Why Antarctic waters are cooling in a warming world

For decades, climate models have predicted that the Southern Ocean surrounding Antarctica should be warming in response to global climate change. But surprisingly, real-world data show something different: much of that ocean has actually cooled over the past forty years. 

Now, scientists at Stanford University believe they’ve found a compelling explanation, and it’s changing how we understand one of the most important regions in Earth’s climate system.

A cooling ocean in a warming climate

The new research, published in the journal Geophysical Research Letters, points to two overlooked but critical factors – meltwater from Antarctica’s ice sheets and increased precipitation. 

“We found that the Southern Ocean cooling trend is actually a response to global warming, which accelerates ice sheet melting and local precipitation,” said Earle Wilson, assistant professor of Earth system science at Stanford’s Doerr School of Sustainability and senior author of the study.

As more ice melts and rain falls around Antarctica, the ocean’s surface becomes fresher and less salty. That reduction in salinity makes the water less dense, preventing the usual vertical mixing that would bring warmer waters from the deep ocean up to the surface. 

“The fresher you make that surface layer, the harder it is to mix warm water up,” Wilson explained. The result is a kind of lid that traps cooler water at the surface – causing the region to cool rather than warm, despite the rising global temperatures.

Changes in Antarctic waters

This cooling effect hasn’t been accurately represented in most state-of-the-art climate models. The researchers argue that this missing factor – freshwater input from melting glaciers – has long been a major blind spot in climate projections. 

This omission could have broad implications. The Southern Ocean is a key player in regulating global climate. It absorbs a significant amount of the carbon dioxide and excess heat generated by human activity. 

“The Southern Ocean is one of the primary places that happens,” said Zachary Kaufman, the study’s lead author and a postdoctoral scholar in Earth system science at Stanford.

Because of this, changes in Southern Ocean temperatures affect much more than just the immediate region. Surface temperature shifts can influence global sea level rise, ocean heat uptake, and even climate phenomena like El Niño and La Niña that affect rainfall patterns as far away as California.

Reconciling global climate mismatches

The discrepancy between what models predict and what observations show in the Southern Ocean is just one piece of a broader puzzle. Current global climate models also fail to simulate cooling trends in the eastern equatorial Pacific and tend to underestimate the frequency of La Niña events. 

At the same time, they often overstate warming in other regions like the Indian Ocean and the western Pacific.

Although warming events have occurred in Antarctic waters over the last eight years, they haven’t erased the long-term cooling trend. 

According to Wilson, if real-world temperature patterns continue to deviate from model predictions, this could reshape scientific expectations for near-term climate impacts. 

Freshwater near the Antarctic margin

The researchers set out to understand just how much the observed cooling could be attributed to freshening – the dilution of salt water by rain and melting ice. Initially, they assumed it wouldn’t matter much where the freshwater was added.

But they were surprised by what they found. Their analysis showed that sea surface temperatures are much more sensitive to freshwater inputs concentrated near Antarctica’s coastline, as opposed to rainfall distributed across the broader ocean. 

“Applying freshwater near the Antarctic margin has a bigger influence on sea ice formation and the seasonal cycle of sea ice extent, which then has downstream impacts on sea surface temperature,” Wilson explained. 

The result was not only unexpected but also highlighted the importance of location in shaping ocean dynamics.

The impact of missing meltwater

In the past, researchers have tried to measure the effects of Antarctic meltwater through so-called “hosing experiments,” where freshwater is added to simulations in various ways. But differences in modeling setups made it difficult to compare results.

To overcome this, the Stanford team used a comprehensive approach. They drew on a new ensemble of simulations from the Southern Ocean Freshwater Input from Antarctica (SOFIA) Initiative and combined this with older models to study ocean density and circulation changes. 

By comparing the modeled sea surface temperatures with actual freshwater inputs between 1990 and 2021, they were able to determine just how much of the temperature mismatch was due to missing meltwater.

“There’s been some debate over whether that meltwater is enough over the historical period to really matter,” Kaufman said. “We show that it does.” Their findings indicate that up to 60% of the mismatch between observed and simulated surface temperatures in the Southern Ocean can be explained by these missing freshwater inputs.

A new lens on climate futures

The implications are far-reaching. “We’ve known for some time that ice sheet melting will impact ocean circulation over the next century and beyond,” Wilson said. 

“Our results provide new evidence that these meltwater trends are already altering ocean dynamics and possibly the global climate.”

As scientists refine their models to include these missing freshwater effects, policymakers and planners will gain a more accurate picture of how the climate is changing – and how to prepare for the challenges ahead.

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