The Largest Current on Earth: Explaining the Antarctic Circumpolar Current

Why Plate Tectonics Alone Didn’t Trigger the World’s Mightiest Ocean Current

For decades, earth scientists viewed the opening and deepening of oceanic gateways between Antarctica, South America, and Australia as the primary trigger for circumpolar circulation. During the transition from the Eocene to the Oligocene epoch roughly 34 million years ago, the slow drift of plate tectonics widened passages such as the Drake Passage and the Tasman Gateway.

Geological evidence and new climate modeling, however, demonstrate that these passages existed without immediately generating the powerful ACC known today. To investigate this mismatch, researchers utilized a high-resolution climate model reflecting Earth’s geography 33.5 million years ago, paired with a detailed model of the early Antarctic ice sheet from a 2024 study published in Science.

“With this PNAS study, we are showing—for the first time—how helpful and important it is to carry out these coupled and relatively high-resolution model simulations for the climate of the deep past,” explains Prof Dr Gerrit Lohmann, an Earth system modeler at the Alfred Wegener Institute and a co-author of the study. These demanding simulations highlight complex interactions among ice, the atmosphere, land surfaces, and the ocean.

The Atmospheric Missing Link: How Westerly Winds Aligned the ACC

The decisive factor in jump-starting the ocean conveyor was the alignment of powerful atmospheric winds. Early in the tectonic drift process, the oceanic gaps sat in the wrong place, leaving the strong westerly winds blowing too far north to push water effectively through the newly formed Tasman Gateway.

Hanna Knahl, a climate modeler and lead author of the study, highlights the necessity of this atmospheric positioning. “There were already indications that the wind in the Tasman Gateway played an important role in the formation of the ACC. Our simulations can clearly confirm this: Only when Australia had moved further away from Antarctica and the strong westerly winds blew directly through the Tasman Gateway, the current could fully develop,” she states.

Before Australia migrated far enough north, early ocean currents fractured rather than forming a continuous loop. While strong water flows churned through the Atlantic and Indian sectors, water hitting the Tasman Gateway deflected northward and dissipated. Meanwhile, the Pacific sector remained heavily stratified and remarkably calm.

Thermal Insulation and Carbon Uptake in the Cenozoic Ice Age

Once the continental migration aligned the oceanic gateways with the prevailing westerly winds, the ACC roared to life. This intensified circulation increased Antarctica’s thermal insulation and significantly boosted the ocean’s carbon uptake.

“This understanding is crucial, as the formation of the ACC has strongly driven carbon uptake by the ocean,” notes Dr Johann Klages, a geoscientist and study co-author. The resulting reduction of greenhouse gases in the atmosphere helped initiate the Cenozoic Ice Age, creating permanently ice-covered polar caps characterized by fluctuating warm and cold periods.

During the Early Oligocene Glacial Maximum, atmospheric carbon dioxide hovered around 600 parts per million (ppm), dropping from roughly 1,000 ppm in the late Eocene. While modern anthropogenic emissions are tracking toward similar atmospheric concentrations by the end of this century, researchers emphasize that past climate mechanics cannot be mapped one-to-one onto modern projections.

Frequently Asked Questions

What is the Antarctic Circumpolar Current (ACC)?

The ACC is the largest ocean current on Earth, flowing entirely eastward around Antarctica and transporting between 140 and 170 million cubic metres of seawater every second through the Drake Passage.

Why did the ACC take so long to form after tectonic gateways opened?

According to the 2026 PNAS study, oceanic passages alone were insufficient. The ACC only developed fully after Australia drifted far enough north for powerful westerly winds to blow directly through the Tasman Gateway.

How does the ancient ACC relate to modern climate change?

By studying how high atmospheric CO2 levels (around 600 to 1,000 ppm) interacted with Southern Ocean circulation and ice sheets 34 million years ago, scientists can better calibrate the complex climate models used to forecast future warming scenarios.