Response of El Nino sea surface temperature variability to greenhouse warming

The destructive environmental and socio-economic impacts of the El Nino/Southern Oscillation(1,2) (ENSO) demand an improved understanding of how ENSO will change under future greenhouse warming. Robust projected changes in certain aspects of ENSO have been recently established(3-5). However, there is as yet no consensus on the change in the magnitude of the associated sea surface temperature (SST) variability(6-8), commonly used to represent ENSO amplitude(1,6), despite its strong effects on marine ecosystems and rainfall worldwide(1-4,9). Here we show that the response of ENSO SST amplitude is time-varying, with an increasing trend in ENSO amplitude before 2040, followed by a decreasing trend thereafter. We attribute the previous lack of consensus to an expectation that the trend in ENSO amplitude over the entire twenty-first century is unidirectional, and to unrealistic model dynamics of tropical Pacific SST variability. We examine these complex processes across 22 models in the Coupled Model Intercomparison Project phase 5 (CMIP5) database(10), forced under historical and greenhouse warming conditions. The nine most realistic models identified show a strong consensus on the time-varying response and reveal that the non-unidirectional behaviour is linked to a longitudinal difference in the surface warming rate across the Indo-Pacific basin. Our results carry important implications for climate projections and climate adaptation pathways.