Description: El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual climate variability in the current climate, influencing ecosystems, agriculture, and weather systems across the globe, but future projections of ENSO frequency and amplitude remain highly uncertain. A comparison of changes in ENSO in a range of past and future climate simulations can provide insights into the sensitivity of ENSO to changes in the mean state, including changes in the seasonality of incoming solar radiation, global average temperatures, and spatial patterns of sea surface temperatures. As a comprehensive set of coupled model simulations is now available for both palaeoclimate time slices (the Last Glacial Maximum, mid-Holocene, and last interglacial) and idealised future warming scenarios (1 % per year CO2 increase, abrupt four-time CO2 increase), this allows a detailed evaluation of ENSO changes in this wide range of climates. Such a comparison can assist in constraining uncertainty in future projections, providing insights into model agreement and the sensitivity of ENSO to a range of factors. The majority of models simulate a consistent weakening of ENSO activity in the last interglacial and mid-Holocene experiments, and there is an ensemble mean reduction of variability in the western equatorial Pacific in the Last Glacial Maximum experiments. Changes in global temperature produce a weaker precipitation response to ENSO in the cold Last Glacial Maximum experiments and an enhanced precipitation response to ENSO in the warm increased CO2 experiments. No consistent relationship between changes in ENSO amplitude and annual cycle was identified across experiments.

Description: Originating in the equatorial Pacific, the El Niño–Southern Oscillation (ENSO) has highly consequential global impacts, motivating the need to understand its responses to anthropogenic warming. In this Review, we synthesize advances in observed and projected changes of multiple aspects of ENSO, including the processes behind such changes. As in previous syntheses, there is an inter-model consensus of an increase in future ENSO rainfall variability. Now, however, it is apparent that models that best capture key ENSO dynamics also tend to project an increase in future ENSO sea surface temperature variability and, thereby, ENSO magnitude under greenhouse warming, as well as an eastward shift and intensification of ENSO-related atmospheric teleconnections — the Pacific–North American and Pacific–South American patterns. Such projected changes are consistent with palaeoclimate evidence of stronger ENSO variability since the 1950s compared with past centuries. The increase in ENSO variability, though underpinned by increased equatorial Pacific upper-ocean stratification, is strongly influenced by internal variability, raising issues about its quantifiability and detectability. Yet, ongoing coordinated community efforts and computational advances are enabling long-simulation, large-ensemble experiments and high-resolution modelling, offering encouraging prospects for alleviating model biases, incorporating fundamental dynamical processes and reducing uncertainties in projections. Key points Under anthropogenic warming, the majority of climate models project faster background warming in the eastern equatorial Pacific compared with the west. The observed equatorial Pacific surface warming pattern since 1980, though opposite to the projected faster warming in the equatorial eastern Pacific, is within the inter-model range in terms of sea surface temperature (SST) gradients and is subject to influence from internal variability. El Niño–Southern Oscillation (ENSO) rainfall responses in the equatorial Pacific are projected to intensify and shift eastward, leading to an eastward intensification of extratropical teleconnections. ENSO SST variability and extreme ENSO events are projected to increase under greenhouse warming, with a stronger inter-model consensus in CMIP6 compared with CMIP5. However, the time of emergence for ENSO SST variability is later than that for ENSO rainfall variability, opposite to that for mean SST versus mean rainfall. Future ENSO change is likely influenced by past variability, such that quantification of future ENSO in the only realization of the real world is challenging. Although there is no definitive relationship of ENSO variability with the mean zonal SST gradient or seasonal cycle, palaeoclimate records suggest a causal connection between vertical temperature stratification and ENSO strength, and a greater ENSO strength since the 1950s than in past centuries, supporting an emerging increase in ENSO variability under greenhouse warming.