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Abstract During the Middle Miocene Climate Transition (MMCT; ∼14.7–13.8 Ma), the global climate experienced rapid cooling, leading to modern‐like temperatures, precipitation patterns, and permanent ice sheets. However, proxy records indicate that atmospheric pCO₂and regional climate conditions (SST, ice volume) were highly variable from 17 to 12.5 Ma and these changes were not always synchronous. Here, we report on a series of middle Miocene (∼16–12.5 Ma) simulations using the water isotope enabled earth system model (iCESM1.2) to explore the potential for multiple equilibrium states to explain the observed decoupling between pCO₂and regional climates. Our simulations indicate that initial ocean conditions can significantly influence deep water formation in the North Atlantic and lead to multiple ocean equilibria. When the model is initiated from a cold state, residual cool surface water temperatures in the North Atlantic intensify Atlantic Meridional Ocean Circulation (AMOC) and inhibit Arctic sea‐ice formation. When initiated from a warm state, the AMOC remains weak. The different ocean states drive differences in equator‐to‐pole sea surface temperature gradients and sea ice distributions through heat redistribution changes. These equilibria cause variations in temperature gradients and sea ice distribution due to changes in heat redistribution. Additionally, changes in ocean circulation and a reduced temperature gradient in the North Atlantic increase North Atlantic precipitation when the AMOC is strong. These findings underscore the importance of the ocean's initial state in shaping regional climate responses to atmospheric pCO₂, potentially explaining regional climate pattern variability observed during the Miocene. 

Abstract. The numerical model GEOCLIM, a coupled Earth system model for long-term biogeochemical cycle and climate, has been revised. This new version (v 7.0) allows a flexible discretization of the oceanic module, for any paleogeographic configuration, the coupling to any General Circulation Model (GCM), and the determination of all boundary conditions from the GCM coupled to GEOCLIM, notably, the oceanic water exchanges and the routing of land-to-ocean fluxes. These improvements make GEOCLIM7 a unique, powerful tool, devised as an extension of GCMs, to investigate the Earth system evolution at timescales, and with processes that could not be simulated otherwise. We present here a complete description of the model, whose current state gathers features that have been developed and published in several articles since its creation, and some that are original contributions of this article, like the seafloor sediment routing scheme, and the inclusion of orbital parameters. We also present a detailed description of the method to generate the boundary conditions of GEOCLIM, which is the main innovation of the present study. In a second step, we discuss the results of an experiment where GEOCLIM7 is applied to the Turonian paleogeography, with a 10 Myr orbital cycle forcings. This experiment focus on the effects of orbital parameters on oceanic O2 concentration, particularly in the proto-Atlantic and Arctic oceans, where the experiment revealed the largest O2 variations.