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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 Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO₂concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high‐latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO₂, and vegetation. The strength of the Southern Ocean‐driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre‐Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC. 

Abstract End of 21st‐century hydroclimate projections suggest an expansion of subtropical dry zones, with Mediterranean and Sahel regions becoming much drier. However, paleobotanical assemblage evidence from the middle Miocene (17‐12 Ma), suggests both regions were instead humid environments. Here we show that by modifying regional sea surface temperatures (SST) in an Earth System Model (CESM1.2) simulation of the middle Miocene, the increased ocean evaporation and integrated water vapor flux overrides any drying effects associated with warming‐induced land‐surface evaporation driven by atmospheric CO₂concentrations. These modifications markedly reduce the bias in the model‐data comparison for this period. A vegetation model (BIOME4) forced with simulated climatologies predicts both regions were dominated by mixed forest, which is largely consistent with the paleobotanical record. This study unveils the potential for wetter subtropical Mediterranean climates associated with warming, presenting an alternative scenario from future drying projections with localized SST warming governing regional climate change. 

Abstract The Miocene (23.03–5.33 Ma) is recognized as a period with close to modern‐day paleogeography, yet a much warmer climate. With large uncertainties in future hydroclimate projections, Miocene conditions illustrate a potential future analog for the Earth system. A recent opportunistic Miocene Model Intercomparison Project 1 (MioMIP1) focused on synthesizing published Miocene climate simulations and comparing them with available temperature reconstructions. Here, we build on this effort by analyzing the hydrological cycle response to Miocene forcings across early‐to‐middle (E2MMIO; 20.03–11.6 Ma) and middle‐to‐late Miocene (M2LMIO; 11.5–5.33 Ma) simulations with CO₂concentrations ranging from 200 to 850 ppm and providing a model‐data comparison against available precipitation reconstructions. We find global precipitation increases by ∼2.1 and 2.3% per degree of warming for E2MMIO and M2LMIO simulations, respectively. Models generally agree on a wetter than modern‐day tropics; mid and high‐latitude, however, do not agree on the sign of subtropical precipitation changes with warming. Global monsoon analysis suggests most monsoon regions, except the North American Monsoon, experience higher precipitation rates under warmer conditions. Model‐data comparison shows that mean annual precipitation is underestimated by the models regardless of CO₂concentration, particularly in the mid‐ to high‐latitudes. This suggests that the models may not be (a) resolving key processes driving the hydrological cycle response to Miocene boundary conditions and/or (b) other boundary conditions or processes not considered here are critical to reproducing Miocene hydroclimate. This study highlights the challenges in modeling and reconstructing the Miocene hydrological cycle and serves as a baseline for future coordinated MioMIP efforts.