Antarctic ozone loss drove unexpected Southern Ocean cooling, climate model shows - Earth - Earth Sciences - Environment

A melting iceberg floats in the sea surrounding Antarctica. Credit: Pixabay.

The Southern Ocean has long stood out as an oddity in the global climate system. While most of the planet's surface oceans have warmed in response to rising greenhouse gases, waters circling Antarctica showed an unexpected tendency to cool during the late 20th and early 21st centuries. This cooling coincided with a period when Antarctic sea ice briefly expanded before its more recent decline, adding to the mystery.

A new modeling study, published in Geophysical Research Letters, helps clarify part of this puzzle.

Using climate model experiments focused specifically on changes in the stratospheric ozone layer, the researchers show that human-driven depletion of ozone over Antarctica likely played a significant role in cooling the Southern Ocean between 1982 and 2005. The findings suggest that changes high in the atmosphere can cascade downward to shape ocean temperatures and even influence sea ice around the continent.

Winds of change

The ozone hole formed largely because of human-made chemicals released in the 20th century. It cools the lower stratosphere, the layer of the atmosphere above where weather happens, and changes the temperature difference between the polar regions and the tropics. Those shifts in temperature help change the strength and position of the strong westerly winds that circle Antarctica.

Shouwei Li of Princeton University and colleagues found that ozone loss strengthens these winds and pushes them closer to the continent. This shift is not limited to the atmosphere but extends down to the ocean surface, where it changes how the wind moves seawater and helps set up conditions that cool the Southern Ocean.

When these winds strengthen and shift poleward, they also change how the ocean moves. One key process is called Ekman transport, in which surface waters are pushed by the wind and gently curved by Earth's rotation. In the Southern Hemisphere, this usually drives surface water northward when the westerly winds become stronger.

The study shows that ozone-driven wind changes enhance this northward movement of surface water south of about 46°S latitude. This carries cold water away from Antarctica and spreads it farther north, helping to cool much of the Southern Ocean surface. At the same time, the changing winds also reshape surface temperature patterns, reinforcing the movement of cold water and strengthening the cooling effect.

Other processes can partly counter this cooling. For example, changes in surface heat exchange with the atmosphere can add heat to the ocean in some regions. But in this case, that warming is not strong enough to overcome the wind-driven cooling, which remains the dominant effect over the study period.

Annual mean sea surface temperature trends over the Southern Ocean from satellite observations, and simulations of stratospheric ozone-only and historical data over 1982–2005. Credit: Li et al, 2026.

Hidden feedbacks

Beyond surface transport, the study also highlights slower processes occurring beneath the ocean's mixed layer, the upper layer of ocean water that is directly influenced by wind and waves. When winds intensify, they can increase upwelling, drawing deeper waters toward the surface.

In the Southern Ocean, those deeper waters can be relatively warmer than the surface, meaning this process can eventually contribute to warming rather than cooling.

However, this warming effect appears weaker and slower than the immediate wind-driven cooling. The researchers describe a two-step response: an initial, rapid cooling driven by horizontal transport of cold water, followed by a more gradual and partial warming linked to vertical mixing and upwelling.

Climate model simulations show that these opposing effects do not cancel each other out on decadal time scales. Instead, the sustained strengthening of wind-driven transport keeps the Southern Ocean cooler than it would otherwise be under greenhouse gas forcing alone.

Reframing a climate paradox

One consequence of this cooling signal is its influence on Antarctic sea ice. The models suggest that ozone-driven cooling contributed to regional sea ice expansion, particularly in areas such as the Ross Sea. This aligns with observations of sea ice growth in certain parts of the Southern Ocean during the satellite era, even as global ice trends moved in the opposite direction.

However, the effect is not uniform. Some regions show sea ice increases, while others still experience declines. This patchwork pattern reflects the complex balance among wind changes, ocean heat transport, and regional feedbacks involving temperature and salinity.

Importantly, the study finds that while ozone depletion contributes to these patterns, it is not strong enough on its own to explain the full observed changes in Antarctic sea ice.

The results also help explain why climate models often show warming in the Southern Ocean over recent decades, even though observations show periods of cooling.

When all major influences are included—greenhouse gases, aerosols, natural variability and ozone changes—the strong warming from greenhouse gases still dominates. Ozone depletion acts like a regional cooling influence, but it is not strong enough to overturn the overall global warming signal.

By isolating the role of ozone depletion, the study highlights a broader point: The Southern Ocean is not responding to a single cause, but to several competing influences acting at the same time. It also shows that the unusual cooling trend is not solely a mystery of ocean circulation, but a fingerprint of human-driven changes. 

by Hannah Bird, Phys.org

edited by Sadie Harley, reviewed by Andrew Zinin 

Source: Antarctic ozone loss drove unexpected Southern Ocean cooling, climate model shows