As if one foot in mouth wasn't enough for the day, Anthony Watts does it again (archived here, cached here). Maybe he wanted to give both his clodhoppers a turn. Whatever - he posted one of his "claim" articles, which was about a new paper published in Geophysical Research Letters. The paper, by Katinka Bellomo and Amy C. Clement from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, was describing evidence of the expected slowdown in the Walker Circulation. Beneath his "claim" headline Anthony wrote:
From the UNIVERSITY OF MIAMI ROSENSTIEL SCHOOL OF MARINE & ATMOSPHERIC SCIENCE and the “correlation is not causation department” comes this paper that suggests GHG’s decrease the Walker Circulation. Only one problem. They don’t give a clear cut mechanism, only tenuous linkage. I suspect Willis will have a thing or two to say about this paper.
Only one problem, the scientists did give a clear cut mechanism. (Actually, they referred to more than one mechanism, although they emphasised the one proposed by Isaac Held and Brian Soden.) As the authors explained right at the very beginning of their paper:
In response to increasing concentrations of greenhouse gases, models simulate a weakening of tropical atmospheric overturning circulation [Held and Soden, 2006; Gastineau et al., 2008; Chou and Chen, 2010; Bony et al., 2013]. The weakening is in very good agreement across models and is projected to manifest primarily as a change in the Walker circulation [Vecchi and Soden, 2007; Chadwick et al., 2013; He et al., 2014; Di Nezio et al., 2013; Kociuba and Power, 2015].In other words, a weakening of the Walker Circulation is expected. (It's already been documented by others previously). And mechanisms have been proposed. What the authors of this new paper have done is measure the effect using different data. They did this by looking at historical observations of cloud cover as a proxy for wind velocity. They found that observations are more consistent with an externally forced response rather than merely being internal variability. Again, just to disappoint deniers who like to think that scientists all say that all the science is done and dusted and they've researched themselves out of a job, the authors include caveats to their work.
Held and Soden  propose that a weakening of overturning circulation is expected because global mean precipitation increases at a slower rate than atmospheric water vapor. This mechanism has been verified in coupled climate models [Vecchi and Soden, 2007; Chadwick et al., 2013]. Other hypotheses have been proposed to explain a weakening of tropical circulation in climate models, including ideas that dry static stability increases at faster rate than the radiative cooling of the troposphere inducing a weakening of the subsidence rate [Knutson and Manabe, 1995]; mean vertical advection of increased low-level moisture stratification in regions of ascent weakens convective mass flux [Ma et al., 2012]; and convective outflow height rises stabilizing the atmosphere and weakening convection [Chou et al., 2009; Chou and Chen, 2010]. In addition, Ma and Xie  show that the pattern of sea surface temperature (SST) change affects the response of the atmospheric overturning circulations.
Why this is important
The reason this is important is because a weakening of the Walker Circulation results in changes in rainfall patterns. It also means equatorial regions will get a lot hotter. Warming will be amplified there.
About the Walker Circulation
The Walker Circulation is the east-west atmospheric circulation pattern over the tropical Pacific. Air rises over the warm pool in the western part of the tropical Pacific and falls in the eastern Pacific. It's kept going by the easterly surface trade winds. The Walker Circulation shifts during ENSO events. In an El Nino it shifts east and in a La Nina it shifts west.
|Source: the Australian Bureau of Meteorology|
How precipitation is affected
There is a very good article by Ray Pierrehumbert and Rasmus Benestad at RealClimate.org, discussing the mechanism by which the Walker Circulation is weakened. They were writing about papers published in 2006 by Vecchi et al and Held and Soden. by I'll try to paraphrase, but recommend you read it yourself, to make sure I've got it right.
- As the surface temperature rises (with global warming), the water vapour content of the boundary layer (just above the ocean) increases a lot, by about 7% per degree of warming. The boundary layer is close to saturation, so as it gets warmer, there's more water in the air.
- Precipitation on the other hand, doesn't increase proportionally. It increases more slowly - by two or three per cent per degree of warming.
- So there's about 7% increase in water vapour content per degree of warming, but only a 2 or 3% increase in precipitation per degree of warming.
- Moisture at the lower levels rises up and rains out as in the animation above.
- To accommodate the lower increase in precipitation compared to the higher increase in water vapour content, the strength of the Walker Circulation decreases. That is, the circulation gets weaker. The parcels of moisture-laden air rise and fall more slowly than before.
A number of important aspects of the hydrological response to warming are a direct consequence of the increase in lower-tropospheric water vapor. Because the increase in strength of the global hydrological cycle is constrained by the relatively small changes in radiative fluxes, it cannot keep up with the rapid increase in lower tropospheric vapor. The implication is that the exchange of mass between boundary layer and the midtroposphere must decrease, and, since much of this exchange occurs in moist convection in the Tropics, the convective mass flux must decrease. In many popular, and in some scientific, discussions of global warming, it is implicitly assumed that the atmosphere will, in some sense, become more energetic as it warms. By the fundamental measure provided by the average vertical exchange of mass between the boundary layer and the free troposphere, the atmospheric circulation must, in fact, slow down. This large-scale constraint has little direct relevance to the question of how tropical storms will be affected by global warming, since the mass exchange in these storms is a small fraction of the total tropical exchange.
In contrast, assuming that the lower-tropospheric relative humidity is unchanged and that the flow is unchanged, the poleward vapor transport and the pattern of evaporation minus precipitation (E − P) increases proportionally to the lower-tropospheric vapor, and in this sense wet regions get wetter and dry regions drier. Since the changes in precipitation have considerably more structure than the changes in evaporation, this simple picture helps us understand the zonally averaged pattern of precipitation change.
Other recent work
The latest IPCC report discusses the above studies, but indicates that the Walker Circulation, while weakening over the twentieth century, strengthened again in recent decades:
A weakening of tropical atmosphere circulation during the twentieth century was documented in observations and reanalyses (Vecchi et al., 2006; Zhang and Song, 2006; Vecchi and Soden, 2007a; Bunge and Clarke, 2009; Karnauskas et al., 2009; Yu and Zwiers, 2010; Tokinaga et al., 2012) and in CMIP models (Vecchi and Soden, 2007a; Gastineau and Soden, 2009). The Pacific Walker circulation, however, intensified during the most recent two decades (Mitas and Clement, 2005; Liu and Curry, 2006; Mitas and Clement, 2006; Sohn and Park, 2010; Li and Ren, 2011; Zahn and Allan, 2011; Zhang et al., 2011a), illustrating the effects of natural variability.
So I guess you could say the future pattern of the Walker Circulation is still up in the air (if you'll forgive me).
From the WUWT comments
As Anthony Watts' good friend, Willis Eschenbach has said (and I paraphrase), Anthony wouldn't know good science from bad if it bit him in the proverbial. He is oblivious to the fact that this topic has been the subject of a lot of research, with quite a few papers published as a result. And his fans are almost as bad, as the first three comments show.
November 5, 2015 at 8:25 am
I think the study of kangaroo farts had more credibility !!
November 5, 2015 at 8:28 am
Correlation does not equal causation – the weakening of the Walker circulation just MAY be due to other factors!!!!!!!!!!
November 5, 2015 at 10:35 am
Too bad these folks are spewing all this speculation as if it were empirical, which when proven false will damage their credibility in the science community.
James at 48 has a random thought. He doesn't explain himself unfortunately, so we'll never know what was rattling around in his head to make him think the work is bogus:
November 5, 2015 at 8:34 am
Hold up. Walker Circulation is totally interwoven with ENSO and PDO. The bogus meter is pegged.
I wonder if timetochooseagain knows where Bob Tisdale gets his sea surface temperature charts from? Or if he's ever wondered how scientists know that there was a warm spell in the Arctic that caused the ice to retreat early last century?
November 5, 2015 at 11:23 am
Ah hahaha oh my god the cloud observations from the freaking ship tracks? Really? Do they have any idea how unreliable that kind of thing is going back a century?.
Of course they do. But “I can’t find this effect in a relatively reliable dataset, so let’s try an unreliable one instead!” is standard fare by now in Climate Science (TM)
November 5, 2015 at 1:25 pm
“In Science, when we understand a system, we can predict its behavior. If not, we do not understand the system.”-John Christy
Climate models simulate a weakening of the Walker circulation in response to increased greenhouse gases, but it has not been possible to detect this weakening with observations …” -Katinka Bellomo and Amy C. Clement
Another admission that these models have no predictive ability. But let’s go ahead and use broken models to implement world policy. (Evil laughter now: Bwa ha ha ha…) Let’s all meet in Paris and take another crack at breaking open that big carbon money bomb.
Reading Anthony's pronouncements and the sort of comments he attracts, I wonder how any sane person can for a second confuse WUWT with a science blog - not even fake sceptics.
References and further reading
- Study of cloud cover in tropical Pacific reveals future climate changes - press release at Science Daily
Held, I. M., and B. J. Soden (2006), "Robust responses of the hydrological cycle to global warming," J. Clim., 19, 5686–5699. doi: http://dx.doi.org/10.1175/JCLI3990.1 (open access)
Vecchi, Gabriel A., Brian J. Soden, Andrew T. Wittenberg, Isaac M. Held, Ants Leetmaa, and Matthew J. Harrison. "Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing." Nature 441, no. 7089 (2006): 73-76. doi:10.1038/nature04744 (pdf here)
On a Weakening of the Walker Circulation - Article by Ray Pierrehumbert and Rasmus Benestad at RealClimate.org, June 2006
Ma, Jian, Shang-Ping Xie, and Yu Kosaka. "Mechanisms for tropical tropospheric circulation change in response to global warming*." Journal of Climate 25, no. 8 (2012): 2979-2994. doi: http://dx.doi.org/10.1175/JCLI-D-11-00048.1