The unexpected and prolonged persistence of warm conditions over the tropical Pacific during the early 1990s can be attributed to an interdecadal climate fluctuation that involves changes in the properties of the equatorial thermocline arising as a result of an influx of water with anomalous temperatures from higher latitudes, The influx affects equatorial sea-surface temperatures and hence the tropical and extratropical winds that in turn affect the influx, A simple model demonstrates that these processes can give rise to continual interdecadal oscillations.