An El Niño-like Future? Investigation and Evaluation of CMIP5 Projections

Under global warming, most global climate models (GCMs) project an El Niño-like sea surface temperature (SST) warming pattern, with enhanced warming in equatorial region, and a reduced zonal SST gradient along the equator. Previous studies have questioned the reliability of these robust responses across GCMs, since most GCMs experience difficulties simulating the observed climatology and ENSO variability in the tropical Pacific. The goal of this study is to evaluate the confidence of such El Niño-like SST pattern in tropical Pacific region among most GCMs through understanding its formation mechanisms. To investigate the possibility of introducing observational constraints, we also compare the formation mechanisms and feedbacks with those for El Niño events.  

We demonstrate that the reduced evaporative damping related with the structure of climatological evaporation and the enhanced greenhouse effect related with increasing water vapor over the equatorial Central Pacific, are the two major contributors of the El Niño-like warming pattern. Furthermore, it is suggested that (1) the biases of GCMs' climatological evaporation may lead to an exaggeration of El Niño-like SST projection, however, the uncertainties in observational data may challenge such conclusion, (2) the global warming scenario and El Niño events have share some common atmospheric climate feedbacks, despite the distinct triggering mechanisms. A process-based comparison with El Niño events in the observational and reanalysis data could be helpful to evaluate our confidence in the El Niño-like SST projection.