Opportunistic Model Intercomparison of the Miocene Ocean Circulation – MioMIP1

During the Miocene epoch (~23-5 Ma), the Earth experienced a notably warmer climate, with global surface temperatures ranging approximately 4°C to 8°C higher than pre-industrial levels, accompanied by atmospheric CO₂ concentrations in the range of 400-800 ppm. Throughout this period, tropical ocean gateways underwent constriction or closure, while high-latitude gateways expanded. These developments likely played a pivotal role in shaping the modern ocean circulation structure, with strong bipolar hemispheric overturning in the Atlantic, although the precise mechanisms remain poorly understood. This study explores Miocene ocean circulation through an opportunistic climate model intercomparison (MioMIP1), encompassing 14 simulations that use different paleogeographies, CO₂ levels, and vegetation distributions. A consistent feature across all models is the fresher-than-modern Arctic and a resulting increased freshwater export to the North Atlantic. Consequently, the Atlantic Meridional Overturning Circulation (AMOC) appears markedly weaker than its modern counterpart in all simulations, ranging from approximately 1 to 16 Sv. However, there is no discernible correlation between the transport of Arctic freshwater to the Atlantic and the strength of the AMOC across the simulations. Similarly, contrary to earlier suggestions, our analysis reveals that neither Panama nor the Tethys gateway exerts a consistent impact on circulation across the simulations. This implies that the influence of these three straits on circulation dynamics also depends on other factors such as background palaeogeography, CO₂ levels, vegetation, or model physics and requires further study. In three out of the 13 simulations, deep overturning in the North Pacific (PMOC) is observed, ranging from approximately 5 to 10 Sv. Notably, in the North Atlantic, the simulations with a higher salinity have a stronger AMOC, and although this is not observed as distinctly in the North Pacific, the simulations with a PMOC exhibit a reduced salinity contrast between the North Pacific and North Atlantic and highlight the salinity feedback in play. A proto-AMOC appears to be developing in most of the simulations, albeit weak. This indicates that while the AMOC began to take shape during the Miocene, it likely attained its modern strength during the late Miocene.