ENSO Change in Climate Projections: Forced Response or Internal Variability?

The El Niño-Southern Oscillation (ENSO) is the dominant driver of interannual variability globally and has effects which are felt in many remote regions of the world. As such it is vital to assess the potential future changes of ENSO. However, there is little consensus on how ENSO sea surface temperature (SST) may change in a future with increasing greenhouse gas emissions and underlying warming (Bellenger et al., 2014; Collins et al., 2010; Guilyardi et al., 2012; Ham & Kug, 2016), with large differences found between different Coupled Model Intercomparison Project 5 (CMIP5) model projections (Collins et al., 2010; Guilyardi et al., 2012). The range of projections of ENSO in the future could be due to differences in model physics, resulting in different projections from different models. However, the role of internal variability must also be considered. Such internal variations can result in different projections from single ensemble members of the same climate model. A large ensemble of a single model can be used to estimate this internal variability, and together with other model ensembles can be used to address uncertainties in model physics.

Two large ensembles are used to quantify the extent to which internal variability can contribute to long-term changes in ENSO characteristics. The range of simulated ENSO amplitude changes in the large ensemble historical simulations encompasses 90% of the Coupled Model Intercomparison Project 5 historical simulations and 80% of moderate (RCP4.5) and strong (RCP8.5) warming scenarios. When considering projected ENSO pattern changes, model differences are also important. We find that ENSO has high internal variability and that single realizations of a model can produce very different results to the ensemble mean response. Due to this variability, 30-40 ensemble members of a single model are needed to robustly compute absolute ENSO variance to a 10% error when 30-year analysis periods are used.