Drivers of Future Indian Ocean Warming and Its Spatial Pattern in CMIP Models

Coupled Model Intercomparison Project phases 5 and 6 (CMIP5/6) projections display substantial inter-model diversity in the future tropical Indian Ocean warming magnitude and spatial pattern. Here, we investigate the underlying physical mechanisms in 46 CMIP5/6 models using an upper-ocean heat budget framework that separates surface net air-sea flux changes into forcing and feedback components. The multi-model mean (MMM) basin-averaged warming is primarily driven by reduced evaporative cooling due to weaker surface winds related to reduction of both summer and winter monsoonal circulations and increased near-surface relative humidity, with inter-model variations in these parameters controlling warming diversity. The MMM warming pattern features a weakening equatorial gradient, resembling a positive Indian Ocean Dipole phase, and a strengthening interhemispheric gradient, both of which also dominate inter-model spread. Ocean dynamics modulate the amplitude of the MMM IOD-like pattern and its inter-model variability through the Bjerknes feedback, which couples the zonal equatorial SST gradient, equatorial winds, and thermocline slope. Interactions with the tropical Pacific may further contribute to this response. Meanwhile, stronger climatological winds enhance evaporative cooling in the Southern Hemisphere, reducing warming there, and strengthening the MMM interhemispheric SST gradient. The diversity in this interhemispheric gradient is linked to variations in cross-equatorial wind changes and their impact on latent heat flux forcing. This interhemispheric gradient strengthening is part of a broader pan-tropical pattern, with similar features in the Pacific and Atlantic Oceans. These findings clarify the relative roles of thermodynamic processes and ocean dynamics in shaping future tropical Indian Ocean warming.