Analyzing the effects of unforced natural variability and anthropogenic forcing on ENSO variability using the Community Earth System Model (CESM)

Abstract:

Understanding how ENSO may change with climate is a major challenge, given the unforced (internal) variability within the system and relatively short observational record. Here we analyze ENSO variability using a new CESM ensemble that samples the effects of internal variability of the coupled Earth system. The CESM experiment consists of 50 ensemble members, featuring historical and future RCP8.5 forcing conditions (1850-2100). Each ensemble member is initialized from unique model states sampled from a ~10,000 year fully coupled unforced equilibrium simulation corresponding to pre-industrial conditions. We use the climate change ensemble and 10,000 year control run to examine ENSO behavior within the model under pre-industrial, historical, and future conditions, and we analyze the effect of anthropogenic forcing on key ENSO statistics. Results indicate that unforced internal variability is a major contributor to ENSO variability, while anthropogenic forcing plays a secondary role.