The last report from the International Panel on Climate Change says in its executive summary
Multi-model averages show a weak shift towards average background conditions which may be described as ‘El Niño-like’, with sea surface temperatures in the central and east equatorial Pacific warming more than those in the west, weakened tropical circulations and an eastward shift in mean precipitation.
All models show continued El Niño-Southern Oscillation (ENSO) interannual variability in the future no matter what the change in average background conditions, but changes in ENSO interannual variability differ from model to model. Based on various assessments of the current multi-model data set, in which present-day El Niño events are now much better simulated than in the TAR, there is no consistent indication at this time of discernible changes in projected ENSO amplitude or frequency in the 21st century.with more detail provided here.
The single figure that tells the main story:
The figure summarizes the output of the different climate models according to two measures. On the horizontal axis, it shows how much the spatial pattern of future warming looks like the spatial pattern we observe during El Nino events. The fact that most of the points are on the right hand side indicates that future warming probably will look "El Nino-like" based only on its spatial distribution. On the vertical axis, the figure shows what happens to "inter-annual variability," which is similar to a normalized standard deviation of a NINO3-like index. The fact that the points are evenly distributed above and below one indicates that we really don't know if El Nino events will become more or less frequent or extreme.