Why Do El Niño and La Niña Events Peak in Boreal Winter?

In December 2012, I had begun a series of posts about El Niño-Southern Oscillation (ENSO). This post starts the return to them.

El Niño and La Niña events typically peak in strength in December and January. Refer to Figure Intro-1. It’s a graph of the weekly sea surface temperature anomalies for the NINO3.4 region of the eastern equatorial Pacific, starting in 1990. The sea surface temperature anomalies of the NINO3.4 region are an often-used index of the frequency, strength and duration of the El Niño and La Niña events. The El Niño events appear as the large upward spikes (warmings) in the sea surface temperature anomalies, and La Niñas show themselves as the downward spikes (coolings). As you’ll note, they normally peak in December and January.

This post addresses why they peak at that time. But in order to show it, we have to discuss sea surface temperatures, not anomalies. Keep that in mind, please.

Also you’ll note there is a difference in the annual cycles of the sea surface temperatures in the eastern and western equatorial Pacific in Figure 4-40. The two cycles per year in the western equatorial Pacific are of course due to the fact that the Sun crosses the equator twice annually. The reasons for single cycle in the eastern equatorial Pacific are still being debated. A good overview of the proposed reasons can be found in the introduction to Wang and Wang (1999).

The following discussion is Chapter 4.7 of my book Who Turned on the Heat?

4.7 ENSO Events Run in Synch with the Annual Seasonal Cycle

We’ll discuss and present what is called the “phase locking” of ENSO events to the seasonal cycle in this chapter.

The sea surface temperature anomalies of the NINO3.4 region are warmest in December during a typical El Niño and coolest in December during a typical La Niña. That is, El Niño and La Niña events normally peak in December.

Let’s look at the normal annual cycle in sea surface temperatures (not anomalies) of the NINO3.4 region (5S-5N, 170W-120E) and the Western Equatorial Pacific (5S-5N, 120E-165E). See Figure 4-40. You’ll notice in the title block that the graph shows the cycle in the “base year” temperatures that are used to determine anomalies. As you’ll recall, that base period data is the average of 30 years of sea surface temperatures for a specific region. NOAA uses the base period of 1971 to 2000. Basically, those two curves show the average annual cycles in sea surface temperatures for those two regions for that 30-year time period. In the NINO3.4 region, the annual cycle for the average year peaks in May and the lowest temperature normally appears in December. The Western Equatorial Pacific, on the other hand, has a double cycle annually, with the warmest waters appearing there in May and November, with November winning by a nose. The sea surface temperature minimums for the Western Equatorial Pacific occur in February and August. The greatest difference between the two curves occurs in November and December.

Let’s recall our ENSO basics. During an El Niño, the NINO3.4 region is being fed warm water from the west Pacific Warm Pool; that is, the Equatorial Counter Current strengthens and carries warm water eastward. As a result, the normal annual sea surface temperature cycle in the NINO3.4 region is now being influenced by the annual cycle of the sea surface temperatures in the West Pacific Warm Pool. This is easily seen in Figure 4-41. It compares NINO3.4 sea surface temperatures (not anomalies) for the period of January1995 to January 2005 and also the series of base year cycles in the sea surface temperatures for the Western Equatorial Pacific (5S-5N, 120E-165E) and the NINO3.4 region. Also note that the La Niña events reach their lowest temperatures in December-January, which is another example of how a La Niña event is simply an exaggeration of an ENSO-neutral (average) phase.

Let’s carry that discussion farther. In the 2008 paper Seasonal Cycle–El Niño Relationship: Validation of Hypotheses, Xiao and Mechoso presented three hypotheses, two of which they confirmed with climate models. Let’s discuss those two. The first hypothesis they confirmed was:

The seasonal warming of the cold tongue in the early part of the calendar year (January–April) favors the initial growth of an event.

The NINO3.4 region captures the western portion of the cold tongue, so the annual cycle in NINO3.4 sea surface temperatures shown in Figure 4.40 above confirms the seasonal warming of the cold tongue region in the early part of the year.

Xiao and Mechoso (2008) also discussed their third hypothesis, which is:

The warm surface waters returning in the western basin from the Northern to the Southern Hemisphere toward the end of the calendar year (November– January) favor the demise of ongoing El Niño events.

The sun passes over the equator twice during the year, at the March and September equinoxes. As shown above in Figure 4-40, the sea surface temperatures in the western equatorial Pacific reach their maximums a few months later in May and November. The warm water is basically following the sun, with a lag, as it progresses between hemispheres. As Xiao and Mechoso (2008) describe, and to put it into terms of the data we’ve presented, the El Niño ebbs as the warm water passes from the western equatorial Pacific into the Southern Hemisphere.

Figure 4-42 presents the same data as Figure 4-41. The two graphs are the same, but the notes are different. I’ve highlighted the evolutions of the 1997/98 and 2002/03 El Niño events in maroon in Figure 4-42 to show that the El Niño events do grow during the normal seasonal upswing in NINO3.4 sea surface temperatures. I’ve also highlighted in green the initial decreases in sea surface temperatures that correspond to the decrease in Western Equatorial Pacific sea surface temperatures. Additionally, I’ve repeated those highlighted periods in Figure 4-43 for the period of January 2004 to May 2012 to show the evolutions and decays of the 2004/05, 2006/07 and 2009/10 El Niño events. In both illustrations, I’ve also highlighted in pink the secondary, but major, decays in the sea surface temperatures that align with the normal seasonal portion of the major drop in annual average NINO3.4 sea surface temperatures.

HHH

RECAP

El Niño events, using NINO3.4 sea surface temperatures (not anomalies) are in phase with the seasonal cycle in the eastern tropical Pacific, the cold tongue region, for the evolution and secondary decay portions of the events. The seasonal peak and decay of the El Niño are in phase with the late year cycle in the western equatorial Pacific. La Niña events appear to simply exaggerate the normal seasonal cycle in the eastern tropical Pacific.

The delayed oscillator mechanism of ENSO, Chapter 4.9, must also be considered when discussing the end of ENSO events.

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INTERESTED IN LEARNING MORE ABOUT THE EL NIÑO AND LA NIÑA AND THEIR LONG-TERM EFFECTS ON GLOBAL SEA SURFACE TEMPERATURES?

Why should you be interested? Sea surface temperature records indicate El Niño and La Niña events are responsible for the warming of global sea surface temperature anomalies over the past 30 years, not manmade greenhouse gases. I’ve searched sea surface temperature records for more than 4 years, and I’ve searched ocean heat content records for more than 3 years, and I can find no evidence of an anthropogenic greenhouse gas signal. That is, the data indicates the warming of the global oceans has been caused by Mother Nature, not anthropogenic greenhouse gases.

I’ve recently published my e-book (pdf) about the phenomena called El Niño and La Niña. It’s titled Who Turned on the Heat? with the subtitle The Unsuspected Global Warming Culprit, El Niño Southern Oscillation. It is intended for persons (with or without technical backgrounds) interested in learning about El Niño and La Niña events and in understanding the natural causes of the warming of our global oceans for the past 30 years. Because land surface air temperatures simply exaggerate the natural warming of the global oceans over annual and multidecadal time periods, the vast majority of the warming taking place on land is natural as well. The book is the product of years of research of the satellite-era sea surface temperature data that’s available to the public via the internet. It presents how the data accounts for its warming—and there are no indications the warming was caused by manmade greenhouse gases. None at all.

Who Turned on the Heat? was introduced in the blog post Everything You Ever Wanted to Know about El Niño and La Niña… …Well Just about Everything. The Free Preview includes the Table of Contents; the Introduction; the beginning of Section 1, with the cartoon-like illustrations; the discussion About the Cover; and the Closing.

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