Winter Sea Ice Drives Southern Ocean CO₂ Variability

Year-round measurements from Ryder Bay in the Antarctic Peninsula are reshaping our understanding of the Southern Ocean's role in the global carbon cycle. Contrary to the long-held focus on summer productivity, recent data from the Rothera Time Series (RaTS) show that winter conditions—particularly sea ice cover—are the dominant drivers of year-to-year carbon dioxide (CO₂) variability in this key oceanic carbon sink.

In colder months, deep ocean mixing brings carbon-rich water to the surface. Whether this CO₂ outgasses or remains trapped depends heavily on sea ice. When ice cover is extensive and persistent, it physically caps the ocean, blocking gas exchange and reducing vertical mixing. In years with thinner or shorter-lasting ice, the barrier disappears sooner, allowing more carbon to be released into the atmosphere. This variability plays a significant role in the Southern Ocean's annual CO₂ balance, but has often gone unaccounted for in global climate models due to limited winter data.

Stratification, Ice Timing, and the Carbon Cycle

The dynamics beneath the ice are as important as the surface changes. RaTS data, spanning over a decade, reveal that sea ice impacts not only gas exchange but also the vertical structure of the water column. Persistent winter ice prevents wind-driven mixing, leading to a more stratified surface layer. This layering helps trap lower-carbon, fresher water near the top, which is further diluted by glacial melt and precipitation. The result: surface waters can actually hold less CO₂ than the atmosphere by late winter—before ice begins to retreat.

In low-ice years, the lack of stratification allows deeper mixing, bringing more dissolved inorganic carbon (DIC) to the surface. If sea ice retreats early, this excess CO₂ is quickly released. The timing of ice melt relative to surface carbon concentrations is therefore critical. In some winters with strong stratification, net annual CO₂ uptake has been recorded at 20–27% higher than in years with minimal ice cover. This suggests that even short-lived shifts in seasonal sea ice can leave a measurable carbon footprint.