From the latest issue of the NIPCC Report:
The Global Warming - ENSO connection
Reference: Ray, S. and Giese, B.S. 2012. Historical changes in El Niño and La Niña characteristics in an ocean reanalysis. Journal of Geophysical Research 117: 10.1029/2012JC008031.
Over the period 1871-2008, in the words of Ray and Giese (2012), "it is widely agreed that the Earth's average temperature has warmed," but they say that "what is far less clear is how this warming trend has altered forms of climate variability such as ENSO," noting, for example, that "Trenberth and Hoar (1996, 1997) argue that there has been an increase in the occurrence of El Niños since 1976" and that they suggest that this change "is due to global warming."
Noting that DelSole and Tippett (2009) have recently demonstrated that meteorological records that are relatively short, on the order of 50 years or less, "are not sufficient to detect trends in a mode of variability such as ENSO," Ray and Giese decided to explore the postulated global warming-ENSO connection in greater detail, by first comparing sea surface temperatures (SSTs) derived from an ocean reanalysis with three widely-used SST reconstructions, after which they used the reanalysis data to evaluate potential changes in ENSO characteristics over the period 1871-2008. And what did their analysis reveal?
"Overall," in the words of the two researchers, "there is no evidence that there are changes in the  strength,  frequency, duration,  location or  direction of propagation of El Niño and La Niña anomalies caused by global warming during the period from 1871 to 2008." Additionally, they write that "climate change is an ongoing process that is expected to continue in the future, but how El Niño and La Niña variability will react to this change is yet unresolved." And especially is this so, in light of the fact that global temperatures appear to have neither risen nor fallen by any significant amount over the past decade and a half, suggesting that even the direction of future global temperature change remains a mystery.
DelSole, T. and Tippett, M.K. 2009. Average predictability time. Part I: Theory. Journal of the Atmospheric Sciences 66: 1172-1187.
Trenberth, K.E. and Hoar, T.J. 1996. The 1990-1995 El Niño-Southern Oscillation event: Longest on record. Geophysical Research Letters 23: 57-60.
Trenberth, K.E. and Hoar, T.J. 1997. El Niño and climate change. Geophysical Research Letters 24: 3057-3060.