New Climate Simulations Unravel the Origins of Earth's Strongest Ocean Current
New climate simulations indicate that the formation of the Antarctic Circumpolar Current (ACC), the world's strongest ocean current, required the alignment of multiple geological and atmospheric factors. Beyond the opening of ocean passageways, the presence and alignment of strong westerly winds were critical for its full development. The ACC plays a significant role in global ocean circulation and climate stability, but contemporary warming trends are observed to be impacting its behavior and raising concerns about its future influence on Antarctic ice sheets.
Overview of the Antarctic Circumpolar Current
The Antarctic Circumpolar Current (ACC) is the most powerful ocean current globally, flowing clockwise around Antarctica. It is estimated to be five times stronger than the Gulf Stream and contributes significantly to global ocean circulation and nutrient distribution.
Factors in ACC Formation: More Than Just Geography
Previous theories suggested the ACC formed approximately 34 million years ago as Australia and South America drifted northward from Antarctica, opening new ocean passageways. However, recent research indicates that this geological shift alone was insufficient for the current's full development.
Scientists at the Alfred Wegener Institute (AWI) conducted climate simulations modeling Earth's conditions from about 33.5 million years ago. These simulations revealed that a strong westerly wind was also necessary for the ACC's complete formation. These winds needed to align and pass directly through the Tasman Gateway, the oceanic expanse situated between Antarctica and Australia's southern coast.
Hanna Knahl, a climate modeler at AWI, stated that simulations confirm the current fully developed only when Australia had moved sufficiently north, aligning the westerly winds directly through the Tasman Gateway.
Research Methodology and Historical Context
The AWI researchers developed climate simulations incorporating data on ocean depth, circulation, atmospheric carbon dioxide levels, wind patterns, and landmass positions for the period around 33.5 million years ago. These models were integrated with data on the evolution of the Antarctic ice sheet to explore mutual influences between ice sheets, ocean currents, and climate.
This period was characterized by Earth's transition from a greenhouse to a cooler icehouse climate, marked by the onset of permanent polar ice caps. During this time, atmospheric CO2 concentrations decreased significantly, from approximately 1,000 parts per million (ppm) to about 600 ppm within less than a million years.
The Evolution of the ACC
While Antarctica became isolated as landmasses drifted, allowing water to circulate around the continent, initial simulations showed a "proto-ACC" that did not complete a full circuit. This early current split and moved northward along the east coasts of Australia and New Zealand before dissipating. This was attributed to interference from winds originating off the East Antarctic Ice Sheet with the westerly winds in the Tasman Gateway.
The full circum-Antarctic circuit was only achieved after Australia's continued northward shift, which aligned the westerly wind belt with the Tasman Gateway, enabling the current to maintain its strength and complete a full loop.
ACC's Role in Climate Stability
Once established, the ACC played a significant role in Earth's climate system. It connects with currents in other oceans, forming a global conveyor belt that transports nutrients and water of varying temperatures. The current also contributes to the preservation of Antarctic ice sheets by isolating them from warmer waters, a process that has continued for millions of years.
Contemporary Impacts and Future Projections
Presently, observed global warming trends are impacting the ACC. The current is migrating southward, increasing the proximity of warmer waters to Antarctic shorelines, which accelerates ice loss. The resulting influx of fresh meltwater is reducing ocean salinity. Recent research and projections suggest this could lead to a potential 20 percent slowdown of the ACC by 2050.
A slowdown could diminish ocean biodiversity and facilitate further warm water access to ice sheets, potentially creating a feedback loop of increased ice loss.
Knahl emphasized the importance of studying past climates to understand future climate scenarios, while cautioning that past conditions cannot be directly applied to future predictions due to differing influences of the ACC in its early stages compared to its fully developed state today.
The research findings were published in the Proceedings of the National Academy of Sciences.