Evolution and Forcing Mechanisms of El Niño over the Last 21,000 Years

Abstract:

The El Niño-Southern Oscillation (ENSO) is earth’s dominant source of interannual climate variability. Yet, ENSO’s response to global warming remains highly uncertain1. To improve our understanding of ENSO’s sensitivity to external climate forcing, it is paramount to determine its past behavior using paleo climate data and model simulations. Paleo climate records document that ENSO has varied considerably since the Last Glacial Maximum (LGM, 21,000 years ago, or 21 ka)2-9 and some datasets suggest a gradual intensification of ENSO for the past ~6,000 years2,5,7,8. Previous attempts to simulate the transient evolution of ENSO have relied on simplified models10 or snapshot11-13 experiments. Here we analyze a series of transient Coupled General Circulation Model (CGCM) simulations forced by changes in greenhouse gasses, orbital forcing and the ice-sheet history throughout the last 21,000 years. Consistent with most paleo ENSO reconstructions, our model simulates an orbitally-induced strengthening of ENSO during the Holocene, which is caused by increasing positive ocean-atmosphere feedbacks. During the early deglaciation, ENSO characteristics change drastically in response to meltwater discharges and the resulting changes of the Atlantic Meridional Overturning Circulation (AMOC) and equatorial annual cycle. Increasing deglacial atmospheric CO2 concentrations tend to weaken ENSO, whereas retreating glacial ice-sheets intensify ENSO. The complex evolution of forcings and ENSO feedbacks and the uncertainties in the reconstruction further highlight the challenge and opportunity for constraining future ENSO responses.