Almost a mea culpa in today’s publication of The impact of global warming on the tropical Pacific Ocean and El NiÃ±o (CSIRO and PDF) by a who’s who of atmospheric circulation research including Vecchi, and CSIRO/BoM researchers Cai and Power.
Therefore, despite considerable progress in our understanding of the impact of climate change on many of the processes that contribute to El NiÃ±o variability, it is not yet possible to say whether ENSO activity will be enhanced or damped, or if the frequency of events will change.
This is illustrated by figure 3, where some CGCMs show an increase in the amplitude of ENSO variability in the future, some show a decrease, some show no statistically significant changes.
Their paper flatly contradicts Power and Smith  that increased El Ninos are caused by weakening of the pressure differential, indicated by the SOI:
We also document what appears to be a concurrent period of unprecedented El Nino dominance. However, our results, together with results from climate models forced with increasing greenhouse gas levels, suggest that the recent apparent dominance might instead reï¬‚ect a shift to a lower mean SOI value.
and that this weakening is caused by anthropogenic global warming by Vecchi et al. :
The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing.
Accurately reproduced by climate model simulations indeed.
The physics of the situation was previously claimed to depend mainly on the strength of the trade winds, related to the tropical atmospheric circulation, called the Walker circulation. The present paper says El Ninos are more complex, but still supports the dubious claim of circulation weakening:
Because of the reduction in the vertical circulation expected as global temperatures rise, we also expect a reduction in the surface trade winds associated with a weaker Walker circulation (figure 1d). Refs 25, 26, 27, 28, 29 and 30 find a consistent picture of weakening trades in both observations and models as measured by the reduction in the east-west mean sea level pressure gradient across the tropical Pacific (figure 1c). In CGCMs, the weakening of the equatorial trade winds drives a reduction of equatorial oceanic circulation, including equatorial Pacific upwelling25,26,31.
As some might remember, it was demonstrated in a series of posts that contrary to their assertion there is no statistically significant evidence of a change in the east-west pressure gradient (as indicated by the Southern Oscillation Index) (Recent Data Show No Weakening of the Walker).
We are not alone. John McLean also commented on the beat-up of the Walker circulation in Ignoring a Natural Event to Blame Humans:
In the last week of September 2007 we had yet another example of a well-recognized natural climate event being ignored in order to sell the notion that mankind is responsible for global warming. Maybe it was deliberate or maybe just ignorance, but you’d think that capable scientists would look closely at prior research and the data and not just be activists for their latest cause.
This time it was Power and Smith, from Australia’s CSIRO and Bureau of Meteorology respectively, who were reporting a weakened Walker Circulation over the last 30 years and a concurrent period of unprecedented El NiÃ±o dominance [note 1], both of which they blamed on human activity.
Last year in May it was Vecchi et al  who told us that the same Walker Circulation had weakened by 3.5% since the mid-1800s and there that there was a just 1% probability that this was due to natural events. Vecchi and Soden  recently continued their line of argument from 2006 by claiming that an ensemble of 23 climate models confirms that weakening of the Walker Circulation is to be expected under anthropogenic warming.
These three papers seem to be the product of researchers lost in their computer simulations and putting the virtual reality of computer models ahead of observational reality.
What they attribute to human activity are natural events that have been well described by other researchers.
McLean concludes that there that there is good evidence the Great Pacific Climate Shift in 1976 changed the upwelling of cold water and moved the Pacific Ocean into a warmer state, which means towards El NiÃ±o conditions, something we showed to be statistically significant in Structural break models of climatic regime-shifts: claims and forecasts.
Despite acknowledging not knowing what the impact of global warming on the tropical Pacific Ocean and El NiÃ±o will be, despite models that produce every possible outcome, despite evidence to the contrary, and being wrong before, they still confidently assert in the abstract that:
Under the influence of global warming, the mean climate of the Pacific region will probably undergo significant changes. The tropical easterly trade winds are expected to weaken;
Why do they have such confidence in the weakening of the atmospheric circulation with global warming? The way the deduce the weakening from the slow increase in precipitation with global warming is as follows:
As the global mean temperature rises, the global mean saturated water vapour pressure should also rise at a rate of approximately 7% per degree K of warming; the Clausiusâ€“Clapeyron relation being close to linear over a few degrees of temperature change. If the relative humidity remains constant, then the global mean vapour pressure should also rise at approximately 7% per degree K and this is observed in different CGCMs (figure 1a). In ref 24, the authors note that the global mean precipitation rate in CGCMs rises at a much smaller rate of only 1.2% per degree K of warming in the experiments analysed here – figure 1b. If we assume that the global mean precipitation rate is proportional to the water vapour mixing ratio multiplied by mass-mixing rate (the rate of exchange of moist boundary layer air with the dryer air above), then the slower rate of global precipitation change in comparison to the faster rate of humidity change implies that there must be a reduction in this mass mixing rate. In association with this reduction in mass flux, there should be a reduction in vertical motion in the main convective regions of the tropics and this should lead to a general reduction in the strength of the atmospheric vertical overturning circulation.
The only problem is, precipitation is not decreasing with warming, and the reduction in wind speed is simply circular reasoning, as described in a brilliant article noted on Pielke Sr’s blog by an environmental lawyer Jason Johnston that :
Quite surprisingly, this is not what climate change models predict. The current set of coupled ocean-atmosphere GCMâ€™s predict substantial increases in atmospheric water vapor (in the range of 7% per degree centigrade) as a consequence of CO2 forced temperature increases. Direct application of the Clausius-Clapyeron relation would imply that such an increase in atmospheric water vapor would lead to a predicted increase in precipitation of roughly the same magnitude, around 6-7% per degree centigrade. However, the climate models predict substantially smaller increases in precipitation, in the range of only a 1-3.5%. The discrepancy between increases in precipitation predicted by climate models and what one would predict based on fundamental thermodynamic relations is in fact even greater than this because, as noted above, other things equal, evaporation should increase as the surface temperature warms.
Climate models do not in fact predict such big increases in evaporation, and it is apparently in large part due to the relatively modest increases in evaporation that such models predict that the models also generate relatively small predicted increases in precipitation. As the models predict that air-sea temperature differentials and relative humidity will remain relatively constant, the only variable that can change to lower the predicted increase in evaporation (and hence precipitation, under the long-term constraint that precipitation, P, equals evaporation, E, in the global, closed system) is the surface wind stress. That is, to get the â€œmuted response of precipitation to global warmingâ€ predicted by GCMâ€™s â€œrequires a decrease in global windsâ€ brought about by changing global atmospheric circulation patterns.
Here Johnston is describing a study by Frank Wentz in How Much More Rain Will Global Warming Bring? finding the observations suggest that precipitation and total atmospheric water have increased at about the same rate over the past two decades. For the same reason, CSIRO scientists have been predicting increased drought in Australia, even though rainfall has increased significantly.
Johnston describes this as yet another example of the climate establishmentâ€™s rhetorical strategy of simply ignoring disconfirming evidence. So what do they say about the argument of Wentz in today’s review the current state of knowledge? They don’t even mention it. Johnston continues:
And when one looks closely at the scientific literature, it turns out that some of the most crucial (and actually testable) predictions or assumptions underlying predictions of dangerous climate change are not in fact being confirmed by observations. Newspapers are full of stories about melting ice sheets; what they neglect to report is that recent work shows that changes in clouds and precipitation â€“ crucial to the predictions of climate models â€“ are not what those climate models have assumed.
This one-sided treatment of disconfirming evidence is endemic:
The cross examination conducted below not only uncovers these findings in the climate science literature, but shows that they are almost completely ignored by the IPCC in its communications to policymakers and the media (Summaries for Policymakers and Technical Summaries that accompany its full Assessment Reports) and by other members of the climate establishment in their popularly accessible reviews and assessments of the state of climate science. Rather than laying out contrasting positions that one finds in the literature, the IPCC and other leading establishment climate scientists either simply ignore or tersely dismiss scientific work that disputes or casts doubt upon the assumptions underlying or projections made by climate models and establishment climate science more generally. My cross examination clearly reveals a rhetoric of persuasion, of advocacy that prevails throughout establishment climate science.