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What’s the hold up, El Niño?

In the July 10update and ENSO discussion, we said the atmospheric part of ENSO doesn’t seem to be responding to the ocean.  El Niño requires that both be in sync and coupled with each other.  Why is the atmosphere acting aloof to the rather warm ocean?  This development may be especially surprising to folks given the rumors and speculation of a very strong El Niño that followed March’s oceanic Kelvin wave.

In June, the Bureau of Meteorology in Australia made an interesting observation that might shed light on the lack of coupling between the ocean and atmosphere (1).  They pointed out that an anomalous sea surface temperature (SST) gradient was not in place across the equatorial Pacific Ocean.  Like the gradient of a hill on a highway, an SST gradient describes a change in temperature across the ocean surface from one location to another (2).  

The maps below show the typical evolution of SST anomalies leading up to wintertime El Niño events (winter is when ENSO is strongest).  In this post, the SST gradient will refer to the difference in SST anomalies.  “Anomalies” are the departures from the average state of the atmosphere or ocean.  Blue shading shows where SSTs are below average, and red shading shows where SSTs are above average.  In this case, the gradient refers to the more negative (cooler) SST anomalies in the western Pacific (blue shading) and the more positive (warmer) SST anomalies in the eastern Pacific (red shading).

Three rows of small global maps showing the evolution of a typical El Niño from April through December

The typical evolution of sea surface temperature (SST) anomalies from April through December associated with El Niño.  These maps were created using lagged regression onto the wintertime Niño-3.4 index using monthly data from ERSSTv3b from 1980-2012.  The SST gradient is shown by the difference in near-to-below-average SST (white or blue) across the far western tropical Pacific and Indonesia and the above-average SST (red) across the central and eastern Pacific.  Map by Michelle L'Heureux, Climate Prediction Center.

So who cares whether “SST gradients” in the equatorial Pacific are in place?  Well, as it turns out, the atmospheric circulation often responds to these SST gradients, and if they do not exist then it is possible that the growth of El Niño will be less energetic or reduced—like a car that will coast downhill (big gradient) on its own but not across a flat stretch of road (no gradient).    

A stronger SST gradient produces stronger winds (3) blowing across the equatorial Pacific.  A weaker gradient results in weaker winds.  This concept can also work in reverse: stronger winds can lead to stronger SST gradients and weaker winds contribute to weaker SST gradients (4). 

Normally, the SST gradient associated with ENSO becomes stronger as the winter approaches.  The figure above shows the growth of the ENSO-related gradient between the western and eastern SST anomalies for the months of April through December.  In April, May, and June, the gradient is typically not very strong, though there is still a difference between the warmer SSTs in the eastern Pacific and the near average SSTs in the western Pacific (near average SSTs shown by the light colors or white regions).  

This year however, the SST gradient is weak- to- nonexistent, as shown by above-average SSTs in both the western and eastern equatorial Pacific Ocean.  

Trio of small global maps showing ocean surface temperature patterns in Apri, May, and June 2014

Recent sea surface temperature (SST) anomalies (or departure from average) in April, May, and June 2014. Unlike the typical El Niño progression, both eastern and far western tropical Pacific SSTs have been above average (warmer). Map by Michelle L'Heureux, Climate Prediction Center.  Data from ERSSTv3b (NOAA NCDC) using a 1981-2010 base period.

Because of these coupled feedbacks (e.g. winds can both cause and be caused by the SST gradient), it is difficult for ENSO forecasters to determine whether the ocean is pushing the atmosphere or the atmosphere is pushing the ocean.  Think of it as a chicken-and-egg problem.  The March 2014 oceanic Kelvin wave resulted in positive (warm) SST anomalies in the eastern Pacific. Normally, those conditions would trigger changes in the winds, pressure, and rainfall

However, near Indonesia and the western Pacific, SSTs have been above average, which is not typical during El Niño development.  It is possible that, in order for the atmosphere to better sync up with the ocean and for this El Niño to emerge and grow, the SST anomalies in the western Pacific and Indonesia will need to become near-to-below average, strengthening the SST gradient (5).   Forecasters will be watching closely for evidence of this change going forward.


(1) In their June 3rd 2014 ENSO wrap-up the Australians wrote “above-average sea surface temperatures (SSTs) also extend into the western tropical Pacific, meaning strong west to east gradients in tropical Pacific SST anomalies are yet to become established.”  Their current ENSO discussions are available here.   We, of course, prefer you look our CPC/IRI ENSO discussion, but we rather enjoy and appreciate the Australian perspective on ENSO. 

(2) Gradients can exist in any direction.  Here we are referring to the “zonal” or east-west SST gradient, but one could also look at the “meridional” or north-south SST gradient. 

(3) If SST anomalies are being compared (as is the case in this post), then the stronger and weaker winds refer to the anomalous winds.  

(4) This example is referred to as the “Bjerknes feedback” and is a positive feedback that helps to grow ENSO events.  “Positive” feedback processes amplify the initial change (i.e. stronger SST gradients --> stronger winds --> stronger SST gradients), whereas “negative” feedbacks will act to dampen/counteract the initial change. 

(5) This is a working hypothesis and requires more rigorous research and attribution.


Bjerknes, J., 1969: Atmospheric Teleconnections from the Equatorial Pacific. Mon. Wea. Rev., 97, 163–172.


Don't know if is just my opinion. But since the globe is getting warm. Shouldn't be already expecting this. I mean. For water to get cooler it will need something that can actually ignite the cooling. But haven't been. I also can state the weather in Honduras haven't been this hot in June-July since i've been here (2002-2014). Finally. I think global temperatures (on the rise) is factor that need to be taken as important when looking for el niño sign. It's very intresting the report that Dr. Jeff Masters present in his blog about average temperatures worldwide. Intresting article.

Over the last 60 years one of the most significant SST trends has been positive trends (warming) over Indonesia/western Pacific. At this point, it's somewhat speculative to mention a link between these SST trends and what we are seeing *this year*, but it is certainly worthwhile to do some research on it .

In reply to by Sem

Today -. What is the probability of occurrence of a phenomenon El Niño?

So one part of the ocean holds El Nino hostage! I always check both CPC and BoM enso updates/indices and they are very informative. And as East Africa, where we usually benefit with enhanced rainfall, we look forward to see whether El Nino 2014 will be a reality.

Great blog! I see a fairly strong Pacific Decadal Oscillation pattern forming in the April through June images. Is it possible that PDO is having an effect on the strengthening of ENSO?

It is difficult for us to untangle how much decadal variability impacts ENSO from month-to-month, so it is hard to say either way.   Also, when monthly Pacific Decadal Oscillation Indices are calculated, we find that much of that variability is due to ENSO.  In other words, on shorter timescales, it likely that much of what you see in the North Pacific SST pattern is caused by ENSO (not the other way around). 

We hope to have a future blog post on this topic in the coming months, so stay tuned.

In reply to by kevin

As the cold artic glaciers melt into the ocean the waters are cooling. When the glaciers are all melted the ocean waters will warm up.

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