A new paper in Nature by Jinbao Li and colleagues, using tree rings to create an El Niño/La Niña (the El Niño-Southern Oscillation, or ENSO) reconstruction of unprecedented fidelity, suggests an increase in ENSO in the second half of the 20th century:

Our data indicate that ENSO activity in the late twentieth century was anomalously high over the past seven centuries, suggestive of a response to continuing global warming. Climate models disagree on the ENSO response to global warming, suggesting that many models underestimate the sensitivity to radiative perturbations.

(For more on the insanely cool usefulness of tree rings, you should buy my book.)