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 in 2015. This month we’re including the 1982/83 and 1997/98 El Niño because they were the strongest El Niños in our short instrument temperature record, comparable to the one this year.
There are a couple of notable things this month. First, NOAA uses the sea surface temperature anomalies of the NINO3.4 region as their primary metric for determining the strength of an El Niño. Based on the weekly and monthly Reynolds OI.v2 data, NINO3.4 region sea surface temperature anomalies are still running slightly ahead of the 1997/98 El Niño. However, as we discussed in the post Is the Current El Niño Stronger Than the One in 1997/98?, the 1997/98 El Niño was a stronger East Pacific El Niño than the one taking place now. Also see the post The Differences between Sea Surface Temperature Datasets Prevent Us from Knowing Which El Niño Was Strongest According NINO3.4 Region Temperature Data, because the results vary depending on the sea surface temperature dataset. And if you still want to argue that the El Niño this year is stronger than the 1997/98 event, see the post Exactly the same, but completely different: why we have so many different ways of looking at sea surface temperature at the NOAA ENSO Blog.
Second, a pocket cooler-than-normal subsurface waters has begun to migrate east along the equator in the Pacific (along the subsurface current known as the Cromwell Current or the Pacific Equatorial Undercurrent). NOAA is showing that as an upwelling Kelvin wave in the Hovmoller diagram of weekly subsurface temperatures for the equatorial Pacific on page 15 of their Weekly ENSO Update, which I’ve included as my Figure 00. Will it initiate a La Niña for 2016/17?
We can also watch the migration of cooler-than-normal subsurface waters along the equatorial Pacific in the animation of subsurface temperature cross-sections from the NOAA GODAS website. See Animation 1, which begins with the pentad (5-day period) centered on October 5 and ends with the pentad of November 29, 2015.
Third, the Southern Oscillation Index for November 2015 has increased to the point that it’s above the threshold of an El Niño. However, daily values have reached well into El Niño territories again over the past 12 days.
Fourth, weekly sea surface temperature anomalies for the NINO3.4 region have recently started to decline, but it’s still too early to tell if that signals we’ve reached the peak of the 2015/16 event.
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 November 2015, and for the weekly data through early-December. 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 17 illustrations and 4 .gif animations so it might take a few moments to load on your browser. Please click on the illustrations to enlarge them. And you may need to click-start the animations.
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 1982 (the two strongest El Niño events during recent decades) as references for 2015. In many respects, the event this year is lagging behind the event of 1997/98.
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 1982 and 1997.
Last, using maps and cross sections available from the NOAA GODAS website, we’ll present animations of sea surface height anomalies, sea surface temperature anomalies, average subsurface temperature anomalies to depths of 300 meters (a.k.a. T300) and equatorial cross-sections of subsurface temperature anomalies, from the start of 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 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)
As of the week centered on December 2, 2015, the sea surface temperature anomalies for the often-referenced NINO3.4 region are above the values reached at the peak of the 1997/98 El Niño. (Again, see the discussions here and here that were linked in the Introduction.) 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 weekly sea surface temperature anomalies for the four NINO regions, Figure 2 compares the goings on this year with the 1997/98 event. While sea surface temperature anomalies in the NINO4 and NINO3.4 regions are higher than they were in 1997, the NINO1+2 and NINO3 regions are still 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.
Note how the NINO3.4 sea surface temperature anomalies (and those of the NINO4 and NINO3 regions) are showing declines over the past few weeks. It’s a little early to tell if we’ve seen the peak there. It could just be weather noise.
Weekly NINO region data before 1991 are not available from the NOAA webpage data here. So in Figure 2 Supplement, we’ll switch to the monthly sea surface temperature anomalies for the four NINO regions for comparisons with the 1982/83 and 1997/98 El Niños. (The monthly Reynolds OI.v2 sea surface temperature data were downloaded from the KNMI Climate Explorer.)
Figure 2 Supplement
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, strong El Niño conditions exist, but they are lagging behind the events of 1982/83 and 1997/98:
Compared to last month, the updated (October-November) MEI has recovered slightly (0.08) to +2.31, but been overtaken by 1982 to now reach the 3rd highest ranking , which is actually only 0.1-0.3 sigma behind 1982 and 1997 at this time of year. The August-September 2015 value of +2.53 remains the third highest overall at any time of year since 1950. It seems noteworthy that the 1997 El Niño event displayed a similar ‘weak’ spell around October as monitored by the MEI, only to recover most of the lost ground in early 1998.
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 October/November were posted on December 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 1982 and 1997.
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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, 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 El Niño so far (through month-to-date December 2015) 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 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 warm water volume in 1982 was comparable at the start of this year but depth-averaged temperature anomalies started off higher in 2015 than in 1982. Both western equatorial datasets now, though, are higher than in both 1982 and 1997.
Both warm water volume and depth-averaged temperature anomalies in the Eastern equatorial Pacific (5S-5N, 155W-80W) in 2015 continue to lag behind the values of 1997, but have been greater than the 1982 values for most of the year. 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.
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 November 2015 Southern Oscillation Index value is -5.6, which is a lesser 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 1982 and 1997…the development years of the 1982/83 El Niño and the 1997/98 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 1997 but is still lagging behind the values in 1982.
Also see the BOM Recent (preliminary) Southern Oscillation Index (SOI) values webpage. The current 30-day running average is continuing to run in ENSO neutral territory, while the 90-day average is still in El Niño conditions. Note, however, in the daily data, the recent return of strong El Niño values (high negative numbers) during the past 12 days.
COMPARISONS OF HOVMOLLER DIAGRAMS OF THIS YEAR (TO DATE) WITH 1982 AND 1997
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 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 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 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 than they were in 1982 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 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 March: 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 they appear early that year 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 this year, the most recent westerly wind burst happened in November.
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 along the equator.
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. In 1982, sea surface temperature anomalies also reached 4.0 deg C, but we have yet to see those values in 2015.
That is, as noted earlier, the 1997/98 was a stronger East Pacific El Niño than the one taking place in 2015.
ANIMATIONS OF GODAS MAPS AND CROSS SECTIONS – JANUARY 2015 TO PRESENT
Animation 2 includes the subsurface temperature anomalies along the equator (2S-2N) for the pentads (5-day averages). The equatorial Indian Ocean is to the left in both Illustrations and the equatorial Atlantic is to the right. We’re interested in the equatorial Pacific in the center.
The recent upwelling (cool) Kelvin wave makes its presence known with the cooler-than-normal subsurface waters that are beginning to migrate east along the Cromwell Current, the Pacific Equatorial Undercurrent. Sea surface temperatures along the equator should start to drop in response over the next few months, initially toward the west and then working eastward.
Animation 3 presents global maps of the depth-averaged temperature anomalies to depths of 300 meters (a.k.a. T300 anomalies). The downwelling Kelvin wave that initiated this El Niño stands out. The impacts of the recent upwelling Kelvin wave are not yet visible. The other noteworthy thing: The Blob in the eastern extratropical North Pacific still makes its presence known in the T300 data. It could re-emerge next year in the sea surface temperature data. We’ll have to watch and see if the upcoming La Niña (assuming one forms) puts an end to The Blob.
Sea surface height anomalies, Animation 4, are often used as a proxy for temperature anomalies from the surface to the ocean floor. The downwelling Kelvin waves this year also stands out. The Blob is also visible.
The sea surface temperature anomaly maps at the GODAS website normally lag by a few weeks, but they’ve caught up recently. Animation 5 shows the sea surface temperature anomaly maps for 2015 from the GODAS website. The impacts of the El Niño are plainly visible in the tropical Pacific. The North Atlantic has yet to show a response, but the Indian Ocean has shown warming. See the time-series graphs of the North Atlantic (here) and the Indian Ocean (here) from the November 2015 sea surface temperature update. The North Pacific (here) is showing the impacts of The Blob and the El Niño, and the sea surface temperatures of the South Pacific (here) have recently surged in response to the El Niño.
Let’s hope a very strong La Niña follows the El Niño this year and finally overcomes the residual effects of “The Blob” on the North Pacific. Even then, there may have been an upward shift in sea surface temperatures there, which would impact the entire east Pacific. We’ll have to keep an eye on it over the next few years.
The most recent update on The Blob is here, dated August 12, 2015. I’ll try to provide an update in the next few weeks. The Blob appears to be dissipating some, but there is still a lot of warm water at depth created by the Blob.
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 El Niño posts in 2014 and from this year’s El Nino series.
A NEW BOOK AND IT’S 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…though not as detailed as in Who Turned on the Heat?