Three-stage response of the equatorial Pacific to CO₂ forcing controlled by shifting trade winds

Abstract Under CO₂ forcing, the ocean thermostat mechanism is expected to induce initial cooling in the equatorial Pacific via the upwelling of cold waters. However, climate models diverge in their initial responses, and the key mechanisms remain unclear. We analyze a large ensemble of 280 abrupt 4 × CO₂ simulations with the MPI-ESM1.2 global climate model to assess the response’s spread and evolution. The response shows three distinct stages: Initial (0–2 years), governed by internal variability; Fast (0–10 years), characterized by a strengthened zonal temperature gradient and intensified easterlies in the equatorial Pacific; and Late (beyond ~50 years), marked by a weakening of the gradient and easterlies. Transitions across stages are not explained by a diminishing ocean thermostat mechanism, which induces persistent cooling, but rather by evolving wind-driven meridional heat transport linked to a waning influence of the land–ocean warming contrast. Under a more realistic forcing of a 1%-CO₂-increase-per-year scenario, Fast dominates for ~60 years, suggesting that fast adjustment processes largely govern the present-day climate.