Uncertainty in the ENSO amplitude change from the past to the future

[1] Due to errors in complex coupled feedbacks that compensate differently in different global climate models, as well as nonlinear nature of El Niño-Southern Oscillation (ENSO), there remain difficulties in detecting and evaluating the reason for the past and future changes in the ENSO amplitude,σ_niño. Here we use physics parameter ensembles, in which error compensation was eliminated by perturbing model parameters, to explore relationships between mean climate and variability. With four such ensembles we find a strong relationship between σ_niño and the mean precipitation over the eastern equatorial Pacific ( ). This involves a two-way interaction, in which the wetter mean state with greater acts to increase the ENSO amplitude by strengthening positive coupled feedbacks. Such a relationship is also identified in 11 single-model historical climate simulations in the Coupled Model Intercomparison Project phase 5 despite mean precipitation biases apparently masking the relationship in the multi-model ensemble (MME). Taking changes inσ_niño and between pre-industrial and recent periods eliminates the bias, and therefore results in a robustσ_niño– connection in MME, which suggests a 10–15% increase in the ENSO amplitude since pre-industrial era mainly due to changing mean state. However, theσ_niño– connection is less clear for their future changes, which are still greatly uncertain.