The timing and impact of onset of Antarctic Circumpolar Current (ACC) on climate and Antarctic ice are unclear. We reconstruct late Eocene to Miocene southern Atlantic surface ocean environment using microfossil remains of dinoflagellates (dinocysts). Our dinocyst records shows the breakdown of subpolar gyres in the late Oligocene and the transition into a modern-like oceanographic regime with ACC flow, established frontal systems, Antarctic proximal cooling, and sea ice by the late Miocene.