How changes in the mean east-west equatorial SST gradient affect ENSO

Using a comprehensive coupled climate model (CESM), we study the dependence of ENSO characteristics on the background state of the tropical Pacific, specifically the mean east-west SST gradient along the equator (ΔT). In a suite of numerical experiments, we modify upper-ocean vertical mixing in the extra-tropics, poleward of 15°N/S, which allows us to vary the supply of cold water feeding equatorial upwelling and controlling temperatures in the eastern equatorial Pacific. The latter factor modifies the zonal SST gradient and related characteristics of the coupled ocean-atmosphere system in the equatorial band, including the atmospheric Walker circulation and the east-west tilt of the ocean thermocline. The imposed changes in the extra-tropics do not affect ENSO dynamics directly, but only through changes in the background state of the coupled system in the equatorial band. In particular, when in the coupled model ΔT is reduced from 5° to 1°C, the period of the simulated ENSO increases from 3 to 5 years, whereas its amplitude decreases by a factor of 4. Despite such a strong reduction in amplitude, the spectral peak associated with ENSO remains statistically significant. Further we show that changes in the tropical background state influence ENSO through two main mechanisms: reduction in the effective coupling between wind and SST anomalies and an increase in the damping of SST anomalies by surface heat fluxes. Ultimately, estimating the magnitude of changes in the mean state of the tropical Pacific necessary to alter ENSO and assessing the relevant physical mechanisms, this study has direct implications for the ongoing debates on how El Niño might change with global warming or in past climates.