Upper-ocean stratification changes control ENSO amplitude shift under sustained global warming

As projected by climate models in the high emission scenarios, the El Niño–Southern Oscillation (ENSO) exhibits a non-monotonic amplitude shift. However, its key drivers remain poorly quantified. Here we introduce a framework using an intermediate coupled model (ICM) that coherently represents mean-state oceanic climatologies in the tropical Pacific, derived from eight selected climate models across three periods (1940–1990, 2040–2090 and 2240–2290). By applying vertical baroclinic mode decomposition to ocean density, we extract wind projection coefficients (pn; n is mode number) governing upper-ocean dynamical responses. The ICM with the explicitly prescribed climatological fields, including stratification and the thermocline structure, successfully reproduces the non-monotonic ENSO shifts, which is illustrated to be primarily driven by opposite changes in p1 and p2 post 2140. Sensitivity experiments further confirm stratification as the dominant modulator. This study establishes a coherent mechanistic framework for disentangling stratification impacts on ENSO in climate model projections under global warming. ENSO’s mid-century strengthening and post-2100 weakening under strong warming is driven mainly by changing tropical Pacific stratification, which produces contrasting adjustments in the feedbacks that regulate ENSO variability.