Changes in ENSO characteristics in CESM1 simulations with considerably altered background climate states

Changes in the tropical Pacific background state can affect interannual variability, i.e. the El Niño-Southern Oscillation (ENSO) by altering feedbacks that control ENSO’s characteristics. Here, the sensitivity of ENSO to the background climate is investigated utilizing two Community Earth System Model version 1 (CESM1) simulations in which the solar constant is altered by ±25 W/m². The resulting stable warm and cold climate mean state simulations differ in terms of ENSO characteristics such as amplitude, frequency, asymmetry and seasonality. Under warm mean state conditions, ENSO reveals a larger amplitude and occurs at higher frequencies than in the cold mean state and control run. The warm run also features an increased asymmetry and a stronger seasonal phase-locking. We relate these changes to the differences in the mean state and the amplifying and damping feedbacks. In the warm run, a shallower mean thermocline results in a stronger subsurface-surface coupling while the cold run reveals reduced ENSO variability due to a reduced Bjerknes Feedback in accordance with a deeper mean thermocline and enhanced mean surface wind stress. A strong zonal advective and Ekman feedback further contribute to the large ENSO amplitude in the warm mean state run. However, in light of the large temperature contrast between the simulations of up to 6 K in the tropical Pacific, the results also highlight the robustness of ENSO dynamics under vastly different climate mean states.