NOTES AND CORRESPONDENCE A Perpetually Running ENSO in the Pliocene?

Difficulties remain with theoretical explanations of the apparent reduced zonal sea surface temperature gradient in the tropical Pacific of the Pliocene. One favored hypothesis is that it was a ‘‘permanent El Niño’’ state, with the warm phase of ENSO remaining fixed over millions of years. Here, an alternative is suggested— that there was a ‘‘perpetually running ENSO’’ with a shorter return time than is observed today, and that the apparently reduced zonal gradient is an alias–rectification of a high-frequency signal governed by the growth patterns of the foraminifera used to provide proxy temperatures. The hypothesis is probably testable in the modern ocean with comparatively modest measurements of foraminifera behavior in time.