A paper well worth reading on the relationship between the Walker circulation and ENSO is Weakening of the Walker Circulation and apparent dominance of El Nino both reach record levels, but has ENSO really changed?, by Scott Power of the Australian Bureau of Meteorology, and Ian Smith of CSIRO. It is very well written and helpful. They aim to prove a notion:
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 reflect a shift to a lower mean SOI value.
The idea is that global warming is causing a weakening of the Walker circulation, and consequently more negative SOI values, on average. Dr Jones has proposed that this is the reason the cumulative sum of SOI is highly correlated with global temperatures. I think the data only show the ‘period of unprecedented El Nino dominance’ between 1976 and 1998 produces the trend, and cSOI largely matches all the wiggles in between.
Perhaps the difference is: a belief in a generalized weakening in the Walker circulation related to GW, vs a specific anomalous period of SOI generating a large part of global warming.
Power and Smith go on to make some other interesting statements. They note the disagreement about ENSO between models, and the relationship between weakening of the Walker circulation and the apparent intensity of the post 1977 period:
The Walker Circulation tends to weaken in climate models forced with increasing greenhouse gases [Tanaka et al., 2004; Vecchi et al., 2006; Meehl et al., 2007].
 On the other hand, there is currently no consensus amongst climate models concerning change in the behaviour of ENSO in response to global warming [Cane, 2005; Collins et al., 2005; Guilyardi, 2006; Nyenzi and Lefale, 2006; Philip and van Oldenborgh, 2006; van Oldenborgh et al., 2005; Zelle et al., 2005; Meehl et al., 2007].
 However, if global warming is largely responsible for the observed decline in the average value of the SOI over the period 1977 â€“ 2006 then the threshold values used to define ENSO events need to be lowered (by approximately 3 SOI units). Under the new thresholds the apparent dominance of El Nino disappears.
This is similar situation whereby the frequency of extreme events, like heatwaves, appear to increase when the trend of average temperatures is increasing. Finally they say:
This simple interpretation gives a result that is consistent with modelling results:
global warming weakens the Walker Circulation and warms the tropical Pacific Ocean, but has little impact on tropical ENSO-driven variability about the new mean-state [Meehl et al., 2007]. While plausible, further research is needed to help quantify the extent to which global warming has in fact driven the unprecedented recent decline in the 30-year average value of the SOI. Southern Oscillation Index (SOI, blue).
However, they did not exclud the possibility that the apparent drift in Walker circulation is an artifact of the ‘period of unprecedented El Nino dominance’. They support the change in the baseline SOI with a structural break model, a methodology that luke claims is not used by real climate scientists and Nick says is ‘technical analysis’.
The break model is consistent with a ‘period of unprecedented El Nino dominance’, not only a weakening in the Walker circulation. To support their hypotheses they would need a significant negative slope on the SOI and other indexes prior to 1977, something that I bet isn’t there — a little fly in the ointment not disclosed by the authors.
It does not affect our study however, as the presence of a ‘drift down’ in the SOI mean would not affect the correlation of the cSOI, as linear regression adjusts the coefficient to compensate for this. This is one of the reasons its unreliable to estimate relative contributions from linear regressions.
Nor does it affect attempts to quantify the effects of the ‘period of unprecedented El Nino dominance’ on global mean temperature — an effect despite acknowledgment in the AR4 does not have been seriously considered as a contributor to the ‘unprecedented’ warming trend since 1960.