 20 deg C isotherm). We can just see the end of a strong downwelling Kelvin wave at the beginning of 1987 and a couple of weaker ones that followed that year. You’ll note how, in 1997 and 2015, the anomalies grew in strength as the Kelvin waves migrated east. That does not mean the Kelvin waves are getting stronger as they traveled east; that simply indicates that the thermocline is normally closer to the surface in the eastern equatorial Pacific than it is in the western portion.
The El Niño conditions were much stronger in 1997 than they were in 1987 and so far in 2015.
Figure 11 presents the Hovmollers for wind stress (not anomalies) along the equator. The simplest way to explain them is that they’re presenting the impacts of the strengths and directions of the trade winds on the surfaces of the equatorial oceans. In this presentation, the effects of the east to west trade winds at various strengths are shown in blues, and the reversals of the trade winds into westerlies are shown in yellows, oranges and reds. To explain the color coding, the trade winds normally blow from east to west; thus the cooler colors for stronger than normal east to west trade winds. The reversals of the trade winds (the yellows, oranges and reds) are the true anomalies and they’re associated with El Niños, which are the anomalous state of the tropical Pacific. (A La Niña is simply an exaggerated normal state.)
The two westerly wind bursts shown in red in the western equatorial Pacific in 1997 are associated with the strong downwelling Kelvin wave that formed at the time. (See the post ENSO Basics: Westerly Wind Bursts Initiate an El Niño.) Same thing with the three westerly wind bursts early in 2015, January through March: they initiated the Kelvin wave this year. Throughout 1997, there was a series of westerly wind bursts in the western equatorial Pacific. There were comparatively few westerly wind bursts in 1987, according to this GODAS reanalysis, and those that did occur were not as strong as we’ve seen in 1997 and 2015. So far in 2015 we’ve had a good number of westerly wind bursts. The most recent one happened in October 2015 and should help to strengthen the El Niño this year.
Figure 12 presents the Hovmollers of wind stress anomalies…just a different perspective. But positive wind stress anomalies, at the low end of the color-coded scale, are actually a weakening of the trade winds, not necessarily a reversal.
NOTE: There are a number of wind stress-related images on meteorological websites. Always check to see if they’re presenting absolute values or anomalies.
And Figure 13 presents the Hovmollers of sea surface temperature anomalies. Unfortunately, the Hovmoller of sea surface temperature anomalies is delayed a few weeks at the GODAS website.
Notice how warm the eastern equatorial Pacific got during the evolution of the 1997/98 El Niño. While the sea surface temperatures this year have reached well above threshold of a strong El Niño, they’ve still well behind those of the 1997/98 El Niño…especially east of 120W (to about 90W), where sea surface temperature anomalies were more than 4.0 deg C at this time.
That is, as noted earlier, the 1997/98 was a stronger East Pacific El Niño than the one taking place in 2015.
GODAS MAPS AND CROSS SECTIONS
NOTE: Next month I will start to present the animations of the GODAS maps and cross sections again to capture the goings-on in 2014 and 2015.
EL NIÑO REFERENCE POSTS
For additional introductory discussions of El Niño processes see:
- An Illustrated Introduction to the Basic Processes that Drive El Niño and La Niña Events
- El Niño and La Niña Basics: Introduction to the Pacific Trade Winds
- La Niñas Do NOT Suck Heat from the Atmosphere
- ENSO Basics: Westerly Wind Bursts Initiate an El Niño
Also see the entire 2014-15 El Niño series. We discussed a wide-range of topics in those posts.
WANT TO LEARN MORE ABOUT EL NIÑO EVENTS AND THEIR AFTEREFFECTS?
My ebook Who Turned on the Heat? 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. 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.
Last year, I lowered the price of Who Turned on the Heat? from U.S.$8.00 to U.S.$5.00. And the book sold well. It continues to do so this year.
A free preview in pdf format is here. The preview includes the Table of Contents, the Introduction, the first half of section 1 (which was provided complete in the post here), a discussion of the cover, and the Closing. Take a run through the Table of Contents. It is a very-detailed and well-illustrated book—using data from the real world, not models of a virtual world. Who Turned on the Heat? is only available in pdf format…and will only be available in that format. Click here to purchase a copy.
My sincerest thanks to everyone who has purchased a copy of Who Turned on the Heat? as a result of the 2014-15 and this year’s El Nino series.