From the AGU Weekly highlights:
El Niño–Southern Oscillation variability persisted in warmer world
Changes in the distribution of sea surface temperature associated with the El Niño–Southern Oscillation (ENSO) cause significant changes in weather. In the past 40 years it has been observed that the frequency and intensity of El Niño events have been increasing. Scientists would like to know what will happen to ENSO variability as the world’s climate warms.
To find out, some have looked to the mid-Piacenzian Warm Period (mPWP), a period about 3.26 to 3.03 million years ago that was about 3° Celsius (5.4° Fahrenheit) warmer than present day and that may be analogous to what can be expected in the future if climate continues to warm. Some studies have suggested that during the mPWP, there was actually no ENSO variability but rather a permanent El Niño state.
To learn more about ENSO variability during the mPWP, Scroxton et al. analyze the isotopic composition of planktonic foraminifera from the eastern equatorial Pacific, as well as ENSO simulations conducted with a coupled ocean atmosphere climate model. Their proxy and model data suggest that interannual ENSO variability did persist during the mPWP, with a mean state similar to a modern El Niño event. Furthermore, they found that during the mPWP, ENSO events may have been more regular and more intense.
Source: Paleoceanography, doi:10.1029/2010PA002097, 2011
Title: Persistent El Niño–Southern Oscillation variation during the Pliocene Epoch
Authors: N. Scroxton: Department of Earth Sciences, University of Oxford, Oxford, UK; Now at Research School of Earth Sciences, Australian National University, Acton, ACT, Australia;
S. G. Bonham: School of Earth and Environment, University of Leeds, Leeds, UK;
R. E. M. Rickaby, S. H. F. Lawrence, and M. Hermoso: Department of Earth Sciences, University of Oxford, Oxford, UK;
A. M. Haywood: School of Earth and Environment, University of Leeds, Leeds, UK.