Sources of uncertainty in the time of emergence of tropical Pacific climate change signal: role of internal variability

Abstract

Determining the time of emergence of anthropogenic climate change signals from natural variability, for both the tropical Pacific mean state and El Niño–Southern Oscillation (ENSO), are critical for early climate warning and adaptation planning. However, there remains considerable uncertainty in estimating those times of emergence in state-of-the-art climate models. In this study, the role of internal variability in the uncertainty of the times of emergence of tropical Pacific annual-mean sea surface temperature (SST) and rainfall, and ENSO-related SST and rainfall are investigated by using three large ensembles of model simulations under the historical and RCP8.5 emission scenarios, as well as the multimodel ensemble from phase 6 of the Coupled Model Intercomparison Project under the historical and SSP5–8.5 emission scenarios. Specific attention is paid to the eastern equatorial Pacific. It is found that internal variability contributes to less than half of the total uncertainty in both the times of emergence of annual-mean SST and rainfall in all the three large ensembles, with more contribution to the latter than to the former. Hence, model differences dominate. Conversely, internal variability contributes to a major part of the total uncertainty in the times of emergence of ENSO-related SST and rainfall in one large ensemble that has sufficient members to show emergent signals. These results imply that we could have a relatively high confidence in claiming that anthropogenic climate change has impacted the annual-mean state, if an emergent annual-mean signal is observed in the real world. However, in claiming changes in ENSO variability, the signal is easily obscured by internal variability.