ENSO Basics: Westerly Wind Bursts Initiate an El Niño

This may seem like an unusual time to have this discussion.  We’re presently in ENSO (El Niño-Southern Oscillation)-neutral conditions, meaning the tropical Pacific isn’t experiencing an El Niño or a La Niña.  And based on the ENSO-forecasting models, NOAA says “ENSO-neutral is expected into the Northern Hemisphere spring 2014”. (See the NOAA weekly ENSO Update.)

But the question “what initiates an El Niño?” was raised again recently on the thread of a blog post at WattsUpWithThat, so I thought I’d bring the answer forward as a post.

This post includes 2 chapters of my book Who Turned on the Heat?  The first is the basic discussion of the transition from ENSO-neutral conditions to El Niño (Chapter 3.5).  I’m providing it for people who aren’t too familiar with the fundamental processes that take place during the evolution of El Niño events.  The second chapter included in this post is the discussion of what initiates an El Niño beyond my usual basic description of “a weakening of the trade winds” (Chapter 4.15).  There are many other topics discussed in Who Turned on the Heat?  See the Table of Contents in the Free Preview.

# # #

3.5 The Transition from ENSO-Neutral to El Niño

While the tropical Pacific is in the ENSO-neutral phase, the trade winds push the warm water to the west so that sea level there is higher than it is in the east.  The sea level would like to be the same height because of gravity, but the trade winds are holding all of that warm water in place. The trade winds are being reinforced by the warmer sea surface temperatures in the western tropical Pacific. Similarly, the warmer waters in the western tropical Pacific are being reinforced by the trade winds. They are providing positive feedback to one another (Bjerknes feedback).  With the positive feedback, the tropical Pacific would tend to stay in the ENSO-neutral mode. Something has to force things to change.

The trade winds in the western Pacific are, like all winds, quite variable. They strengthen and weaken with changes in weather. The size and shape of the Pacific Warm Pool varies in response.  However, under “normal” conditions, the trade winds continue to hold the warm water in the western tropical Pacific.

Suppose the trade winds were to weaken to the point, and weaken long enough, that they could no longer hold the warm water in place in the west Pacific Warm Pool.  The water would slosh to the east.

That’s how an El Niño event starts. Figure 3-10 shows the ENSO-neutral conditions changing to El Niño.

As shown in Figure 3-10, the Equatorial Countercurrent in the Pacific carries the warm water eastward.  Normally, it’s a relatively small current compared to the North and South Equatorial Currents, but during an El Niño, the Pacific Equatorial Countercurrent becomes much larger. See Figure 3-11.

Figure 3-11 presents maps that show the direction of the currents in the central portion of the tropical Pacific. The eastward-flowing Equatorial Countercurrent is shown in dark blue. The westward-flowing North and South Equatorial Currents are shown in the off-color green. The top map shows the relatively small Equatorial Countercurrent in December 1996, which was an ENSO-neutral month.  The bottom map shows that it’s much larger near the peak of the 1997/98 El Niño in December 1997.  The maps are available through the NASA Ocean Motion website, at their OSCAR webpage.

A couple of years ago, I used those maps of the tropical Pacific Ocean currents to create a series of animations that I presented on YouTube.  The animations capture the strengthening of the Equatorial Countercurrent during the transition from ENSO-neutral phase to the 1997/98 El Niño phase and its subsequent weakening as the El Niño event transitions back toward ENSO-neutral.  Because there are multiple animations showing different portions of the tropical Pacific, I’ll refer you to the post Equatorial Currents Before, During, and After The 1997/98 El Niño.

Figure 3-12 compares two ENSO-related variables:  western equatorial Pacific trade wind strength and NINO3.4 sea surface temperature anomalies, our ENSO index.  The NINO3.4 sea surface temperature anomalies have been scaled and inverted (multiplied by a scaling factor of -2.0) so that the variations in both datasets are in the same direction.  That is, the El Niño events are now the large downward spikes.  The western equatorial Pacific trade wind data is from the NOAA/Climate Prediction Center Monthly Atmospheric & SST Indices webpage.  There it’s identified as “850 mb Trade Wind Index (135°East-180°West) 5°North-5°South West Pacific”.  The Trade Wind Index data presented in the graph are the anomalies, which are the second group here. As illustrated, the El Niño events are preceded by significant drops in western equatorial Pacific trade wind strength. 

  RECAP

In Figure 3-11, we confirmed that the Equatorial Countercurrent enlarges during an El Niño, carrying the warm water from the Pacific Warm Pool eastward.  The dark green curve of the trade wind anomalies leads the sea surface temperature in Figure 3-12.  This confirms that a weakening of the trade winds in the western tropical Pacific happens a number of months before the NINO3.4 sea surface temperature anomalies register the El Niño event. In other words, it takes a couple of months after the weakening of the west Pacific trade winds for the enlarged Equatorial Countercurrent to carry the warm water east so that it warms the sea surface temperatures of the NINO3.4 region.

We’ll expand on this discussion, introducing a phenomenon called a Kelvin wave, in “Chapter 4.8 Subsurface Temperature and Temperature Anomaly Variations in the Equatorial Pacific And an Introduction to Kelvin Waves” [not included with this blog post].

# # #

4.15 Further Discussion on What Initiates an ENSO Event

When Anthony Watts cross posts one of my blog posts about ENSO at his widely read blog WattsUpWithThat, a question that’s often asked is “What initiates an El Niño?”  My reply is typically something to the effect of: An El Niño event is initiated by a weakening of the tropical Pacific trade winds.  This allows the warm water that had been held in place in the west Pacific Warm Pool to slosh east.

Sometimes there’s the follow-up question “What causes the trade winds to relax?” My usual reply is:  There are a number of causes and they vary.

This doesn’t satisfy some people who are looking for a single definite answer, but, unfortunately, it’s true. There are numerous scientific papers that discuss this fact.  In this chapter, we’ll present relatively simple descriptions of the many factors that cause the relaxing of the trade winds.

A phenomenon known as a Westerly Wind Burst (WWB), also known as a Westerly Wind Event (WWE), accompanies the relaxed trade winds. If you wanted to investigate this further, those would be the phrases to use in your searches.  There are multiple causes of Westerly Wind Bursts, including:

1. Cross-equatorial tropical cyclones in the western tropical Pacific. This refers to a time when one tropical cyclone exists north of the equator in the western tropical Pacific, while, at the same time, another tropical cyclone exists there but south of the equator. The tropical cyclone winds in the Northern Hemisphere rotate counterclockwise and in the Southern Hemisphere they’re clockwise.  Between them, the winds would be traveling from east to west west to east.     These are discussed in Keen (1982) The Role of Cross-Equatorial Tropical Cyclone Pairs in the Southern Oscillation.

2. A single cyclone and series of cyclones in the western tropical Pacific. These are discussed in Hartten (1996) Synoptic settings of westerly wind bursts.

3.  Cold surges from mid-latitudes, discussed in Harrison (1984) The appearance of sustained equatorial surface westerlies during the 1982 pacific warm event

4. Convective cloud clusters associated with the Madden–Julian oscillation (MJO).  Refer to Zhang (1995) Atmospheric Intraseasonal Variability at the Surface in the Tropical Western Pacific Ocean.

As noted earlier, there are a plethora of other papers that discuss these factors. There is a good overall discussion in Vecchi and Harrison (2000) Tropical Pacific Sea Surface Temperature Anomalies, El Niño, and Equatorial Westerly Wind Events.

Then, after you’ve digested all of the factors that can trigger El Niño events, there’s a study that could shift your understanding once again. The paper is Yu et al (2003) Case analysis of a role of ENSO in regulating the generation of westerly wind bursts in the Western Equatorial Pacific. Yup, you read that title correctly. Yu et al (2003) found that ENSO can create favorable background conditions for westerly wind bursts.  In other words, ENSO has the built-in ability to trigger itself.

RECAP

El Niño events are initiated by the relaxation of the trade winds associated with a single or a series of Westerly Wind Bursts in the western tropical Pacific. The causes of Westerly Wind Bursts are tropical cyclones (individual, multiple, and cross equatorial), cold surges from the mid-latitudes, and convection associated with the Madden–Julian oscillation (MJO), or a combination of them. To complicate things, there are indications that ENSO can create the background conditions that promote Westerly Wind Bursts.

[End of reprints from Who Turned on the Heat?]

Something to ponder before you argue that ENSO can’t create the conditions that help to initiate an El Niño, consider that Yu et al (2003) are considering low-frequency ENSO-neutral conditions to be a part of ENSO.

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 31+ years, not manmade greenhouse gases.  I’ve searched sea surface temperature records for more than 5 years now, and I’ve searched ocean heat content records for more than 4 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.

For a further discussion, see the essay (pdf) titled The Manmade Global Warming Challenge. (It’s 42MB, but it’s free and worth the download time.)

Last year, I 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.

Please buy a copy. (Credit/Debit Card through PayPal.  You do NOT need to open a PayPal account.) Simply scroll down to the “Don’t Have a PayPal Account” purchase option.  It’s only US$8.00.  Sales of my ebooks allow me to continue my research into human-induced and natural climate change and to continue to blog here at Climate Observations and at WattsUpWithThat?

Regards