Holocene trends in tropical Pacific sea surface temperatures and the El Niño-Southern Oscillation

PAGES News, Vol.14 • No 3 • December 2006 Introduction The El Niño-Southern Oscillation (ENSO) dominates modern climate variability and considerable eff ort has been invested in reconstructing its history. Much of this effort has focused on the Holocene for two important reasons: (1) Holocene boundary conditions have been similar to present-day (the orbital confi guration being an important exception) providing a useful analog for long-term ENSO dynamics in an interglacial climate; and (2) a growing number of studies indicate signifi cant Holocene ENSO changes, including a marked decline in activity in the early-middle Holocene (e.g. Moy et al., 2002). If correct, the latter implies that strong ENSO reorganizations can arise from gradual shifts in background climate conditions, which has important implications for the future. However, a number of challenges continue to hinder adequate understanding of the Holocene evolution of ENSO, including: (1) lack of optimally located continuous ENSO archives with annual or sub-annual resolution; (2) lack of a rigorous theoretical underpinning of how ENSO depends on the background (i.e. time-averaged) climate state; and (3) slow progress toward realistic modeling of tropical ocean-atmosphere dynamics, particularly as it applies to the equatorial annual cycle. As a consequence, our understanding of Holocene ENSO variability lags behind that of other tropical climate systems such as the monsoons and the Intertropical Convergence Zone (ITCZ).