Abstract: Under the background of global warming, the ocean mixed layer depth (MLD) undergoes significant changes, which in turn influence weather, climate, and marine ecosystems. However, due to differences in study regions and the definitions of MLD adopted, existing studies show considerable inconsistencies in the spatial patterns of MLD responses to warming, and a coherent global-scale pattern remains unclear. Based on WOA23 dataset and the multi-model ensemble mean under the highest emission scenario (SSP5-8.5) of the Coupled Model Intercomparison Project Phase 6 (CMIP6), this study employs a simplified energy balance model (EBM) to quantify the spatial distribution of effective heat capacity (EHC) in the global ocean, starting from the regulatory role of MLD in modulating the seasonal amplitude of surface air temperature. The inferred changes in EHC are then used to derive the spatial variations in MLD, thereby refining the method for defining MLD. Results indicate that, under warming, MLD generally shoals in low- and mid-latitude regions while deepening significantly in high-latitude areas. The latitudinal differences in the dominant contributions of temperature and salinity to seawater density are identified as the primary drivers of the heterogeneous spatial responses of MLD and the limitations of existing MLD definitions. This study advances the understanding of oceanic responses to global warming and contributes to improving our ability to predict and assess oceanic feedbacks in future weather, climate evolution, and marine ecosystem changes.