Guest Post by Bob Tisdale
In the recent post Say Good-Bye to the 2015/16 El Niño, we illustrated and discussed how the weekly sea surface temperature anomalies of the NINO3.4 region of the equatorial Pacific had dropped below NOAA’s +0.5 deg C threshold for El Niño conditions for the week centered on May 18th. Weekly sea surface temperature anomalies for that region are now below zero.
We noted in the April update that both Australia’s Bureau of Meteorology (BOM) and the U.S.’s NOAA have issued La Niña alerts for the ENSO season of 2016/17. (BOM notice is here, and NOAA advisory is here.) Sea surface temperature anomalies for the equatorial Pacific are rapidly moving in the direction.
The questions now: If a La Niña forms and persists, how strong
with will the 2016/17 La Niña be? Will it last for a single-season, or will it be a multiyear La Niña like the 1998-01 La Niña that followed the 1997/98 El Niño, or will there be back-to-back La Niñas like we saw in 2010/11 and 2011/12 (See the current version of NOAA’s Oceanic NINO Index) and in 2007/08 and 2008/09 when NOAA was using the ERSST.v3b data for its Oceanic NINO Index)?
Back to your regularly scheduled update…
ENSO METRIC UPDATES
This post provides an update on the progress of the evolution and decay of the 2015/16 El Niño with monthly data through the end of April 2016, and for the weekly data through late-May, 2016. The post is similar in layout to the updates that were part of the 2014/15 El Niño series of posts here and the series of posts about the 2015/16 El Niño here. The remainder of the post includes a bunch of illustrations and a gif animation, 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 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 1997/98 and 1982/83 where possible (the two strongest El Niño events during recent decades) as references for 2015/16.
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 for the 2015/16 El Niño to the El Niños of 1982/83 and 1997/98.
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 anomalies for the NOAA/CPC data are referenced to the base years of 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)
Note that the horizontal red (positive anomalies) and blue (negative anomalies) lines in the graphs are the present readings, not the trends.
The sea surface temperature anomalies for the easternmost NINO1+2 region have recently been cycling below and above zero. The anomalies for NINO3 and NINO3.4 regions have recently turned negative.
NOTE: The NINO4 region should follow soon. In response to the recent upwelling Kelvin wave, the subsurface waters below the equatorial Pacific have become cooler than normal for the most part. That is, the warmer-than-normal subsurface waters below the equatorial Pacific have mostly departed. See Supplemental Figure 1, which is the most-recent cross section of equatorial subsurface temperature anomalies from the NOAA-GODAS website here.
Supplemental Figure 1
EL NIÑO EVOLUTION COMPARISONS FOR NINO REGION SEA SURFACE TEMPERATURE ANOMALIES
Using weekly sea surface temperature anomalies for the four NINO regions, Figure 2 compares the 2015/16 El Niño with the 1997/98 event. (That weekly data start in January 1990, so we can’t include the 1982/83 El Niño.) While sea surface temperature anomalies in the NINO4 and NINO3.4 regions peaked higher than in 1997, the NINO1+2 and NINO3 regions lagged 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.
We also showed in the post here that the differences between sea surface temperature datasets and their uncertainties keep us from knowing which El Niño was strongest.
The NINO region sea surface temperature anomalies are continuing to show declines as the El Niño transitions to La Niña. The weekly data are impacted by “weather noise” so we might expect to see another couple of upticks from time to time, but the 2015/16 El Niño decayed and is transitioning to La Niña on schedule. El Niños are tied to the seasonal cycle and typically peak in November to January. See the post here.
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”.
Compared to last month, the updated (March-April) MEI has stabilized (up by 0.11) at +2.07, continuing just below a Top-3 ranking for the second month in a row. The preceding nine-month run in the Top-3 is tied with 1982-83 for its duration, while 1997-98 kept this level going for a full 12 months. No other El Niño since 1950 even exceeded three months at that level. The August-September 2015 MEI of +2.53 represents the peak of the 2015-16 event, and was exceeded only during the 1982-83 and 1997-98 events. The overall evolution of the 2015-16 El Niño has been most similar to 1997-98, as monitored by the MEI.
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 March/April were posted on May 6th. Figure 3 presents a graph of the MEI time series starting in Dec/Jan 1979. And Figure 4 compares the evolution in 2015/16 to the reference El Niños of 1982/83 and 1997/98.
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According to NOAA’s Multivariate ENSO Index, the 2015/16 El Niño was weaker than the 1982/83 and 1997/98 events.
EL NIÑO EVOLUTION COMPARISONS WITH TAO PROJECT SUBSURFACE DATA
IMPORTANT NOTE: The 1982 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 (a computer model), not observations-only-based data. [End note.]
The NOAA Tropical Atmosphere-Ocean (TAO) Project website includes the outputs of a reanalysis for two temperature-related datasets for the waters below the surface of the equatorial Pacific. See their new Upper Ocean Heat Content and ENSO webpage for descriptions of the datasets and for a link to the data presented in the following graphs. 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.
Note: After a recent temporary reformatting, the TAO Project website has returned their data webpage to its original format. They’re available here. [End note.]
In the following three illustrations, we’re comparing reanalysis outputs for the evolution of the 2015/16 El Niño so far (through April 2016) with the outputs for the evolutions of the 1982/83 and 1997/98 El Niños. The Warm Water Volume outputs are the top graphs and the depth-averaged temperature (T300) 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. The western equatorial Pacific supplies the warm water for an El Niño. Claims that El Niños are becoming stronger due to human-induced global warming are obviously not supported by the subsurface data from the western equatorial Pacific. The warm water volume in 1982 was comparable at the start of 2015 but depth-averaged temperature anomalies started off higher in 2015 than in 1982. Notice how there was a much greater decline in 1997/98 than 2015/16. That indicates more warm water migrated eastward from the western tropical Pacific during the 1997/98 event than in 2015/16….another indication that the 2015/16 El Niño was weaker that the one in 1997/98.
Both warm water volume and depth-averaged temperature anomalies in the Eastern equatorial Pacific (5S-5N, 155W-80W) in 2015/16 had lagged behind the values of 1997/98, but had been greater than the 1982/83 values for most of the event. See Figure 6. Both the warm water volume and T300 have recently fallen into line with those seen in 1997/98 and are now lower than the values in 1982/83.
Once again, with the noticeable differences between the 1997/98 and 2015/16 events, data contradict claims that the 2015/16 El Niño was stronger that the event of 1997/98.
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. Both the warm water volume and the subsurface T300 data show greater drops in 1997/98 than in 2015/16, suggesting that more heat was released from equatorial Pacific in 1997/98 than in 2015/16.
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 temperature-based indices. The April Southern Oscillation Index value is -22.0, which is a greater negative value than the threshold of El Niño conditions. (The BOM threshold for El Niño conditions is an SOI value of -8.0.) In other words, according to the SOI, we were in back in El Niño conditions last month. Figure 8 presents a time-series graph of the SOI data. The BOM SOI data provide more indications that the 2015/16 event was comparable to or weaker than many El Niño events.
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 in 2015/16 to those in 1982/83 and 1997/98. 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 once again fallen behind the values in 1997 and is comparable to the values in 1982.
Also see the BOM Recent (preliminary) Southern Oscillation Index (SOI) values webpage. The 30-day running average has been running in ENSO-neutral conditions for the past week. Will it stay there?
COMPARISONS OF HOVMOLLER DIAGRAMS OF THIS EL NIÑO (TO DATE) WITH 1982/83 AND 1997/98
NOTE: For the following illustrations, I’ve extended the Hovmoller diagrams by splicing the 2016 portions of the most recent ones onto 2015 so that we can compare the evolutions and decays of the El Niños. [End note.]
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 1982 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 first-year Januarys at the top and second-year 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 1982, 1997 and 2015, were caused by downwelling Kelvin waves, which push down on the thermocline (the 20 deg C isotherm). You’ll note how the anomalies grew in strength as the Kelvin wave migrated east. That does not mean the Kelvin wave is getting stronger as it 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. In this illustration, we’re looking at anomalies, not absolute values.
Based on thermocline depth anomalies, the El Niño conditions were much stronger in 1997/98 than they were in 1982/83 and in 2015/16.
The recent change to shallower-than-normal anomalies was initiated by an upwelling Kelvin wave. The values in recent times appear to be lagging behind those in 1983 and 1998.
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 east to west trade winds. The reversals of the trade winds (the yellows, oranges and reds) are the unusual events and they’re associated with El Niños, which are the abnormal 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 April: they initiated the Kelvin wave this year. Throughout 1997, there was a series of westerly wind bursts in the western equatorial Pacific. Same thing occurred in 2015. There were comparatively few westerly wind bursts early in 1982, and the bursts early in 1982 appear to have been weaker than those in 1997 and 2015, according to this GODAS reanalysis. But there was a strong westerly wind burst later in 1982. Returning to 2015/16, the most recent westerly wind burst happened in January 2016.
Based on what happened in 1983 and 1998, we may not expect to see another westerly wind burst until October-December of 2016, and then the westerly wind bursts would likely be weak by comparison to those that occurred during the El Niños.
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. [End note.]
And Figure 13 presents the Hovmollers of sea surface temperature anomalies along the equator.
Notice the extremely high sea surface temperature anomalies in the eastern equatorial Pacific during the peak of the 1997/98 El Niño. While the sea surface temperatures in 2015/16 had reached well above threshold of a strong El Niño, they were 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. In 1982/83, sea surface temperature anomalies also reached 4.0 deg C, but we never reached those values in 2015/16.
That is, as noted earlier, the 1997/98 was a stronger East Pacific El Niño than the 2015/16 event.
Currently, sea surface temperature anomalies are where we would expect them for the transition from El Niño to La Niña conditions.
THE SLOW MOVING ROSSBY WAVES NORTH AND SOUTH OF THE EQUATOR
In the March ENSO update, we discussed how the most recent downwelling Kevin wave (initiated by the January 2016 Westerly Wind Burst) appeared to split the pocket of subsurface warm waters in the eastern equatorial Pacific in February 2016, creating two pockets of warm subsurface waters…one north of the equator and another south of the equator. Those two pockets of warm subsurface waters (leftovers from the El Niño) are now migrating westward very slowly…very slowly (as Rossby waves). In Animation 1, I’ve included an animation of the sea level residual maps from the JPL website here, starting in December 2015, through the most recent map dated May 22nd.
During the upcoming posts about the 2016/17 La Niña, we’ll discuss the impacts of that leftover warm water.
The last downwelling Kelvin wave (started in January 2016) was followed two months later by a strong upwelling Kelvin wave. Thus, the negative sea level residuals along the equator.
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.
My ebook Who Turned on the Heat? – The Unexpected Global Warming Culprit, El Niño-Southern Oscillation IS NOW FREE. Click here for a copy (23MB .pdf).
A NEW EBOOK AND IT TOO IS FREE
I also published On Global Warming and the Illusion of Control (25MB .pdf) back in November. The introductory post is here. It also includes detailed discussions of El Niño events and their aftereffects in Chapter 3.7…though not as detailed as in Who Turned on the Heat?