Background trade winds and climate sensitivity control the eastern equatorial Pacific warming pattern.

Climate models robustly project enhanced warming in the eastern equatorial Pacific (EEP) by the late 21st century, producing a mean El Niño-like pattern that strongly influences atmospheric circulation and El Niño-Southern Oscillation dynamics. Here, we show that the magnitude of this pattern depends primarily on the model's strength of background Pacific trade winds and the effective climate sensitivity (EffCS), which together explain ∼74% of the intermodel variance in future EEP warming. Perturbation experiments reveal the underlying mechanism: Stronger trade winds produce larger east-west contrasts in sea surface temperature (SST) and low-cloud cover. Under radiative forcing, this amplifies positive low-cloud and Bjerknes feedbacks, leading to a greater reduction in the east-west SST gradient. These results provide an emergent constraint on future changes in the zonal SST gradient. Specifically, an observation-based easterly wind stress of 0.043 newtons per square meter and an EffCS of 3.0°C yields an estimated reduction in the zonal SST gradient of 0.73 under SSP5-8.5.