Evolution and forcing mechanisms of El Niño over the past 21,000 years

A simulation of the evolution of El Niño Southern Oscillation in the past 21,000 years in a state-of-the-art climate model shows the complex response mechanisms of El Niño to external climate forcings and poses further challenges to our understanding and projection of El Niño in the future. Palaeoclimate evidence has suggested that the El Niño Southern Oscillation (ENSO) — the most important driver of interannual climate variability — has swung from periods of intense variability to relative quiescence. Zhengyu Liu and colleagues use a series of climate simulations to show that since the Last Glacial Maximum, consistent with palaeoclimate data interpretations, orbital changes have tended to strengthen the ENSO. Variations in ocean circulation, increasing CO2, and retreating ice sheets all influenced ENSO, but at different times and sometimes in offsetting directions. For example, increasing CO2 tended to weaken ENSO, but this influence was opposed by ENSO-enhancing effects from retreating ice. The simulations demonstrate that the ENSO state at any one time reflects the net impact of a series of different climate forcings. The El Niño Southern Oscillation (ENSO) is Earth’s dominant source of interannual climate variability, but its 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 behaviour by using palaeoclimate data and model simulations. Palaeoclimate records show that ENSO has varied considerably since the Last Glacial Maximum (21,000 years ago)2,3,4,5,6,7,8,9, and some data sets suggest a gradual intensification of ENSO over the past ∼6,000 years2,5,7,8. Previous attempts to simulate the transient evolution of ENSO have relied on simplified models10 or snapshot11,12,13 experiments. Here we analyse a series of transient Coupled General Circulation Model simulations forced by changes in greenhouse gasses, orbital forcing, the meltwater discharge and the ice-sheet history throughout the past 21,000 years. Consistent with most palaeo-ENSO reconstructions, our model simulates an orbitally induced strengthening of ENSO during the Holocene epoch, 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 in the Atlantic Meridional Overturning Circulation 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.