Answer to the Question Posed at Climate Etc.: By What Mechanism Does an El Niño Contribute to Global Warming?

Late last week, Judith Curry published a post written by Donald Rapp titled El Ninos and La Ninas and Global Warming. Donald referred to my work a number of times in that post. That, of course, created the expected responses from warmists who can’t grasp or refuse to understand the subtleties of ENSO. I made a quick appearance there and promised to answer a question by one of the bloggers. The following is that detailed answer. It appears on that Climate Etc. thread here. (At the time this post was published, my comment there was awaiting moderation.)

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My apologies for not replying earlier. I was doing things that help pay my bills.

I see that the clueless usual suspects are misrepresenting my work and expressing their total misunderstanding of ENSO. But that’s their choice, and it’s a waste of my time responding to their nonsense.

I did promise GaryM that I would answer his question, if he were to rephrase it. And his rephrased question was “By what mechanism does an El Nino ‘contribute to global warming?’”

Through numerous mechanisms, as I stated in my earlier reply to GaryM.

First, in the tropical Pacific, ENSO serves as a chaotic, sunlight-fueled, naturally occurring, recharge-discharge oscillator. That’s one of the fundamental discussions about ENSO contained in Trenberth et al. (2002). They write (my boldface):

“The negative feedback between SST and surface fluxes can be interpreted as showing the importance of the discharge of heat during El Niño events and of the recharge of heat during La Niña events. Relatively clear skies in the central and eastern tropical Pacific allow solar radiation to enter the ocean, apparently offsetting the below normal SSTs, but the heat is carried away by Ekman drift, ocean currents, and adjustments through ocean Rossby and Kelvin waves, and the heat is stored in the western Pacific tropics. This is not simply a rearrangement of the ocean heat, but also a restoration of heat in the ocean.”

The NODC Ocean Heat Content data for the tropical Pacific clearly shows the recharge phase during specific La Niña events. They include the 1973-76 La Niña, the 1995/96 La Niña and the 1998-01 La Niña. See the graph here. The impacts of those La Niña events are hard to miss. In fact, the vast majority of the warming of ocean heat content in the tropical Pacific was during La Niña events. I discussed the processes through which the ocean heat content for the tropical Pacific warms in that fashion in the post here, with support from a number of papers and datasets.

ENSO also causes ocean heat from the tropical Pacific to be distributed poleward and into the Indian Ocean. This can be plainly seen in the animation of JPL sea level residual maps here after the end of the 1997/98 El Niño. The impact of the slow-moving downwelling Rossby wave traveling from east to west is blatantly obvious. It’s carrying ENSO residuals back to the western tropical Pacific as the 1998-01 La Niña is forming. So while the trailing (recharging) La Niña is taking place, the left-over warm water from the El Niño is being distributed to the adjoining ocean basins. And where’d all that warm water come from? The 1995/96 La Niña, of course, as confirmed by the data in the graph linked earlier (here).

And that brings us to the discharge phase. An El Niño discharges heat stored in the form of warm water from below the surface of the West Pacific Warm Pool, or as Trenberth et al. (2002) noted “the heat is stored in the western Pacific tropics.” That discharged warm water rises to the surface through upwelling and releases heat to the atmosphere during the El Niño (primarily through evaporation), and then all of the remaining warm water that was discharged by the El Niño is subsequently redistributed around the global oceans at the conclusion of the El Niño, some on the surface and some below the surface of the oceans. Let’s rephrase that because it’s important. The discharge phase is the release of “the heat [that] is stored in the western Pacific tropics”, which is taking subsurface warm water from the West Pacific Warm Pool and initially relocating it into the eastern tropical Pacific where it is upwelled to the surface and then subsequently redistributed at the conclusion of the El Niño. One of the byproducts of that discharge of warm water from “western Pacific tropics” is the release of heat to the atmosphere, and that’s what many persons focus on, but it’s only a portion of the discharge phase.

Which brings us to the Trenberth “big jumps” in response to the strong El Niño events of 1986/87/88, 1997/98 and 2009/10. (I initially discussed Trenberth’s “big jumps” article here.) Those “big jumps” are plainly evident in the sea surface temperature anomalies of the South Atlantic, Indian and West Pacific Ocean. See the graph here. Two things are stand out in that graph: (1) the warming of the sea surface temperatures for the South Atlantic-Indian-West Pacific occurs during the El Niño events and (2) the sea surface temperatures there do not respond proportionally during the trailing La Niñas. Those upward shifts are the long-term responses to the discharge phases of ENSO that occurs during strong El Niños. As part of the discharge phase of ENSO, the El Niño takes warm water from below the surface of the western tropical Pacific and places it on the surface (warm water that was created by the increased sunlight during the prior recharging La Niña). The discharged warm water floods into the East Pacific, where it temporarily raises sea surface temperatures during the El Niño, but causes little long-term trend there. And at the end of the El Niño, the warm water is redistributed by the renewed trade winds, ocean currents and the downwelling Rossby wave into the West Pacific, Indian Ocean and eventually the South Atlantic. The East Pacific represents about 33% of the surface of the global oceans, and the South Atlantic-Indian-West Pacific covers another 52%. That leaves the North Atlantic, which has another mode of natural variability called the Atlantic Multidecadal Oscillation. The Atlantic Multidecadal Oscillation, according to NOAA, can contribute to or suppress global warming. And so far, the only global surface warming we’ve seen was in the South Atlantic-Indian-West Pacific subset and that warming was caused by discharge of sunlight-created warm water released from below the surface of the West Pacific Warm Pool during El Niño events.

Speaking of the North Atlantic, and back to GaryM’s basic question, an El Niño causes surface temperatures in many regions outside of the tropical Pacific to warm not as a result of the direct transfer of heat, but as a response to changes in atmospheric circulation. The processes through which those changes occur were discussed in minute detail in Trenberth et al. (2002), which was why I linked that paper for GaryM earlier.

There are a few less-technical discussions in Trenberth and Fasullo (2011). Here’s a portion where they confirm that ENSO acts as a sunlight-fueled recharge-discharge oscillator:

“Typically prior to an El Niño, in La Niña conditions, the cold sea waters in the central and eastern tropical Pacific create high atmospheric pressure and clear skies, with plentiful sunshine heating the ocean waters. The ocean currents redistribute the ocean heat which builds up in the tropical western Pacific Warm Pool until an El Niño provides relief (Trenberth et al. 2002).”

Then Trenberth and Fasullo (2011) explain why some portions of the oceans warm in response to an El Niño (my boldface):

“But a major challenge is to be able to track the energy associated with such variations more thoroughly: Where did the heat for the 2009–2010 El Niño actually come from? Where did the heat suddenly disappear to during the La Niña? Past experience (Trenberth et al. 2002) suggests that global surface temperature rises at the end of and lagging El Niño, as heat comes out of the Pacific Ocean mainly in the form of moisture that is evaporated and which subsequently rains out, releasing the latent energy. Meanwhile, maximum warming of the Indian and Atlantic Oceans occurs about 5 months after the El Niño owing to sunny skies and lighter winds (less evaporative cooling), while the convective action is in the Pacific.”

Isn’t that curious? That’s two papers by Trenberth which state that temporary blasts of sunlight associated with ENSO warm the oceans of the tropical Pacific, the Atlantic Ocean and the Indian Ocean. And what processes can’t climate models simulate properly? ENSO and teleconnections. No wonder they come up with the wrong answer.

Looking at the ocean heat content data for the tropical Pacific (here), provides the answer to Trenberth and Fasullo’s question “Where did the heat for the 2009–2010 El Niño actually come from?” The supply of warm water for the 2009/10 El Niño was initially created during the 1998-01 La Niña, and it replenished the warm water released by the 1997/98 El Niño—warm water that was created during the 1995/96 La Niña. And according to two Trenberth papers, that warm water is created by temporary blasts of sunlight.

One of the key questions in that quote that Trenberth and Fasullo failed to answer was, “Where did the heat suddenly disappear to during the La Niña?” It didn’t disappear. The JPL sea level animation of sea level residual maps linked earlier (here), and the graph of sea surface temperatures of the South Atlantic-Indian-West Pacific subset (here) show quite plainly that the left-over warm water from the 1986/87/88, 1997/98 and 2009/10 El Niño events was redistributed within the oceans to form the Trenberth “big jumps”.

And as Trenberth and Fasullo (2011) note, while the tropical Pacific is releasing heat to the atmosphere, the Atlantic and Indian Oceans are gaining heat due (1) weaker surface winds, which results in less evaporative cooling, and (2) the resulting reduction in cloud cover which allows sunlight to penetrate the oceans to depth. The effect of ENSO on the sea surface temperatures of the tropical North Atlantic is discussed in Wang (2005), and it helps confirm the statement by Trenberth and Fasullo.

I provided graphs in a recent post (here) and in my book Who Turned on the Heat? that confirm Trenberth and Fasullo’s claim that ENSO does in fact cause variations in the ocean heat content of those ocean basins. Those graphs compared an ENSO index to ARGO-era ocean heat content for the Indian Ocean and the tropical Atlantic. The Indian Ocean to depths of 700 meters (here) warms in response to the El Niños but does not cool proportionally during La Niñas. The failure to cool proportionally is likely a result of the warm water released by the El Niño being distributed into the Indian Ocean by the Rossby waves and the Indonesian Throughflow during those strong La Niñas.

For the tropical North Atlantic (here), we can see that the ocean heat content there warmed in response to the 2009/10 El Niño and cooled during the La Niñas before and after it.

Let’s look again at the tropical Pacific. In the animation here, we can watch ENSO distribute heat from the tropical Pacific to the adjoining ocean basins. It doesn’t take much of a brain trust to figure out what will happen during a period when El Niño events dominate ENSO. Global surface temperatures and ocean heat content (even remotely in the North Atlantic) will warm. They have to.

I’ve only glanced at the some of the arguments on this thread, those that argue against what I have presented over the past 4 to 5 years. They’re repeats of arguments I have replied to over the years. Examples are here and here. And some of the comments on this thread were from persons with limited grasps of the truth.

To close this comment, this is not a theory I created. In fact, I haven’t presented a theory. I present data…data which illustrates, supports and expands on known processes presented in peer-reviewed papers.

Thanks again to Donald Rapp and Judith Curry for raising this important topic. And thanks to GaryM for asking a simple question that required a detailed answer. And I will again apologize for taking so long to reply to GaryM’s question, but I had other obligations.

It’s unlikely that I’ll be back to this thread, so if you have questions that were not answered by this reply or questions that were not answered in any of the blog posts linked to it, please feel free to ask it at my blog Climate Observations.

[End of comment on Climate Etc.]

The closing paragraph was of course about returning to the thread at Climate Etc. And this is the post at Climate Observations linked for those with questions.