This post provides an update on the progress of the evolution of the 2014/15 El Niño. Included are updates of the weekly sea surface temperature anomalies for the four most-often-used NINO regions. Also included are updates of the GODAS map-based animations of sea surface height anomalies, T300 anomalies (depth-averaged temperature anomalies to 300 meters), sea surface temperature anomalies, and the cross sections of temperature anomalies at depth along the equator. GODAS only includes the last 3 months in the animations at their website. These animations start in January 2014 for the full progress of the events.
We compared the evolution of the 2014/15 El Niño to the 1982/83 and 1997/98 El Niños in the third post in this series. The evolution of this El Niño is still being hyped by comparing it to the strong 1997/98 El Niño. See Kevin Trenberth’s YouTube video here. So I’ve updated those graphs. And since we’ve been watching the downwelling (warm) Kelvin wave as it makes 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 TAO 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 1997. This will serve as a background for a general discussion of what should happen next as this El Niño evolves, regardless of how strong this El Niño eventually becomes.
NINO REGION TIME-SERIES GRAPHS
Figure 1 includes the weekly sea surface temperature anomalies of the 4 most-often-used NINO regions of the equatorial Pacific. (Yes, Virginia, there are more NINO regions.) 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)
While the +0.5 deg C El Niño threshold really only applies to the NINO3.4 region, I’ve highlighted +0.5 deg C in red on all four graphs. As of last week, with the exception of the NINO3.4 region, the sea surface temperature anomalies in all four NINO regions are warmer than +0.5 deg C. But the NINO3.4 region anomalies are within spitting distance of that threshold. It won’t take much to push them over.
EL NIÑO EVOLUTION COMPARISONS FOR NINO REGION SEA SURFACE TEMPERATURE ANOMALIES
Using weekly sea surface temperature anomalies for the NINO3.4, NINO3 and NINO1+2 regions, Figure 2 updates and expands on the comparisons of the evolutions of this El Niño with the 1982/83 and 1997/98 events. As you’ll recall, the NINO3.4 and NINO1+2 comparisons were originally provided in the post 2014/15 El Niño – Part 3 – Early Evolution – Comparison with 1982/83 & 1997/98 El Niño Events. I’ve added the NINO3 region for this post. NINO3.4 and NINO3 region sea surface temperature anomalies this year are still in the ballpark of the two earlier strong El Niños. And in the NINO1+2 region, the temperature anomalies have broken away sharply from the 1982/83 El Niño evolution, but they’re still far below the values at this time for 1997/98 El Niño. We’ll have to keep an eye on the NINO1+2 data, because they’re an indicator of an East Pacific El Niño, which are stronger than Central Pacific El Niños.
In the first post in this series, we discussed a number of animations of maps and animations of equatorial cross sections available from the NOAA Global Ocean Data Assimilation System (GODAS) website. Each cell of the animation is a 5-day (pentadal) average. Those animations ran from January 3rd to March 29th. The following are updates, again starting in January 3rd. GODAS only maintains their animations for 3 months. I’ve stored the maps from the first of the year and will continue to add maps as time progresses. That way we can watch the El Niño unfold from the beginning and then try to keep track of the warm water when El Niño is over.
Animation 1 provides the sea surface height anomalies and the depth-averaged temperature anomalies for the top 300 meters (T300) side by side. With the update, we can see that the downwelling Kelvin wave has reached the coast of South America. (Thump.) Please click them to enlarge them.
Animation 2 is a similar side-by-side comparison, but on the left are maps of sea surface temperature anomalies and on the right are the H300 maps. In less than two months, we’ve gone from La Niña conditions in the sea surface temperature anomalies of the eastern equatorial Pacific to the threshold of El Niño conditions. Considering the immensity the Pacific, that was quite a remarkable feat of nature.
And Animation 3 is an update of the cross sections of temperature anomalies at depth along the equator.
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 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)
- Equatorial Pacific (5S-5N, 120E-80W)
Keep in mind that the longitudes of 120E-80W stretch 160 deg, almost halfway around the globe.
In the following three graphs, we’re comparing data for the evolution of the 2014/15 El Niño so far (through month-to-date May 2014) with the data for the evolutions of the 1982/83 and 1997/98 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.
Let’s start with the Western Equatorial Pacific (5S-5N, 120E-155W), Figure 3. The warm water volume and depth-averaged temperature data show the Western Equatorial Pacific had slightly less warm water or was slightly cooler this year than during the opening months of 1997. But 2014 had more warm water or was warmer than 1982.
In the second post in this series, we showed that the ocean heat content for the entire eastern tropical Pacific (24S-24N, 180-80W), for the depths of 0-700 meters, was cooler now than it was in 1997. (See the graph here.) The warm water volume and depth-averaged temperature data shown in Figure 4 for the eastern equatorial Pacific also show lower warm water volume and lower depth-averaged temperatures in 2014 than in 1997.
As a result, across the entire equatorial Pacific, Figure 5, warm water volume is lower and depth-averaged temperatures are less in 2014 than they were in 1997. Then again, they’re higher than they were in 1982.
Keep in mind, though, that both the 1982/83 and 1997/98 events were strong El Niños.
COMPARISONS OF HOVMOLLER DIAGRAMS OF THIS YEAR (TO DATE) WITH 1997
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 6. It presents the Hovmoller diagrams of thermocline depth anomalies (the depth of the isotherm at 20 deg C) with 2014 on the left and 1997 on the right. GODAS, unfortunately, furnishes the illustrations (not the data) in different dimensions for the two years. The vertical (y) axis in both is time with the Januarys for both years at the top and Decembers at the bottom. The horizontal (x) axis is longitude, so, moving from left to right, 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 anomaly data are color-coded according to the scales below the Hovmollers.
Figure 6 is presenting the depth of the 20 deg C isotherm along a band from 2S to 2N. The positive anomalies, working their way eastward since the beginning of 2014, were caused by the downwelling Kelvin wave, which pushes 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 moves 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 1997/98 El Niño was preceded by two downwelling Kelvin waves shown in the right-hand Hovmoller in Figure 6. The first one that began in 1996 wasn’t very strong, but the second one that began a few months later in 1997 was enough to kick start the 1997/98 El Niño.
Note how the thermocline continued to drop in the eastern equatorial Pacific as 1997 progressed. The 1997/98 El Niño was a freak. So much warm water flooded from the western tropical Pacific into the eastern portion that the normal warm water distribution along the equator reversed. That is, normally there is more warm water in the western portion than in the eastern portion of the equatorial Pacific so that the thermocline slopes upward from west to east, but at the peak of the 1997/98 El Niño, there was more warm water in the central and eastern portion than the west, with the slope of the thermocline growing downward from west to east. (See the cross section from ECMWF here.)
Figure 7 presents the 2014-to-date and 1997 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 surface 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.
The two westerly wind bursts shown in red in the western equatorial Pacific in 2014 are associated with the downwelling Kelvin wave, and the westerly bursts early in 1997 are associated with the Kelvin waves that year. Note how in 1997 as time progressed from June through November that the negative wind stresses decreased, with the neutral whites expanding eastward, and with repeated westerly wind bursts in the western equatorial Pacific. Those westerly wind bursts throughout the summer and fall of 1997 continued to help push warm water from the western equatorial Pacific into the east, strengthening the 1997/98 El Niño.
Figure 8 presents the Hovmollers of wind stress anomalies…just a different perspective. Note how there were more positive wind stress anomalies in the western equatorial Pacific in 1997 than there have been so far this year. The westerly wind bursts this year were earlier, but the westerly wind bursts in 1997 were stronger and longer.
And Figure 9 presents the Hovmollers of sea surface temperature anomalies. Unfortunately, the Hovmoller of sea surface temperature anomalies is delayed a few weeks. But as we’ve seen in the comparison graphs in Figure 2, the sea surface temperature anomalies of the NINO3.4 and NINO3 regions in 2014 are tracking with those of 1997, and that the sea surface temperature anomalies this year in the NINO1+2 region are less than they were at this time in 1997. As you’ll note in the Hovmoller for this year, it didn’t take long for the La Niña conditions in the eastern equatorial Pacific to disappear.
As you’ll recall, the surface temperatures (absolute) and the strength of the trade winds are coupled. The temperature difference between the western and eastern tropical Pacific (warmer in the west than in the east) depends on the strength of the trade winds (blowing from east to west), and the strength of the trade winds depend on the temperature difference between the western and eastern tropical Pacific. The stronger the trade winds, the greater the temperature difference, and the greater the temperature difference, the stronger the trade winds. The temperature difference and the trade winds reinforce one another with positive feedback. That positive feedback is called Bjerknes feedback.
Now, in the wake of the downwelling Kelvin wave, as the warmer-than-normal subsurface waters are upwelled to the surface, the temperature difference (absolute) between the eastern and western equatorial Pacific is decreasing. The trade winds will weaken in response, allowing more warm water from the West Pacific Warm Pool to migrate eastward, which decreases the temperature difference more, which further weakens the trade winds, etc.; that is, the positive feedback between the trade winds and the surface temperature gradient (absolute) will reinforce the decrease in the temperature difference between the western and eastern tropical Pacific by forcing more warm water from west to east. And as a result, the surface temperatures and anomalies in the eastern equatorial Pacific will rise.
That feedback will eventually kick in to allow the 2014/15 El Niño to strengthen, if it hasn’t started already. The only questions now are how strong the El Niño will become and how long El Niño conditions will last. Everything depends on the weather in the tropical Pacific, which is why no two El Niño events are the same.
EARLIER POSTS IN THIS SERIES
- The 2014/15 El Niño – Part 1 – The Initial Processes of the El Niño.
- The 2014/15 El Niño – Part 2 – The Alarmist Misinformation (BS) Begins
- The 2014/15 El Niño – Part 3 – Early Evolution – Comparison with 1982/83 & 1997/98 El Niño Events
- The 2014/15 El Niño – Part 4 – Early Evolution – Comparison with Other Satellite-Era El Niños
- The 2014/15 El Niño – Part 5 – The Relationship Between the PDO and ENSO
- The 2014/15 El Niño – Part 6 – What’s All The Hubbub About?…
And 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
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.
I’ve lowered the price of Who Turned on the Heat? from U.S.$8.00 to U.S.$5.00. 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. Thanks. Book sales and tips will hopefully allow me to return to blogging full-time once again.