Suppression of El Nino during the mid-Holocene by changes in the Earth's orbit

A number of recent reports have interpreted paleoproxy data to describe the state of the tropical Pacific, especially changes in the behavior of the El Nino-Southern Oscillation (ENSO), over the Holocene. These interpretations are often contradictory, especially for the eastern tropical Pacific and adjacent areas of South America. Here we suggest a picture of the mid-Holocene tropical Pacific region which reconciles the data. ENSO variability was present throughout the Holocene but underwent a steady increase from the mid-Holocene to the present. In the mid-Holocene, extreme warm El Nino events were smaller in amplitude and occurred less frequently about a mean climate state with a cold eastern equatorial Pacific and largely arid coastal regions as in the present climate. This picture emerges from an experiment in which a simple numerical model of the coupled ocean-atmosphere system in the tropical Pacific was driven by orbital forcing. We suggest that the observed behavior of the tropical Pacific climate over the mid- to late Holocene is largely the response to orbitally driven changes in the seasonal cycle of solar radiation in the tropics, which dominates extratropical influences.