6.5.3 How and Why Has the El Niño-Southern Oscillation Changed Over the Present Interglacial?

High-resolution palaeoclimate records from diverse sources (corals, archaeological middens, lake and ocean sediments) consistently indicate that the early to mid-Holocene likely experienced weak ENSO variability, with a transition to a stronger modern regime occurring in the past few thousand years (Shulmeister and Lees, 1995; Gagan et al., 1998; Rodbell et al., 1999; Tudhope et al., 2001; Moy et al., 2002; McGregor and Gagan, 2004). Most data sources are discontinuous, providing only snapshots of mean conditions or interannual variability, and making it difficult to precisely characterise the rate and timing of the transition to the modern regime.

A simple model of the coupled Pacific Ocean and atmosphere, forced with orbital insolation variations, suggests that seasonal changes in insolation can produce systematic changes in ENSO behaviour (Clement et al., 1996, 2000; Cane, 2005). This model simulates a progressive, somewhat irregular increase in both event frequency and amplitude throughout the Holocene, due to the Bjerknes feedback mechanism (Bjerknes, 1969) and ocean dynamical thermostat (Clement and Cane, 1999; Clement et al., 2001; Cane, 2005). Snapshot experiments conducted with some coupled GCMs also reproduce an intensification of ENSO between the early Holocene and the present, although with some disagreement as to the magnitude of change. Both model results and data syntheses suggest that before the mid-Holocene, the tropical Pacific exhibited a more La Niña-like background state (Clement et al., 2000; Liu et al., 2000; Kitoh and Murakami, 2002; Otto-Bliesner et al., 2003; Liu, 2004). In palaeoclimate simulations with GCMs, ENSO teleconnections robust in the modern system show signs of weakening under mid-Holocene orbital forcing (Otto-Bliesner, 1999; Otto-Bliesner et al., 2003).