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Widening Channels and Westerly Winds Together Formed Earth’s Strongest Current
A critical ocean current that regulates Antarctica’s climate may have formed only once continents separated and winds aligned with new ocean passageways, according to a new study published in the Proceedings of the National Academy of Sciences of the United States of America.
Today, the Antarctic Circumpolar Current transports more than 100 times as much water as all of Earth’s rivers combined and, critically, insulates the Antarctic Ice Sheet from heat at lower latitudes. A clear picture of the origins of this current can help scientists further understand the relationships between contemporary ocean dynamics, the global climate, and ice formation in Antarctica.
“It’s very interesting to learn more about this current, how it developed, and what role it played in the climate change that was happening at that time,” said Hanna Knahl, a paleoclimatologist and doctoral student at the Alfred-Wegener-Institut in Germany and lead author of the new study.
The Birth of a Current
About 34 million years ago, Earth was undergoing a climatic shift, now known as the Eocene-Oligocene transition, during which atmospheric carbon dioxide decreased and the planet cooled.
Earth’s tectonic plates in the Southern Ocean moved away from each other, opening and deepening bodies of water such as the Tasmanian Gateway and the Drake Passage, which separate Antarctica, Australia, and South America.
For years, scientists hypothesized that the alignment of these newly formed waterways, along with westerly winds, could have channeled ocean water and spurred the formation of the Antarctic Circumpolar Current.
“The exact position of the westerly winds and their relative position to the [ocean] gateways have to click together.”
To test that hypothesis, Knahl and her colleagues simulated conditions of the early Oligocene Southern Ocean with a coupled model that included ocean dynamics, atmosphere and wind patterns, temperatures, ice sheet growth, and precipitation. The research team compared these simulations to data from actual Antarctic sediment cores and scans of the ocean floor.
Results confirmed that westerly winds were necessary for the Antarctic Circumpolar Current to form.
“The exact position of the westerly winds and their relative position to the [ocean] gateways have to click together,” Knahl said.
Joanne Whittaker, a marine geophysicist at the University of Tasmania who was not involved in the new study, was a coauthor of a 2015 study that proposed westerly wind alignment played a role in the formation of the current. Knahl’s study presents a more sophisticated model of the early Oligocene Southern Ocean and is a great next step in the investigation of the current’s origins, Whittaker said.
“They did a really nice job of taking a range of different people’s work and linking it all together,” she said.
Oligocene Understandings
“If you can have a model that works in the past, it’s going to give you confidence that it’s going to work for the future, as well.”
Scientists often use Earth’s past behavior to better understand how Earth systems may behave in the present or future. “If you can have a model that works in the past,” Whittaker explained, “it’s going to give you confidence that it’s going to work for the future, as well.”
The Eocene-Oligocene transition is a key to understanding the relationship between atmospheric carbon, ocean dynamics, and the glaciation of Antarctica, Whittaker said. Knowing how the current’s behavior affected carbon uptake millions of years ago helps scientists model how the present current’s behavior might also affect atmospheric carbon.
In addition to carbon uptake, the new research hints at how changes in westerly winds may influence the advance and retreat of the Antarctic Ice Sheet. Some modeling and proxy data indicate the westerly winds that spurred the Antarctic Circumpolar Current’s formation 34 million years ago have shifted in the past century and may continue to shift in the future. Understanding the role these winds initially played in the current’s development may shed light on the current’s present ability to guard the Antarctic Ice Sheet from warmer air masses.
There are still Oligocene patterns that require more research to sort out, though. For example, modeling in the new study showed interesting asymmetries in the timing of the development of different parts of the Antarctic Circumpolar Current, Knahl said. Scientists know from proxy data and modeling that similar asymmetry exists in the history of the Antarctic Ice Sheet; the ice sheet in East Antarctica began to form about 7 million years before the ice sheet began to form in West Antarctica.
“It could be interesting to see if there’s a connection between the asymmetries that we see here,” Knahl said. “Are they linked, or were they more or less independent?”
—Grace van Deelen (@gvd.bsky.social), Staff Writer
