Paleogeography and boundary condition sensitivities in mid-Miocene climate simulations with CESM1.2

The Langhian (15.98–13.82 Ma) was a stage of the mid-Miocene characterized by atmospheric CO₂ levels higher than those of the present day and significantly warmer surface temperatures. Growing interest in the mid-Miocene arises from its potential as an analog for future climate scenarios. In this study, we conducted Langhian simulations using the climate model CESM1.2 and a new and unpublished geography, comparing them to simulations submitted for the Miocene Modeling Intercomparison Project Phase 1 (MioMIP), which utilizes the geography of Burls et al. (2021). The global mean surface temperature anomaly is similar for both geographies, averaging +5.5°C relative to pre-industrial levels, but exhibits strong local differences due to variations in ice sheets, topography, and bathymetry. A notable feature of our simulations is significant cooling in the northern Atlantic Ocean, driven by a collapse of the Atlantic meridional overturning circulation. Conversely, a strong Pacific meridional overturning circulation emerges, a phenomenon less commonly observed in other Miocene simulations. We further explore the sensitivity of the Langhian climate by varying CO₂ concentrations, removing the Antarctic ice sheet, adjusting cloud parametrization, and incorporating dynamic vegetation. This study reveals a wide range of climate responses, emphasizing the critical influence of geography and other uncertain boundary conditions in achieving realistic Miocene climate simulations and improving data-model comparisons.