Once upon a time, a weakening of the East-West Pacific overturning circulation – called the Walker circulation – was regarded in climate science as a robust response to anthropogenic global warming. This belief was based on studies in 2006 and 2007 using climate models.
Together with a number of El Nino events (that are associated with a weakening of the Walker circulation) the alarm was raised in a string of papers (3-6) that global warming was now impacting the Pacific Ocean and that the Walker circulation would further weaken in the 21st century, causing more El Ninos and consequently more severe droughts in Australia.
These types of alarms in the context of a severe Australian drought gave rise to an hysterical reaction of building water desalination plants in the major capital cities in Australia, all but one now moth-balled, and costing consumers upwards of $290 per year in additional water costs.
In 2009 I did a study with Anthony Cox to see if there was any significant evidence of a weakening of the Walker circulation when autocorrelation was taken into account. We found no empirical basis for the claim that observed changes differed from natural variation, and so could not be attributed to Anthropogenic Global Warming.
Since 2009, a number of articles show that, contrary to the predictions of climate models, the Walker has been strengthening (7-12). A recent article gives models a fail: “Observational evidences of Walker circulation change over the last 30 years contrasting with GCM results” here.
The paper by Sohn argues that inceases in the frequency of El Nino cause the apparent weakening of the Walker Circulation, not the other way around, and it is well known that climate models unsuccessfully reproduce such trends.
The problems with models may rest in their treatment of mass flows. In “Indian Ocean warming modulates Pacific climate change” here, they find that
“Extratropical ocean processes and the Indonesia Throughflow could play an important role in redistributing the tropical Indo-Pacific interbasin upper-ocean heat content under global warming.”
Finally from an abstract in 2012 “Reconciling disparate twentieth-century Indo-Pacific ocean temperature trends in the instrumental record” here:
“Additionally, none of the disparate estimates of post-1900 total eastern equatorial Pacific sea surface temperature trends are larger than can be generated by statistically stationary, stochastically forced empirical models that reproduce ENSO evolution in each reconstruction.”
Roughly translated this means there is no evidence of any change to the Walker ciriculation beyond natural variation – weakening or otherwise.
Nice to be proven right again. The “weakening of the Walker Circulation” is another scary bedtime story for global warming alarmists, dismissed by a cursory look at the evidence.
1. Held, I. M. and B. J. Soden, 2006: Robust responses of the hydrological cycle to global warming. J. Climate, 19, 5686–5699.
2. Vecchi, G. A. and B. J. Soden, 2007: Global warming and the weakening of the tropical circulation. J. Climate, 20, 4316–4340.
3. Scott B. Power and Ian N. Smith. Weakening of the walker circulation and apparent dominance of el ni˜no both reach record levels, but has enso really changed? Geophys. Res. Lett., 34, 09 2007.
4. Power SB, Kociuba G (2011) What caused the observed twentieth-century weakeningof the Walker circulation? J Clim 24:6501–56514.
5. Yeh SW, et al. (2009) El Niño in a changing climate. Nature 461(7263):511–514.
6. Collins M, et al. (2010) The impact of global warming on the tropical Pacific Ocean and El Niño. Nat Geosci 3:391–397.
7. Li G, Ren B (2012) Evidence for strengthening of the tropical Pacific Ocean surfacewind speed during 1979-2001. Theor Appl Climatol 107:59–72.
8. Feng M, et al. (2011) The reversal of the multidecadal trends of the equatorial Pacific easterly winds, and the Indonesian Throughflow and Leeuwin Current transports. Geophys Res Lett 38:L11604.
9. Feng M, McPhaden MJ, Lee T (2010) Decadal variability of the Pacific subtropical cells and their influence on the southeast Indian Ocean. Geophys Res Lett 37:L09606.
10. Qiu B, Chen S (2012) Multidecadal sea level and gyre circulation variability in the northwestern tropical Pacific Ocean. J Phys Oceanogr 42:193–206.
11. Merrifield MA (2011) A shift in western tropical Pacific sea-level trends during the 1990s. J Clim 24:4126–4138.
12. Merrifield MA, Maltrud ME (2011) Regional sea level trends due to a Pacific trade wind intensification. Geophys Res Lett 38:L21605.
13. “Observational evidences of Walker circulation change over the last 30 years contrasting with GCM results BJ Sohn, SW Yeh, J Schmetz, HJ Song – Climate Dynamics, 2012 – Springer
14. Indian Ocean warming modulates Pacific climate change Jing-Jia Luoa,b,c,1, Wataru Sasakia, and Yukio Masumotoa
15. “Reconciling disparate twentieth-century Indo-Pacific ocean temperature trends in the instrumental record”. Solomon, A. & Newman, M. Nature Clim. Change 2, 691–699 (2012).”