The Past Half-Century of ENSO Behavior
Nicholls, N. 2008. Recent trends in the seasonal and temporal behaviour of the El Niño-Southern Oscillation. Geophysical Research Letters 35: 10.1029/2008GL034499.
In an attempt to resolve the riddle, Nicholls examined "trends in the seasonal and temporal behaviour of ENSO, specifically its phase-locking to the annual cycle over the past 50 years," where phase-locking, in his words, "means that El Niño and La Niña events tend to start about April-May and reach a maximum amplitude about December-February," which is why he examined trends in ENSO indices for each month of the year. So what did he find?
The Australian researcher determined "there has been no substantial modulation of the temporal/seasonal behaviour of the El Niño-Southern Oscillation" -- as measured by (1) the sea surface temperature averaged across the region 5°S-5°N by 120°W-170°W, and (2) the Southern Oscillation Index (the non-standardized difference between sea level pressures at Tahiti and Darwin) -- over the past 50 years, during what he describes as "a period of substantial growth in the atmospheric concentrations of greenhouse gases and of global warming."
The fact that Nicholls found that "the temporal/seasonal nature of the El Niño-Southern Oscillation has been remarkably consistent through a period of strong global warming" clearly repudiates the early climate-model-derived inferences of Timmermann et al. (1999), Collins (2000a,b), and Cubasch et al. (2001) that global warming will increase both the frequency and intensity of ENSO events -- which projections (not surprisingly) followed fast on the heels of the powerful 1997-98 El Niño that was subsequently described by some as "the strongest in recorded history" (Jimenez and Cortes, 2003).
Collins, M. 2000a. Understanding uncertainties in the response of ENSO to greenhouse warming. Geophysical Research Letters 27: 3509-3513.
Collins, M. 2000b. The El Niño Southern Oscillation in the second Hadley center coupled model and its response to greenhouse warming. Journal of Climate 13: 1299-1312.
Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P., Dai, X., Maskell, K. and Johnson, C.I. (Eds.). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the 3rd Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK, pp. 525-582.
Jimenez, C.E. and Cortes, J. 2003. Coral cover change associated to El Niño, eastern Pacific, Costa Rica, 1992-2001. Marine Ecology 24: 179-192.
Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E. 1999. Increased El Niño frequency in a climate model forced by future greenhouse warming. Nature 398: 694-696.