Ocean salinity feedback amplifies extreme El Niño

The El Niño-Southern Oscillation (ENSO) is a major driver of interannual global climate variability, influencing various regions of the world. Ocean salinity is known to respond to ENSO events, but whether and how it may feed back onto the ENSO events is not fully understood. Through observational analysis and partial coupling experiments with CESM2.1, we find that ocean surface and subsurface salinity can actively influence the development of El Niño and La Niña events, particularly extreme El Niño that has a strong salinity footprint. When the equatorial Pacific salinity is restored to model climatology, the amplitude of extreme El Niño events significantly weaken, suggesting a positive ocean salinity feedback during the El Niño development. This result is robust for a wide range of large-ensemble simulations starting from various ocean initial conditions. The underlying physical mechanisms of the positive salinity feedback involve both ocean heat uptake and ocean zonal current changes in response to the salinity-induced density changes. Our results demonstrate that ocean salinity changes may not only influence the triggering of El Niño events, as previous studies suggest, but also affect the development of El Niño events as a dynamical feedback.