New climate simulations suggest that the world’s most powerful ocean current emerged only after a unique combination of geological and atmospheric changes came together, rather than forming suddenly.
The Antarctic Circumpolar Current (ACC) is the strongest ocean current on Earth, flowing clockwise around Antarctica. About five times more powerful than the Gulf Stream, it links major ocean circulation systems that transport heat, water, and nutrients across the globe, playing a crucial role in regulating Earth’s climate.
Tectonic Shifts Alone Could Not Explain the ACC’s Birth
Scientists have long believed the ACC began around 34 million years ago when Australia and South America drifted away from Antarctica, creating new ocean gateways. However, the new research indicates that these tectonic shifts alone were not enough to trigger the current.
The simulations reveal that the ACC only became established after strong westerly winds began sweeping through the Tasman Gateway, the stretch of ocean separating Antarctica from southern Australia. These persistent winds provided the force needed to drive the powerful current.
“There were already indications that the wind in the Tasman Gateway played an important role in the formation of the ACC,” said Hanna Knahl, a climate modeler at Germany’s Alfred Wegener Institute (AWI). “Our simulations clearly show that only after Australia had moved farther from Antarctica and the strong westerly winds flowed directly through the Tasman Gateway could the current fully develop.”
Reconstructing the Origins of the Antarctic Circumpolar Current
Although the ACC is a key component of Earth’s climate system, it remains relatively understudied because it circulates through some of the planet’s most remote waters. To better understand its evolution and future behavior, AWI researchers reconstructed Earth’s climate as it existed about 33.5 million years ago, around the time the current is thought to have formed.
Their models incorporated ancient ocean depths and circulation patterns, atmospheric carbon dioxide concentrations, wind conditions, and the positions of the continents, allowing the team to recreate the conditions that gave rise to the world’s strongest ocean current.
The researchers then combined these climate models with reconstructions of the Antarctic ice sheet’s growth to examine how the expanding ice influenced ocean circulation—and how the evolving currents, in turn, shaped Antarctica’s climate.
That era was a period of significant upheaval in Earth’s history. The planet was shifting from a warmer greenhouse state toward a colder icehouse climate, marked by the development of permanent polar ice sheets.
Over less than one million years, atmospheric CO₂ levels fell dramatically, declining from roughly 1,000 parts per million (ppm) to about 600 ppm.
This transformation was accompanied by other major geological changes. As Australia and South America moved northward, Antarctica became fully separated from the other continents, creating a pathway for ocean currents to flow around the continent.
An Early Antarctic Current Falls Short of Becoming the Modern ACC
Even so, these conditions were not yet sufficient to establish the Antarctic Circumpolar Current (ACC) in its modern form. The simulations indicated that an early version of the ACC, known as a “proto-ACC,” had begun to develop, but it was unable to complete a continuous circuit around Antarctica. Instead, the current divided and flowed northward along the eastern coasts of Australia and New Zealand, where it eventually weakened and disappeared.
The main obstacle appears to be the interaction between winds flowing off the East Antarctic Ice Sheet and the westerly winds passing through the Tasman Gateway. This conflict weakens the current, preventing it from maintaining the strength needed to complete a full circuit around Antarctica. A fully developed ACC could only form once Australia moved farther north.
The researchers note that their model results reinforce earlier studies showing that the establishment of a complete Antarctic Circumpolar Current was only possible after Australia migrated northward to a position where the belt of westerly winds and the Tasman Gateway were aligned at similar latitudes.
Once the Antarctic Circumpolar Current (ACC) became fully established, it played an important role in regulating Earth’s climate. By linking with ocean currents in other regions, it became part of a global circulation system that moves nutrients, heat, and water with varying temperatures around the planet. Most importantly, this powerful current acts as a barrier around Antarctica, preventing warmer ocean waters from reaching the ice sheets and helping preserve them for millions of years.
Global Warming Pushes the ACC Into Uncharted Territory
However, ongoing global warming may now be altering the behavior of the ACC. The current is shifting southward, allowing warmer waters to move closer to Antarctica’s coast, which contributes to faster ice loss.
As Antarctic ice melts, the added freshwater reduces the ocean’s salinity near the continent. Recent studies indicate that this change could slow the ACC by around 20 percent by 2050, potentially harming marine biodiversity while also allowing more warm water to reach the ice sheets—creating a feedback loop that could accelerate further melting.
“To make reliable predictions about Earth’s future climate, we need to examine the planet’s past using simulations and historical data. Studying periods when Earth experienced warmer conditions and higher atmospheric CO₂ levels than today helps us better understand how the climate system responds to change,” Knahl explains.
“However, past climate conditions cannot simply be applied directly to the future. Our research demonstrates that the Antarctic Circumpolar Current, during its early stages of development, affected the climate in very different ways compared with the fully established ACC that exists today.”
Read the original article on: sciencealert
Read more:Robotic Bird Addresses the Biggest Aerodynamic Weakness of Drones