This thesis investigates the structure and drivers of large-scale ocean circulation during the early and middle Miocene (~20–12 Ma), a period of major tectonic and climatic change. Ocean circulation during this time remains poorly constrained, particularly regarding the transition from a Pacific-dominated overturning regime to the modern Atlantic Meridional Overturning Circulation (AMOC). Using a combination of ensemble simulations and targeted sensitivity experiments with a fully coupled climate model, this work explores how surface freshwater fluxes, wind forcing, ocean gateways, and orography influenced both meridional overturning and horizontal circulation.

The first study focuses on the meridional overturning circulation (MOC) using 14 simulations from the MioMIP1 ensemble. These reveal a consistently weak or absent AMOC and, in some cases, a strong Pacific MOC (PMOC). Overturning strength and structure are closely linked to net surface freshwater fluxes, with fresher basins exhibiting weaker circulation. The Arctic is markedly fresher than today in all Miocene simulations, while the Southern Ocean supports deep overturning comparable to the modern, exerting greater global influence in the absence of strong northern cells. Early Miocene palaeogeography, including a deeper Panama Seaway and lower Tibetan Plateau, likely favoured a PMOC over AMOC.

The second study examines horizontal circulation patterns, including wind-driven gyres, gateway transports, and the Antarctic Circumpolar Current (ACC). Miocene simulations show weaker gyres in the Atlantic and South Pacific and stronger gyres in the North Pacific relative to pre-industrial (PI) conditions, consistent with differences in wind stress curl and basin geometry. Gateway transport evolves with the closure of the Tethys Seaway and shoaling of the Panama Seaway, with Panama transport reversing from westward to eastward as the Tethys closes, consistent with island rule predictions. ACC transport is generally weaker than in PI, reflecting reduced Southern Hemisphere westerlies, though some simulations show enhancement under stronger meridional density gradients or model-dependent feedbacks.

The third study investigates orographic effects on the MOC using middle Miocene simulations with modified elevations of the Tibetan Plateau and Rocky Mountains. Unlike studies using modern boundary conditions, results show that orographic changes only modestly affect PMOC strength through altered atmospheric circulation and freshwater routing and are insufficient to initiate a strong AMOC. Lower CO₂ levels do not qualitatively alter the overturning regime, suggesting that orography exerted a regional rather than dominant control on Miocene ocean circulation.

Together, these studies highlight the dynamic nature of Miocene ocean circulation and the combined roles of freshwater forcing, wind patterns, and tectonic boundary conditions in shaping global overturning. The Miocene regime represents a transitional state between Eocene greenhouse circulation and the modern configuration, retaining key dynamical differences despite near-modern continental geometry. These findings provide insight into the evolution of ocean circulation across the Neogene and lay the groundwork for future studies of the late Miocene, when the modern AMOC likely emerged.