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 (p_n; 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.