The Atlantic Meridional Overturning Circulation (AMOC) exerts a key control on inter-basin water exchange, yet its influence on Indonesian Throughflow (ITF) transport during the Middle Miocene Climate Optimum (MMCO) remains poorly understood. Using coupled climate simulations with FGOALS-g3 and a generalized island rule adapted to Miocene paleogeography, we show that AMOC variability modulates Pacific outflow via a dynamic interplay between the Panama Throughflow (PTF) and ITF. A strong AMOC drives Pacific water into the Atlantic through the PTF, enhancing compensatory upwelling in the North Pacific. In contrast, weakened or collapsed AMOC reverses the PTF, strengthening the ITF and reducing Pacific upwelling. This anti-phased PTF–ITF seesaw reveals a mechanistic link between Atlantic overturning and Pacific–Indian exchange. Our findings provide a theoretical and dynamical framework to interpret how AMOC state shifts could have reorganized global ocean circulation through the open seaways during Miocene.

The Atlantic Meridional Overturning Circulation (AMOC) exerts a key control on inter-basin water exchange, yet its influence on Indonesian Throughflow (ITF) transport during the Middle Miocene Climate Optimum (MMCO) remains poorly understood. Using coupled climate simulations with FGOALS-g3 and a generalized island rule adapted to Miocene paleogeography, we show that AMOC variability modulates Pacific outflow via a dynamic interplay between the Panama Throughflow (PTF) and ITF. A strong AMOC drives Pacific water into the Atlantic through the PTF, enhancing compensatory upwelling in the North Pacific. In contrast, weakened or collapsed AMOC reverses the PTF, strengthening the ITF and reducing Pacific upwelling. This anti-phased PTF–ITF seesaw reveals a mechanistic link between Atlantic overturning and Pacific–Indian exchange. Our findings provide a theoretical and dynamical framework to interpret how AMOC state shifts could have reorganized global ocean circulation through the open seaways during Miocene.