���Ĵ�ý

For decades, most of the world’s oceans have been warming in step with climate change. But the Southern Ocean around Antarctica is an outlier. Since the 1950s, its surface temperature has risen more slowly than that of other oceans, even declining over a 25-year span starting in the 1980s.

“There’s a so-called suppressed warming over that region, and people have been trying to understand why,” says Shineng Hu, an assistant professor of climate dynamics at the Nicholas School of the Environment.

This suppressed warming is particularly evident in summer. As a result, the Southern Ocean’s seasonal cycle — the difference between summer and winter sea surface temperatures — has also dramatically weakened. In other words, temperature fluctuations from season to season have become less prominent.

As the interface between the atmosphere and the deeper ocean, the sea surface experiences and mediates exchanges of moisture and heat. Previous research suggested that summertime westerlies — winds that blow around Antarctica — mix up Southern Ocean waters, moving heat from the surface to deeper depths.

Hu and international collaborators sought to understand whether and how the westerlies contribute to the dramatic dampening of the Southern Ocean’s seasonal cycle in the context of human-caused climate change.

Using historical data and climate models, the team ran comprehensive analyses to home in on potential mechanisms underlying the weakening seasonal cycle. Then they conducted model experiments to see how the seasonal cycle changed when they reduced the strength of the westerlies.

Reporting in , they found that, with accelerated climate change, westerly winds have been intensifying during summer, causing more ocean mixing.

“When the winds are intensified, they stir the ocean more actively than the case with calm winds,” Hu explains. As a result, “heat absorbed by the surface from the warming atmosphere is brought into the subsurface, which means that the surface water warms less.”

The effect is less pronounced in winter, which translates to a smaller difference between summer and winter sea surface temperatures, according to the authors. The stronger summertime westerlies are likely driven by changes in greenhouse gas emissions and ozone, based on other research.

As for the heat that was pushed below the Southern Ocean surface — it doesn’t just disappear.

“Large-scale ocean circulation can transport that ocean heat throughout the rest of the world ocean in the subsurface,” Hu says. “It’s not visible at the surface level, but the heat is stored in there. So, after decades or centuries, that heat may come back to the ocean surface, but maybe in another region.”

For his part, Hu and team are further exploring relationships between heat uptake in the Southern Ocean and its effects on surface waters.

“The Southern Ocean represents one of the key ocean windows for the ocean to receive the heat from the warming atmosphere,” Hu says. “We are now investigating how different climate models capture the effect of ocean heat uptake on this Southern Ocean sea surface warming. This is a critical step to better understand how much the Southern Ocean and global surface temperatures will rise in the coming decades.”