A Different Perspective of the Equatorial Pacific and ENSO Events

It’s difficult to express how enormous El Niño and La Niña events are. Often, when discussing them, I’ll display a map of the tropical Pacific (see example here) and write something to the effect of, the equatorial Pacific or the tropical Pacific stretches almost half way around the globe.  But that really doesn’t show up very well with a map.

I was cleaning out a catch-all drawer in my desk recently and happened on a protractor. (I even have a pocket calculator and a slide rule in there.  No abacus, though.)  It occurred to me that maybe a graduated half-circle would help to put the equatorial Pacific and El Niño-Southern Oscillation (ENSO) events into perspective for some readers.

To create Figure 1, I borrowed the image of a protractor from a website, started the longitudes with 110E to the left and ended with 70W to the right, and added a couple of blobs to indicate Indonesia in the west and South America in the east.  If we consider this a partial cross-section, we’re looking north.  I’ve also identified the longitudes for the region used in the Cold Tongue Index (180-90W) and the longitudes I typically present for the western equatorial Pacific (120E-165E).  The sea surface temperature anomalies for the Cold Tongue Region (known as the Cold Tongue Index) are a commonly used index for the strength, timing and duration of El Niño and La Niña events.  I used the Cold Tongue Index for this post in place of the more commonly used NINO3.4 sea surface temperature anomalies because, as you can see, the Cold Tongue Region stretches ¼ of the way around the globe.  The longitudes for the western equatorial Pacific stretch half that—or 1/8 of the length of the equator.

Figure 1

Figure 1

(As a reference, the same illustration with the NINO3.4 region identified is here.)

A graph of the sea surface temperature anomalies of the western equatorial Pacific (6S-6N, 120E-165E) and the Cold Tongue Region (6S-6N, 180-90W) is shown in Figure 2. A huge volume of naturally created warm water is transported from the western equatorial Pacific eastward to the Cold Tongue Region during an El Niño.  We can see that sea surface temperatures in the Cold Tongue Region warm drastically during El Niños and cool during La Niñas—nothing new with that.  During the evolution of the1997/98 El Niño, so much naturally created warm water was transported east that sea surface temperature anomalies for the Cold Tongue Region (which stretches ¼ of the way around the globe) warmed more than 3.0 deg C (about 5.4 deg F) from January to December 1997.

Figure 2

Figure 2

On the other hand, the sea surface temperature anomalies for the western equatorial Pacific cool during El Niños and warm during La Niñas.  But as we can see in Figure 2, the cooling in the western equatorial Pacific is much less than the warming in the Cold Tongue Region during El Niños, and there’s much less warming in the western equatorial Pacific than there is cooling in the Cold Tongue Region during La Niñas.  So the cooling (warming) in the western equatorial Pacific is not enough to offset the warming (cooling) in the Cold Tongue Region, especially when we consider the Cold Tongue Region is twice the size of the western equatorial Pacific.  That’s one of the indications that much of the warm water that fuels an El Niño comes from below the surface of the western tropical Pacific.

In Figure 2 above, the magnitude of the variations in the eastern equatorial Pacific (Cold Tongue Region) mask something that happened in the western equatorial Pacific during the 1995/96 La Niña.  So in Figure 3, I’ve scaled the Cold Tongue Region data by a factor of 0.25 and smoothed both datasets with 13-month running-average filters.  Now, standing out quite clearly is the upward shift in the sea surface temperature anomalies of the western equatorial Pacific that happened in response to the 1995/96 La Niña.  Other than the dip and rebound associated with the 1997/98 El Niño, the sea surface temperatures of the western equatorial Pacific have been relatively flat since 1996.

Figure 3

Figure 3

I’ve been showing the same upward shift in the ocean heat content data of the tropical Pacific for more than 3 years. I’ve added scaled tropical Pacific ocean heat content data (24S-24N, 120E-90W) for the depths of 0-700 meters to the comparison graph in Figure 4. (The ENSO-related variations in tropical Pacific ocean heat content take place in the top 300 meters, so we’ve captured them with this depth.) In response to the major El Niño and La Niña events, the larger variations in the sea surface temperature anomalies of the western equatorial Pacific and the ocean heat content for the entire tropical Pacific mimic one another.  But there appears to be an additional seasonal component in the sea surface temperature anomalies of the western equatorial Pacific.

Figure 4

Figure 4

A REMINDER ABOUT THE 1995/96 LA NIÑA

As you’ll recall from earlier posts, when looking at the sea surface temperature anomalies of the Cold Tongue Region, the 1995/96 La Niña was not strong, but there were unusually strong trade winds in the western equatorial Pacific.  According to McPhaden (1999) The Evolution of the 1997/98 El Niño:

For at least a year before the onset of the 1997–98 El Niño, there was a buildup of heat content in the western equatorial Pacific due to stronger than normal trade winds associated with a weak La Niña in 1995–96.

Stronger trade winds reduce cloud cover, which allows more sunlight to warm the tropical Pacific. All of that warm water then collects in the west Pacific Warm Pool.

And as you’ll further recall, the 1995/96 La Niña created the warm water that fueled the 1997/98 El Niño. In turn, the 1997/98 El Niño released enough of that naturally created warm water from below the surface of the western tropical Pacific to temporarily raise the sea surface temperatures of the East Pacific (90S-90N, 180-80W) about 0.5 to 0.6 deg C. See Figure 5. Keep in mind the East Pacific from pole to pole covers about 33% of the surface area of the global oceans.  And there was enough naturally created warm water left over from the 1997/98 El Niño to cause the sea surface temperatures of the Atlantic, Indian and West Pacific oceans (the rest of the global oceans) to warm about 0.19 deg C, Figure 5—where they remained until the next strong El Niño in 2009/10.

Figure 5

Figure 5

##########

Figure 6

Figure 6

FURTHER INFORMATION

If this subject is new to you, refer to my illustrated essay “The Manmade Global Warming Challenge” [42MB], which was introduced here.

For a much more-detailed discussion, refer to my ebook Who Turned on the Heat, (US $8.00), which was introduced in the post here.

SOURCES

The Reynolds Optimum Interpolation sea surface temperature data (v2) is available through the NOAA NOMADS website and the KNMI Climate Explorer.  The KNMI Climate Explorer is also the source of the NODC Ocean Heat Content data (0-700m) used in this post.

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About Bob Tisdale

Research interest: the long-term aftereffects of El Niño and La Nina events on global sea surface temperature and ocean heat content. Author of the ebook Who Turned on the Heat? and regular contributor at WattsUpWithThat.
This entry was posted in El Nino-La Nina Processes. Bookmark the permalink.

15 Responses to A Different Perspective of the Equatorial Pacific and ENSO Events

  1. Neville. says:

    Bob, just thought I’d show you a new study that Luke has just linked to at the Thatcher post at Jennifer Marohasy’s blog.

    http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1863.html This is another post that claims the lost heat is to be found in the top 700 metres of the oceans.

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  3. Don B says:

    I like the protractor image – that helps give a perspective.

  4. Bryan S says:

    A couple questions: You highlight the step up pattern in response to the movement of warm water from below the surface to the surface (first in the east Pacific, then in the west again). If the 1997/98 El Nino caused the warmth after 2001 relative to 1996/97, where did that heat come from? You mention strong trade winds in the west Pacific… but what in particular caused that? I’ve always thought there was a distinction to be made between “ocean driven” ENSO events and Atmospheric driven ENSO events. The strong El Ninos that followed the El Chichon and Pinatubo eruptions were driven by a slowing of the trade winds in response to aerosol cooling… that’s why we had El Nino but no appreciable change in ocean heat content in the west. Following that thought train… could the El Nino of 1997/98 ultimately have been the response of some warming mechanism that Pinatubo could have had (that only becomes apparent once the cooling sulfates fall out of the stratosphere)? By that logic, you could hypothesize that the 1986/87 El Nino was the equivalent of the 1997/98 El Nino to the eruption of El Chichon in 1982. I wish i were a scientist, because I am very curious about a possible link between the super Nino of ’97/98 and the eruption of Pinatubo. There seems to be a pattern when you have a large tropical volcanic eruption… that almost immediately an El Nino will form.. but a different kind of El Nino… the planet doesn’t warm during this kind of El Nino… but it still disrupts weather patterns greatly. It is then followed eventually by an unusually strong build up of ocean heat content even if ENSO kind of dodges between neutral and La Nina or even more El Nino. After Pinatubo, the longer duration cooling event of the two major volcanoes, western Pacific SSTs and heat content fell during 1991/92 and 1992/93, but things turned briefly La Nina-ish going into 1994 and western Pacific SSTs shot back up… the cooling associated with the 1994/95 El Nino was pitiful compared to the heat the “neutral” Nina in 1993/94 put into the western Pacific… then followed again by more heating from strong trade winds in 1995 going into 1996. It’s as if maybe major tropical volcanic eruptions could ultimately have an overall warming impact that is longer lasting than the initial cool period. I always appreciate your insight on this issue.

  5. Bryan S says:

    One more thing.. Do you think an El Nino is likely to form this spring into summer? It seems like this past winter acted more like La Nina than anything with heat continuing to build up in the west Pacific… aside from these short summer stints that seem to take the breath out of the west Pacific warmth (in summer 2008 and 2011, and again in 2012)… but we seem kinda due for another El Nino now.

  6. Bob Tisdale says:

    Bryan S says: “If the 1997/98 El Nino caused the warmth after 2001 relative to 1996/97, where did that heat come from?

    The warm water for the 1997/98 El Niño came from the 1995/96 La Niña. We have to look at ocean heat content data for the tropical Pacific to confirm that:

    Stronger trade winds reduce cloud cover, which allows more downward shortwave radiation (sunlight) to warm the tropical Pacific to depth.

    Bryan S says: “You mention strong trade winds in the west Pacific… but what in particular caused that?”

    For that answer I have to refer you to McPhaden 1999. He goes into a very detailed explanation of the events that led up to the 1997/98 El Niño:
    http://lightning.sbs.ohio-state.edu/geo622/paper_enso_McPhaden1999.pdf

    Bryan S says: “I’ve always thought there was a distinction to be made between ‘ocean driven’ ENSO events and Atmospheric driven ENSO events.”

    ENSO events are coupled ocean-atmosphere processes. They provide positive feedback to one another. I’ve never heard of the distinction you’re making.

    Bryan S says: “The strong El Ninos that followed the El Chichon and Pinatubo eruptions were driven by a slowing of the trade winds in response to aerosol cooling… that’s why we had El Nino but no appreciable change in ocean heat content in the west.”

    An interesting thought. I’ve never seen that presented anywhere. Have you confirmed it with data?

    There have been many attempts to determine the impacts of strong volcanic eruptions on El Niño events. I looked at those papers 5 or 6 years ago when I first began studying ENSO—one was written by Michael Mann. There was lots of speculation, but I don’t recall anything conclusive—due to the frequency of El Niños and coincidence of the rarer explosive volcanic eruptions like El Chichon and Mount Pinatubo. I don’t recall any of the papers looking at magnitudes of El Niño events following strong volcanic eruptions though, but the problem in any research about ENSO is the lack of records the further back in time you go. Equatorial Pacific sea surface temperature sampling was very rare before the opening of the Panama Canal in 1914, so trying to analyze NINO3.4 data after the big volcanos in the late 1800s is difficult at best. And there are two periods of data for the ocean heat content in the tropical Pacific: before and after the TAO project, which has been complete since the early 1990s. Before those buoys were in place, one can’t expect the variability to be comparable to variability afterwards.

    Bryan S says: “One more thing.. Do you think an El Nino is likely to form this spring into summer?”

    I don’t make predictions, but the model mean of the NOAA and IPI/CPC ENSO-forecasting models are showing ENSO neutral through next winter:
    http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
    Then again, they still have to get past the springtime prediction barrier.

    Bryan S says: “…but we seem kinda due for another El Nino now.”

    We may have switched to a new epoch, when short La Niñas dominate—La Niñas that are intermixed with small El Niño events. Let’s hope for that for a decade or two to suppress the global warming caused by the large El Niños. Then we can welcome a few big El Ninos and watch temperatures rise again. Hopefully I’ll be around that long.

    Regards

  7. Espen says:

    Bryan S, I like your ideas. In fact, I’ve been wondering about the same myself: What if the game-changing 95-96 La Niña was actually a consequence of a “disturbance in the force” caused by the Pinatubo eruption? We know that both El Chichon and Pinatubo caused more disturbance than just the initial cooling: After the initial warming spikes in the lower stratosphere, there was a cooling of the stratosphere to a lower level than before the eruptions.

  8. Bob says:

    Dear Bob,
    In response to your post “ENSO 2013 – Boy or Girl?” at wattsupwiththat.com, I have been researching analogue ENSO sequences for this year. I found one, so thought you might like to expand on it (maybe another thought provoking post?).
    Present: Monthly Nino 3.4 series Jan 2004 – March 2013
    Analog: Monthly Nino 3.4 series Jan 1883 – March 1892
    Will the present follow the analog series out to 1895?

    Thankyou Bob and keep up the good work.

  9. Bob Tisdale says:

    Bob, interesting find. I’ll write up a post about it.

    Regards

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