During the early Eocene, the southern tip of South America was joined or adjacent to Antarctica. This configuration engendered warm surface currents from the Atlantic and Pacific oceans to travel south, skimming Antarctica and shedding their heat. These currents moderated Antarctic temperatures, and thus those of the entire planet.
As South America slid north, the seaway around Antarctica broadened and deepened. The emergence of a Southern Ocean created a circular current around the southernmost continent: the Antarctic Circumpolar Current.
This gyre became less susceptible to heat transfer from the warmer south-flowing currents. As Antarctica cooled during the middle Eocene, so did the whole planet.
Continental glaciers formed and sea level dropped. Planetary albedo (reflective power) increased. Global cooling strengthened as deep ocean passages grew deeper: between Antarctica and Australia, and South America and Antarctica.
Antarctic offshore surface waters started to sink as they grew cooler than those below. Horizontal current flow changes were accompanied by vertical seawater reorganization. This had 2 noteworthy effects.
1st, warmer nutrient-rich waters rose offshore. Phytoplankton blooms ensued, which fostered a new ecosystem. For one, early oceanic whales – basilosaurids – went extinct, as they were outcompeted by new whale species that had especially adapted to the new abundance of food.
2nd, coupled with tectonic events in the North Atlantic, an ocean conveyor system developed in the ocean basins that provided aeration, thus opening the ocean floor to colonization.