ENSO Activity and Climate Change
Langton, S.J., Linsley, B.K., Robinson, R.S., Rosenthal, Y., Oppo, D.W., Eglinton, T.I., Howe, SS., Djajadihardja, Y.S. and Syamsudin, F. 2008. 3500 yr record of centennial-scale climate variability from the Western Pacific Warm Pool. Geology 36: 795-798.
Among other things, the authors report that "basin stagnation, signaling less El Niño-like conditions, occurred during the time frame of the Medieval Warm Period (MWP), from ca. 1000 to 750 years BP," which was "followed by an increase in El Niño activity that culminated at the beginning of the Little Ice Age ca. 700 years BP." Thereafter, their record suggests that "the remainder of the Little Ice Age was characterized by a steady decrease in El Niño activity with warming and freshening of the surface water that continued to the present." In addition, they say that "the chronology of flood deposits in Laguna Pallcacocha, Ecuador (Moy et al., 2002; Rodbell et al., 1999), attributed to intense El Niño events, shows similar century-scale periods of increased [and decreased] El Niño frequency."
The nine researchers conclude that "the finding of similar century-scale variability in climate archives from two El Niño-sensitive regions on opposite sides of the tropical Pacific strongly suggests that they are dominated by the low-frequency variability of ENSO-related changes in the mean state of the surface ocean in [the] equatorial Pacific." And that "century-scale variability," as they describe it, suggests that global warming typically tends to retard El Nino activity, while global cooling tends to promote it.
Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson, D.M. 2002. Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature 420: 162-165.
Rodbell, D.T., Enfield, D.B., Newman, J.H., Seltzer, G.O., Anderson, D.M. and Abbott, M.B. 1999. An ~15,000-year record of El Niño-driven alluviation in southwestern Ecuador. Science 283: 516-520.