Large-Scale Ocean Circulation During the Early and Middle Miocene: Insights from MioMIP1 Simulations

The early and middle Miocene (~20–12 Ma) was a time of major global tectonic and climatic reorganisation; yet the structure and evolution of large-scale ocean circulation during this period remain poorly understood. In particular, the timing and mechanisms governing the transition from a Pacific-dominated overturning regime to the modern Atlantic Meridional Overturning Circulation (AMOC) are still debated. Here, we investigate both meridional overturning and wind-driven horizontal circulation during the Miocene using an ensemble of 14 fully coupled climate model simulations from the MioMIP1 framework.

Across all simulations, Atlantic overturning is weak or absent, while some early Miocene simulations exhibit evidence of a Pacific Meridional Overturning Circulation (PMOC). Differences in Northern Hemisphere overturning strength and structure are strongly linked to net surface freshwater fluxes, with basins receiving greater freshwater input exhibiting weaker overturning. In all simulations, the Arctic is substantially fresher than today, and the Southern Ocean supports robust deep overturning that is comparable in strength to the modern but dominates the global MOC in the absence of strong northern overturning cells.

Relative to pre-industrial simulations, the Atlantic and South Pacific wind-driven gyres are generally weaker during the Miocene, while the North Pacific gyres are stronger. These changes are consistent with differences in wind stress curl and basin geometry. Antarctic Circumpolar Current (ACC) transport is typically weaker than modern, consistent with weakened Southern Hemisphere westerlies.

Simulations with earlier Miocene palaeogeographies tend to exhibit westward flow through the Panama Seaway when the seaway was deeper, and the Tethys Seaway was open. These configurations are also more likely to simulate a PMOC compared to later palaeogeographies within the ensemble. With the closure of the Tethys Seaway and shoaling of the Panama Seaway in middle Miocene configurations, flow through the Panama Seaway becomes eastward, consistent with previous studies, and evidence for a PMOC disappears.

Together, these results highlight the importance of surface freshwater forcing, wind stress patterns, and evolving ocean gateways in shaping Miocene ocean circulation and underscore the transitional nature of the Miocene between earlier greenhouse climates and the modern ocean state.