The El Niño-Southern Oscillation (ENSO) is the dominant and most consequential climate variation on the planet.  ENSO varies from a type of events with a sea surface temperature (SST) anomaly centre in the equatorial central Pacific, to another with an anomaly centre in the equatorial eastern Pacific (Niño3, 5°S-5°N, 150°W-90°W), referred to as CP and EP regimes, respectively. How ENSO may change under greenhouse warming has been plagued by a persistent lack of inter-model agreement in the response of Niño3 SST. Here we find a robust future increase in EP El Niño variability in climate models participating in the fifth phase of Coupled Model Inter-comparison Project (CMIP5) that simulate the two distinct regimes and the associated nonlinear processes. We show that the EP El Niño pattern and its anomaly centre differ vastly from one model to another, therefore cannot be represented by the spatially fixed Niño3 SST index. The robust increase in EP El Niño SST variability occurs at the anomaly centre unique to each model.  Greenhouse warming-induced intensification in stratification of the equatorial Pacific upper ocean enhances ocean-atmosphere coupling, conducive to the growth of the EP SST anomalies that typically govern strong El Niño events.