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. The reference years for comparison graphs in this post are 1997 and 2014, which are the development years of the strongest recent El Niño and the last El Niño. I have not included animations in this post. In their place, I’ve compared present-day maps from the NOAA GODAS website to the same time in 2014.
Note: In addition to the standard time-series presentations of global, NINO3.4, hemispheric and ocean basin sea surface temperature anomalies, I’ve also added an updated graph of the sea surface temperature anomalies for The Blob to the standard format of the monthly sea surface temperature updates at my website, starting with the April 2015 update. I have not posted the update for the July anomalies yet.
There are two 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 running a tick ahead of the 1997/98 El Niño. But there’s still a ways to go before the peak of this event. We’ll illustrate the sea surface temperature-based indices in a moment.
Second, there appears to have been yet another westerly wind bust in the western tropical Pacific recently.
The Hovmoller diagram of the surface zonal wind stress along the equator from the NOAA GODAS website, Figure I-1, is showing yet another westerly wind burst during late-July/early-August 2015 in the western equatorial Pacific.
Looking at the Earth.nullschool.net website map of winds in the western tropical Pacific from July 30th here, it appears the westerly wind burst is associated with the early development of typhoon Soudelor/Hanna.
For those new to westerly wind bursts, the following is an introduction of how they are related to El Niño events. I’ve borrowed it from my May 18, 2015 post The Recent Westerly Wind Burst in the Western Equatorial Pacific Could Help to Strengthen the 2015/16 El Niño.
The “normal” state of the tropical Pacific has trade winds blowing from east to west, which pushes the sun-warmed water to the west. As a result, the surfaces of the western tropical Pacific are naturally warmer than those in the east.
Westerly wind bursts are temporary weather-related reversals of that normal wind flow. That is, the winds blow from west to east for a few weeks. The westerly wind bursts help to move some of the warm water back to the central and eastern equatorial Pacific, strengthening the eastward flow of the equatorial countercurrent. Westerly wind bursts are also known to initiate downwelling Kelvin waves, which are also associated with additional warm waters being carried eastward below the surface along the equator by the Cromwell Current (a.k.a. the Pacific equatorial undercurrent).
One of the factors that kept the 2014/15 El Niño from strengthening last year was the absence of westerly wind bursts after the initial ones early in the year. See Figure I-2, which is the same as Figure 11 later in the post. It includes Hovmoller diagrams of surface zonal wind stress along the equator for 2014 on the left, 2015-to-date in the center, and 1997 as a reference for a powerful El Niño on the right. In 1997, there were numerous westerly wind bursts over the course of the year, helping to strengthen it into a “super El Niño”.
So, if Mother Nature wants the El Niño to continue strengthen, she’ll continue to provide westerly wind bursts in the western equatorial Pacific.
ENSO METRIC UPDATES
This post provides an update on the progress of the evolution of the 2015/16 El Niño (assuming one continues into next year) with monthly data through the end of June 2015, and for the weekly data through early August. 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 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 and 2014 (a very strong El Niño and the last El Niño) as references for 2015. The 1997/98 El Niño was extremely strong, while the 2014/15 event was extremely weak and intermittent.
And since there is another downwelling (warm) Kelvin wave making its way east along the equator in the Pacific, 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 2014.
Last, we’ll compare maps and cross sections (2014 and 2015) from the GODAS website of a number of ENSO-related metrics.
NINO REGION TIME-SERIES GRAPHS
Note: The weekly NINO region sea surface temperature anomaly data for Figures 1 and 2 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 August 5, 2015, the sea surface temperature anomalies for the easternmost NINO1+2 region were about 2.6 deg C, and that’s a decline over the past month. The westernmost NINO4 region is also showing a decline in sea surface temperature anomalies. On the other hand, the NINO3.4 and NINO3 regions are showing increases in recent weeks.
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 and 2014/15 events. All of the NINO regions this year are warmer than during the same times of the 2014/15 El Niño, and, while the NINO1+2 is lagging slightly behind the 1997/98 El Niño, the other regions are comparable to or warmer than the 1997/98 El Niño. Then again, we started this year in weak El Niño conditions, while we didn’t during the two reference years.
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:
In the context of strong El Niño conditions lasting continuously since March-April 2015, this section features a comparison figure with the classic set of strong El Niño events during the MEI period of record.
While the updated (June-July) MEI has dropped slighty (by 0.09 standard deviations in one month) to +1.97, it is now reaching the 2nd highest ranking, surpassed only by 1997 at this time of year. The MEI has hovered around +2 standard deviations for two months running, highest overall since 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 June/July were posted back on August 4th. 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 2014.
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EL NIÑO EVOLUTION COMPARISONS WITH TAO PROJECT SUBSURFACE DATA
The NOAA Tropical Atmosphere-Ocean (TAO) Project website includes data 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 data for the evolution of the 2015/16 “season” so far (through month-to-date August 2015) with the data for the evolutions of the 1997/98 and 2014/15 El Niños. The Warm Water Volume data are the top graphs and the depth-averaged temperature data 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 and 2014. Both western equatorial datasets now, though, are higher than in 1997 but less than 2014.
Both warm water volume and depth-averaged temperature anomalies in the Eastern equatorial Pacific (5S-5N, 155W-80W) have fallen behind the values of 1997, but are greater than the 2014 values. See Figure 6.
The total of the TAO project eastern and western equatorial subsurface temperature-related data, 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 2014, 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 July 2015 Southern Oscillation Index value is -14.7, 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.) 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 2014…the development years of the 1997/98 and 2014/15 El Niños. 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 this year is ahead of the values in 2014, but behind 1997.
Also see the BOM Recent (preliminary) Southern Oscillation Index (SOI) values webpage. For the past week (through August 10), SOI values have dropped out of El Niño conditions, returning to “normal” conditions over the past two days. However, the current 30-day running average is a greater negative value than the -8.0 threshold of an El Niño based on the Southern Oscillation Index, as is the 90-day average.
COMPARISONS OF HOVMOLLER DIAGRAMS OF THIS YEAR (TO DATE) WITH 1997 AND 2014
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 date 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 2014 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, 2014 and 2015, were caused by downwelling Kelvin waves, which push down on the thermocline (the 20 deg C isotherm). You’ll note how, each year, 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.
The El Niño conditions were much stronger in 1997 than they were in 2014 and so far in 2015.
An upwelling (cool) Kelvin wave followed the initial downwelling (warm) Kelvin wave in 2014 and suppressed the development of the El Niño last year. So far that has not happened in 2015.
Figure 11 presents the 2015-to-date along with the 1997 and 2014 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 2014 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. We didn’t see the additional westerly wind bursts later in 2014, which suppressed the evolution of the 2014/15 El Niño. So far in 2015 we’ve had a number of westerly wind bursts. The most recent one happened in late-June/early-August of 2015 and helped to strengthen the El Niño this year.
We’ll need more westerly wind bursts this year, too, in order for this El Niño to continue to develop throughout the 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. Refer again, also, to the comparison graphs in Figure 2.
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 the threshold of a strong El Niño, they’ve still got a lot of work to do to reach the strength of the 1997/98 El Niño.
GODAS MAPS AND CROSS SECTIONS – EARLY AUGUST 2014 AND 2015
As opposed to presenting animations from NOAA’s GODAS website of maps and cross sections of a number of metrics as I did in the 2014/15 El Niño series, I thought it would be better (more informative) to compare the most recent maps and cross sections from this year to those from the same time last year. So let’s start with the cross sections of temperature anomalies along the equator.
Figure 14 compares the subsurface temperature anomalies along the equator (2S-2N) for the pentads (5-day averages) centered on August 1, 2015 (left) and August 1, 2014 (right). 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 illustrations confirm what was shown in the depth-averaged temperature anomaly graphs in Figures 5 and 6, where east and west are divided at 155W. The subsurface temperature anomalies in the western equatorial Pacific are cooler this year than last, but in the eastern equatorial Pacific, they’re warmer this year. By August 2014, an upwelling (cool) Kelvin wave had traveled east and lowered the subsurface temperature anomalies along the equatorial Pacific, and another downwelling (warm) Kelvin wave in 2014 was about to begin…fueled by warm water that had been recirculated back to the west from the initial downwelling Kelvin wave that year.
Figure 15 presents global maps of the depth-averaged temperature anomalies to depths of 300 meters (a.k.a. T300 anomalies). Looking at the tropical Pacific as a whole, not just the equator, the downwelling Kelvin wave this year has definitely reached the shores of South America. This year’s Kelvin wave has traveled eastward into an eastern tropical Pacific that’s warmer than last year, a product of the additional downwelling (warm) Kelvin waves later in 2014. Keep in mind, though, that the downwelling (warm) Kelvin wave this year started later than in 2014 and that there was an upwelling (cool) Kelvin wave last year by this time that suppressed it. Also note that the western tropical Pacific is much cooler this year than last. Are those cool anomalies in the west setting up for a strong La Niña next year? We’ll have to wait and watch.
Sea surface height anomalies, Figure 16, are often used as a proxy for temperature anomalies from the surface to the ocean floor. They are showing lower sea levels in the western tropical Pacific this year than last and showing that the downwelling Kelvin wave has arrived in a warmer eastern tropical Pacific.
The sea surface temperature anomaly maps at the GODAS website lag by a few weeks. Figure 17 shows the sea surface temperature anomaly maps for 2014 and 2015 for the pentads centered on August 2nd. The sea surface temperature anomalies along the equatorial Pacific are warmer this year than last, concentrated this year just east and west of the dateline. The eastern North Pacific is also warmer this year, with the remnants of “The Blob” and the coastally trapped Kelvin wave(s) from last year.
Let’s hope a very strong La Niña follows the El Niño this year and finally overcomes the 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.
I’ll provide an update on The Blob in a few days.
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.