Southern Ocean freshening stalls deep ocean CO2 release in a changing climate

Southern Ocean freshening has strengthened stratification since the 1990s, trapping CO₂-rich deep waters 40m shallower below surface—temporarily buffering carbon sink weakening.

A 2025 study reveals that Southern Ocean freshening since the 1990s has strengthened density stratification, preventing CO₂-enriched deep waters from reaching the surface despite increased upwelling—with subsurface CO₂ fugacity rising by ~10 µatm as circumpolar deep water moved 40 meters shallower and replaced winter water between 100-200 meters depth, temporarily buffering the model-predicted weakening of the Southern Ocean carbon sink.

Published by Léa Olivier and F. Alexander Haumann from the Alfred Wegener Institute, the research analyzes circumpolar hydrographic observations from seven repeated sections across the Southern Ocean south of 55°S, spanning data from the 1990s to 2019. Using quality-controlled biogeochemical measurements from the GLODAP database covering 1972-2021, the researchers isolated circulation-driven changes by reconstructing dissolved inorganic carbon (DIC) from total alkalinity to remove effects of anthropogenic CO₂ uptake, air-sea gas exchange, and biological activity. The study demonstrates that while climate models predicted increased CO₂ outgassing from enhanced wind-driven upwelling of carbon-rich deep water, observed surface freshening created a stratification barrier that trapped high-CO₂ waters in the subsurface, explaining the observed reinvigoration of the carbon sink in the 2000s rather than its expected weakening.

Key Findings: Stratification Traps Carbon-Rich Waters Below Surface

Subsurface CO₂ Fugacity Increased 10 µatm Across All Seven Sections

Analysis of circumpolar sections reveals consistent positive fCO₂ anomalies averaging 10.0 µatm across all sections, with regional values ranging from 2.4 µatm to 17.0 µatm between the 1972-2013 climatology and the most recent sections after 2013. The anomalies appear exclusively within a specific subsurface layer between roughly 70-200 meters depth, with average subsurface fCO₂ ranging from 470-480 µatm—well above current atmospheric levels of ~420 µatm, indicating these upwelled deep waters would cause CO₂ release if reaching the surface.

The consistency of subsurface anomalies across seven geographically diverse regions from the Atlantic to Pacific sectors suggests a robust, large-scale, long-term change rather than regional variability. In contrast, the top 100 meters shows greater variability and lacks distinct patterns, likely reflecting seasonal differences between sections (sampled from late spring to early autumn) and increased regional heterogeneity in surface waters during summer when biological activity and air-sea gas exchange are particularly active.

Circumpolar Deep Water Shoaled by Average of 40 Meters Since 1990s

Analysis of water mass distributions reveals that upper circumpolar deep water (uCDW), naturally enriched in DIC at concentrations around 2,260 µmol kg⁻¹, is progressively replacing overlying winter water (WW) in the 100-200 meter layer. The depth limit below which water contains an 80% uCDW fraction has become shallower by an average of 40 meters across all sections, ranging from 17.6 meters in one section to 83.3 meters in another.

The recent sections since 2013 consistently show uCDW reaching shallower depths compared to the 1990s. Waters with fCO₂ higher than 500 µatm now occur at shallower depths, matching the shallower occurrence of waters with oxygen levels below 215 µmol kg⁻¹. This shoaling brings carbon-rich uCDW closer to the surface, with fCO₂ considerably higher than atmospheric levels (~570 µatm at depths greater than 500 meters), explaining the positive subsurface fCO₂ anomalies.

Winter Water Freshening Creates Stratification Barrier

The observed freshening of the winter water core reaches up to -0.3 in salinity units in the Atlantic sector, with maximum anomalies across sections ranging from -0.17 to -0.37. This circumpolar freshening trend south of 55°S, consistent with previous observational and modeling studies, represents surface waters becoming fresher (S < 34.5), colder (T < -1°C), and containing less DIC (~2,210 µmol kg⁻¹ compared to uCDW's ~2,260 µmol kg⁻¹).

While temperature changes in winter water vary between sections, freshening emerges as a consistent circumpolar pattern. The freshening affects both physical and biogeochemical characteristics: winter water is becoming less alkaline and more oxygenated (up to 50 µmol kg⁻¹ increase in some sections), further increasing its biogeochemical distinction from underlying uCDW. The reduced winter water density intensifies stratification between WW and the shoaling uCDW, with long-term trends in uCDW (increasing salinity and temperature, up to +0.2°C warming) further reinforcing this gradient.

Salinity Anomaly Dipole Reflects Water Mass Replacement Pattern

Comparing recent sections with the climatology reveals a clear salinity anomaly dipole: fresher winter water near the surface overlying saltier water below as winter water is replaced by upwelling circumpolar deep water. This dipole structure demonstrates the competing effects of surface freshening (creating stratification) and deep water shoaling (bringing carbon-rich waters upward), with the stratification currently winning by preventing surface expression of the subsurface changes.

The intermediate layer between 90% winter water and 90% uCDW now exhibits properties more similar to uCDW, including increased temperature and salinity, higher DIC levels, greater alkalinity, and lower oxygen concentrations. However, this intermediate layer occupies a smaller volume as it is progressively replaced by shoaling uCDW, indicating the upward encroachment of deep water despite the surface cap.

Surface and Subsurface Anomalies Remain Decoupled Through 2019

As of the last repeat section in 2019, surface and subsurface fCO₂ anomalies remain mostly decoupled, with the signal confined below 100 meters in most sections. This decoupling between layers underscores stratification's role in isolating subsurface anomalies and temporarily stalling their propagation to the surface, preventing the anticipated increase in CO₂ outgassing despite enhanced upwelling of carbon-rich deep water.

The analysis method, which reconstructs DIC from total alkalinity to remove anthropogenic CO₂, biological productivity, and air-sea gas exchange influences, isolates long-term ocean circulation and mixing changes. This reveals that uCDW and high-fCO₂ waters are being trapped below the surface by enhanced stratification, preventing the model-predicted weakening of the Southern Ocean carbon sink and explaining the observed reinvigoration of the sink in the 2000s.

Freshening Driven by Multiple Mechanisms Over Multiple Decades

The circumpolar decrease in surface salinity reflects increased precipitation over evaporation, enhanced northward export of sea ice, and possibly increased export of glacial meltwater from the continental shelf. While anthropogenic forcing contributes to this freshening trend, which has been ongoing since the 1980s and confirmed with biogeochemical observations since the 1990s, the variability of surface stratification remains difficult to predict due to natural variability influences.

The influence of the Southern Annular Mode (SAM) on Southern Ocean freshening on decadal to multi-decadal timescales remains uncertain. In 2016, a strong change in sea-ice regime occurred as the period of growing sea-ice cover suddenly ended, with low sea-ice extent years observed since. This transition coincides with a recent reversal in surface salinity trends from freshening to salinification, which has weakened upper-ocean stratification across the circumpolar Southern Ocean.

Why This Matters: Reconciling Models and Observations

The Olivier and Haumann study provides a mechanistic explanation that reconciles previously contradictory findings between model projections and observational studies regarding the Southern Ocean carbon sink evolution. Early modeling efforts predicted sink weakening due to increased upwelling driven by strengthening westerly winds, while observational studies reported a reinvigoration of the sink in the 2000s, creating a fundamental disconnect in understanding.

The research demonstrates that upwelling of carbon-rich waters has indeed strengthened over recent decades, consistent with model predictions of enhanced overturning circulation under more positive phases of the Southern Annular Mode driven by increasing atmospheric CO₂ and ozone depletion. However, the expected weakening of the carbon sink has been delayed because freshening-driven increased stratification is trapping high-DIC waters in the subsurface, preventing their ventilation to the atmosphere.

The findings align with paleoclimate studies suggesting lower atmospheric CO₂ levels when deep-water masses are isolated from the atmosphere, highlighting stratification's importance in controlling carbon cycling. During past glacial periods, enhanced Southern Ocean stratification limited deep-water upwelling and ventilation, effectively isolating CO₂-rich deep waters from the surface and reducing outgassing, with some reconstructions also suggesting enhanced biological pump efficiency under more stratified conditions.

The identification of physical mechanisms behind vertical decoupling provides a new framework for understanding current trends and anticipating future transitions in the ocean carbon sink. The results suggest models may underestimate the strength and persistence of surface stratification, which critically affects CO₂ release from subsurface sources. By showing how stratification changes can both mask and drive considerable shifts in the ocean's carbon cycle, the research reveals fundamental controls on the Southern Ocean carbon sink evolution.

Critical Uncertainties and Future Implications

The 2016 transition toward lower sea-ice extent coinciding with surface salinity reversal from freshening to salinification suggests a possible shift toward a less stratified upper ocean with profound implications for CO₂ ventilation. Increased surface salinity potentially indicates a transition that could lead to elevated CO₂ outgassing as stratification weakens, allowing trapped subsurface carbon-rich waters to reach the surface and exchange with the atmosphere, thus accelerating global warming.

Recent biogeochemical Argo float studies revealed unexpected wintertime CO₂ outgassing, emphasizing the central role of subsurface carbon-rich waters in driving seasonal variability. The shoaling of uCDW by 40 meters and increased subsurface fCO₂ by 10 µatm indicate that while stratification currently prevents surface outgassing, the carbon-rich waters are positioned progressively closer to the surface. If stratification weakens through continued salinification or other mechanisms, these waters could rapidly ventilate, causing the model-predicted sink weakening to emerge suddenly.

One proposed mechanism for the post-2016 salinification involves increased upward mixing of relatively warm and salty uCDW, though changes in sea-ice formation and melt could also affect upper-ocean salinity. The biogeochemical impact of this shift remains uncertain due to limited carbonate data after 2016, underscoring the urgent need for sustained year-round observations to refine attribution and improve projections.

The study relies on seven repeated hydrographic sections, most sampled during summer months, limiting assessment of changes in surface air-sea CO₂ fluxes. Wintertime sections would be particularly valuable for resolving carbon dynamics in the upper ocean and validating stratification-driven suppression of surface outgassing. The spatial and temporal sparsity of carbonate system data also limits capacity to conduct formal attribution analyses separating anthropogenic forcing contributions from natural variability.

While observed biogeochemical changes—enhanced upper-ocean stratification, freshening, and shoaling of CO₂-rich uCDW—align with mechanisms attributed to anthropogenic forcing including greenhouse gas emissions and stratospheric ozone depletion, internal variability in atmospheric and oceanic circulation could also contribute. Given observational gaps, particularly in time series and winter coverage, the respective roles of natural variability versus long-term forced trends in the upper 100 meters cannot be fully disentangled, highlighting the need for comprehensive monitoring to predict how ongoing climate change might alter the delicate balance between upwelling-driven carbon supply and stratification-driven carbon isolation in the Southern Ocean.

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