Fig. 4: Simulated ocean and atmosphere processes driving extreme El Niño across past and future climate states.

a, Growth rate of positive SST anomalies over the central equatorial Pacific driven by zonal displacements of the western Pacific warm pool, in which values during June–September capture the strength of the positive Bjerknes feedback driving the growth of El Niño events. The growth rates are computed over the equatorial Niño–3.4 region (170° W–120° W; 2.5° S–2.5° N) in which air–sea coupling is the strongest. This narrower region isolates the effect of variations in equatorial currents on zonal thermal advection. b, Relationship between the mean amplitude of simulated El Niño events and the strength of the Bjerknes feedback (June–September growth rates) across climatic states. c, Strength of the Bjerknes feedback across climatic states (black) and the contribution from mechanical coupling (magenta), wind–SST coupling (blue) and thermal coupling (green). The amplitude of ENSO variability (Fig. 1) in each interval is highly correlated with the strength of the Bjerknes feedback. d, Processes controlling the strength of the Bjerknes feedback across climatic states. The averaged mixed layer depth over the equatorial Niño–3.4 region (blue) controls the strength of mechanical coupling. The extent of the warm pool, measured by the longitude of its eastern edge (orange), controls the strength of the wind–SST coupling. The edge of the warm pool is defined as the easternmost longitude along the equator exhibiting ascending motion in the mid-atmosphere (Methods). The depth of the mixed layer is defined based on a 0.5-K threshold. These climatic features are ultimately related to the strength of the trade winds across the equatorial Pacific (dark blue; 2.5° S–2.5° N)—a measure of the strength of the Pacific Walker circulation.