The role of upper–deep ocean heat exchange in shaping warming patterns and climate feedbacks in CMIP6 models

Surface warming patterns modulate climate feedbacks. Ocean heat exchange redistributes heat between the upper and deep ocean, shaping warming patterns. However, its influence on climate feedbacks remains unclear. We explore this issue in the CMIP6 abrupt-4×CO2 experiment by dividing the simulation into fast, intermediate and slow phases and using an effective upper–deep ocean heat exchange coefficient (γ) inferred from a two-layer energy balance model. The intermediate phase stands out as the period when ocean heat exchange leaves its strongest imprint on warming patterns and climate feedbacks. Models with stronger ocean heat exchange (high-γ ensemble) differ from those with weaker exchange (low-γ ensemble) in both background ocean state and response, exhibiting weaker mean-state stratification and a stronger Atlantic Meridional Overturning Circulation (AMOC) favoring stronger heat penetration, subsurface warming, and distinct warming pattern and feedback evolution. Across the three phases, the low-γ ensemble sustains intensified warming in the tropical eastern Pacific and Southern Ocean alongside subdued warming in the Northern Hemisphere extratropics. However, the high-γ ensemble experiences a warming pattern reversal over these regions, tied to stronger intermediate-phase heat penetration and AMOC weakening. This centennial-scale warming pattern reversal, overlooked in previous work, partly mirrors the millennial-scale reversal previously found in multi-millennial abrupt-4×CO2 simulations. The high-γ ensemble also exhibits a larger increase and later peak change in net climate feedback. These findings highlight ocean heat exchange and associated ocean mean-state differences as key controls on time-evolving warming patterns and climate feedbacks.