Configuration of circum-Antarctic circulation at the last green- to icehouse climate transition

During the Eocene–Oligocene Transition ( ∼ 34.4 to 33.7 Ma), major climatic and tectonic changes initiated Earth’s current icehouse climate. Plate motion intensified mountain building, reduced atmospheric CO 2 , and triggered global cooling. Crucially, the opening of the Tasman Gateway and Drake Passage initiated the formation of the Antarctic Circumpolar Current (ACC), today Earth’s strongest ocean current. However, the ACC’s initial structure during its onset remains poorly understood, limiting our understanding in controlling global circulation patterns and heat distribution at the time. Here, we present data-validated, high-resolution coupled climate–ice sheet simulations of the Early Oligocene Glacial Maximum ( ∼ 33.7 to 33.2 Ma), showing that early ocean–atmosphere circulation around Antarctica was closely tied to Southern Ocean gateway geometry and the presence of the early Antarctic Ice Sheet. Unlike today, however, these gateways were not aligned with the westerly wind belt, hence limiting ACC development and promoting strong Antarctic amplification. We conclude that orogenesis, CO 2 drawdown, and Southern Hemisphere gateway opening alone were insufficient to establish a strong ACC ∼ 34 Ma. Only the later alignment of westerly winds with open oceanic gateways enabled the enhancement of interhemispheric overturning circulation, promoted carbon uptake, and the long-term stabilization of Earth’s icehouse climate.