Evolution of the El Nino-Southern Oscillation in the late Holocene and insolation driven change in the tropical annual SST cycle

Abstract

South American lake sediment records indicate that El Nino events in the eastern equatorial Pacific (EEP) became more frequent after 3000calendar years BP. The reason for this evolution of ENSO behavior remains in question. An important trigger for ocean–atmosphere state switching in the tropical ocean is the annual cycle of sea surface temperature south of the equator along the margin of South America. This annual cycle can be reconstructed from the oxygen isotope records of the surf clam Mesodesma donacium. We provide evidence that these isotope records, as preserved in archeological deposits in coastal central Peru, reflect seasonal paleo-SST. We find that the annual SST cycle in the eastern equatorial Pacific became larger over the 4500–2500calendar year BP interval. This is consistent with increased ENSO variability. The magnification of the annual SST cycle can be attributed to changing insolation, indicating that ENSO is sensitive to the intensity and seasonal timing of solar heating of the southern EEP.