Externally Forced ENSO Diversity Changes in CESM: From the Last Millennium to the 21st Century

ENSO diversity is known to be sensitive to changes in background climate conditions, as well as atmosphere/ocean feedbacks. However, the degree to which shifts in ENSO characteristics can be attributed to external climate forcings remains unknown. Efforts to assess these changes in a multi-model framework are subject to uncertainties due to differing model physics, and internal ENSO variability necessitates the use of multiple ensemble members. The Community Earth System Model Last Millennium Ensemble (LME) and Large Ensemble (LE) are two new community resources ideally suited to this problem, providing many realizations of the climate of the 850-2100 period with a combination of both natural and anthropogenic climate forcing factors. Analysis of the LME reveals that greenhouse gas (GHG) and anthropogenic aerosol influences on ENSO tend to counteract one another; ENSO amplitude increases due to greenhouse gas forcing and decreases in response to aerosol effects. Changes in Eastern and Central Pacific (EP and CP) El Nino show similar opposing GHG/aerosol effects: GHGs tend to strengthen both EP and CP El Nino, terminate EP El Nino events more efficiently, and enhance CP El Nino persistence, while the opposite is true for simulations with aerosol forcing only. These changes are driven by the zonal advective and upwelling feedbacks, which strengthen in the eastern Pacific under GHG and weaken under aerosol forcing. When both GHG and aerosol influences are included, the LME shows no 20th century change in overall ENSO diversity. This implies that projected future ENSO diversity changes are highly sensitive to the relative magnitude of GHG and aerosol forcings, and analysis of the LE 21st century projection ensembles is used to investigate the range of expected 21st century ENSO diversity changes.