Orbital controls on the El Nino/Southern Oscillation and the tropical climate

The global synchroneity of glacial-interglacial events is one of the major problems in understanding the link between Milankovitch forcing and the cli mate of the late Quaternary. In this study we isolate a part of the climate system, the tropical Pacific, and test its sensitivity to changes in solar forcing associated with changes in the Earth's orbital parameters. We use a simplified coupled ocean-atmosphere model that is run for the past 150,00 0 years and forced with Milankovitch changes in the solar insolation. This system responds primarily to the presessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distr ibution of solar radiation even while the annual mean insolation is roughly constant. The mean response to the precessional forcing is due to an inter action between an altered seasonal cycle and the El Nino/Southern Oscillati on (ENSO). Changes in the ENSO behavior result in a mean tropical climate c hange. The hypothesis is advanced that such a change in the tropical climat e can generate a globally synchronous climate response to Milankovitch forc ing.