The El Niño–Southern Oscillation (ENSO) in the tropical Pacific Ocean is of key importance to
global climate and weather. However, state-of-the-art climate models still disagree on the ENSO’s response
under climate change. The potential role of atmospheric ozone changes in this context has not been
explored before. Here we show that differences between typical model representations of ozone can
have a first-order impact on ENSO amplitude projections in climate sensitivity simulations. The vertical
temperature gradient of the tropical middle-to-upper troposphere adjusts to ozone changes in the upper
troposphere and lower stratosphere, modifying the Walker circulation and consequently tropical Pacific
surface temperature gradients. We show that neglecting ozone changes thus results in a significant increase
in the number of extreme ENSO events in our model. Climate modeling studies of the ENSO often neglect
changes in ozone. We therefore highlight the need to understand better the coupling between ozone,
the tropospheric circulation, and climate variability