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

Due to errors in complex coupled feedbacks that compensate differently in different global climate models, as well as nonlinear nature of El Nino-Southern Oscillation (ENSO), there remain difficulties in detecting and evaluating the reason for the past and future changes in the ENSO amplitude, sigma(nino). 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 sigma(nino) and the mean precipitation over the eastern equatorial Pacific ((P) over bar (nino)). This involves a two-way interaction, in which the wetter mean state with greater (P) over bar (nino) 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 in sigma(nino) and (P) over bar (nino) between pre-industrial and recent periods eliminates the bias, and therefore results in a robust sni sigma(nino)-(P) over bar (nino) connection in MME, which suggests a 10-15% increase in the ENSO amplitude since pre-industrial era mainly due to changing mean state. However, the sni sigma(nino)-P-nino connection is less clear for their future changes, which are still greatly uncertain. Citation: Watanabe, M., J.-S. Kug, F.-F. Jin, M. Collins, M. Ohba, and A. T. Wittenberg (2012), Uncertainty in the ENSO amplitude change from the past to the future, Geophys. Res. Lett., 39, L20703, doi:10.1029/2012GL053305.