Recent measurements showing shifts in Antarctic ocean salinity are sparking debate, but a look at the data reveals a complex picture of this critical region.
Southern Ocean Salinity Trends Reverse, Puzzling Scientists
New data indicates a reversal in the freshening trend of the Southern Ocean, impacting sea ice and offering clues to climate dynamics.
Key takeaways:
- Ocean salinity around Antarctica, which had been freshening, has shown a reversal in trend since 2016.
- This shift coincides with a dramatic decrease in Antarctic sea ice extent, moving from record highs to record lows.
- Scientists are working to understand the complex interplay of factors driving these changes in the Southern Ocean.
- While some earlier projections suggested muted climate impacts in Antarctica, decades of anomalous cooling and sea ice increases were observed until around 2015/2016.
The Southern Oceans, encircling Antarctica and linked by the Antarctic Circumpolar Current, have presented a climate puzzle for decades. Early climate projections from the 1980s anticipated muted climate change signals in this region compared to the Arctic. Surprisingly, Antarctica experienced cooling until about 2015/2016, coupled with an increase in sea ice extent, outcomes not predicted by these initial models.
During this same period, southern ocean winds showed a long-term increase, largely attributed to the polar ozone hole and rising CO2 levels. The Antarctic ice sheet, particularly the West Antarctic Ice Sheet and the Antarctic Peninsula, experienced mostly steady mass loss, though a slight gain was noted in the center of the East Antarctic ice sheet. Scientists theorized that changing winds might be responsible for the increased sea ice, though models did not fully support this. Another hypothesis suggested that a freshwater meltwater anomaly led to increased stratification and higher sea ice levels, a view with some supporting evidence from Schmidt et al. in 2023.
However, since 2016, the situation has flipped dramatically. Antarctic sea ice has transitioned from record highs to record lows, and the freshening trend in ocean salinity appears to have reversed. Recent assessments of remote sensing and Argo measurements, detailed by Silvano et al. in 2025, confirm this reversal in salinity trends.
Navigating the Speculation
This new salinity data has garnered attention, with some initial reports suggesting potentially global impacts. While the observed changes in Antarctic sea ice did contribute to record global temperatures in 2023 and 2024, the broader implications of the salinity shift are still being unpacked. The changes in salinity itself are not dramatic, but the reversal in the trend is certainly noteworthy.
Understanding the normal processes in the Southern Ocean is key. The region experiences a significant seasonal cycle, with sea ice expanding vastly in winter, primarily near the coast. This ice then drifts northward, accumulating snow and thickening until it melts in warmer waters. Deep water formation, crucial for ocean circulation, typically occurs around the continent, driven by salt rejection during sea ice formation. This establishes a poleward surface circulation and a northward deep circulation, maintaining strong stratification in the ocean. This fundamental circulation pattern is unlikely to reverse.
It is important to note that the clearest salinity trends are observed away from the coast and areas of deep water formation.
The Buoyancy Factor
Oceanic ‘buoyancy’ is gained or lost through various processes. Adding heat or freshwater reduces density and increases buoyancy. Conversely, extracting heat or forming sea ice, which leaves brine behind, increases density and removes buoyancy. Freshwater input comes in many forms: meltwater from ice sheets, melting icebergs, rainfall, snowfall, and even melting sea ice further north. Shifts in ice sheet melt rates, calving events, or the paths of storm systems can all influence salinity. The sea ice cover itself plays a direct role, with snow falling on ice having a less pronounced impact on surface salinity than snow falling directly into the ocean.
Unraveling the Mystery
The exact causes behind the recent rapid turnaround in Southern Ocean salinity remain a subject of ongoing research. While the freshening trend observed until 2015 might be linked to increased freshwater from ice sheets and shelves, the subsequent reversal is less clear. Scientists are exploring whether a decrease in meltwater or a shift in storm tracks originating from the tropics could be contributing factors, alongside a complex interplay between sea ice, salinity, winds, and stratification.
Developing accurate models for this region is challenging. It requires incorporating detailed histories of ice sheets and shelves, high-resolution ocean data, ice-shelf/ocean interactions, and precise wind observations, along with improved cloud and aerosol data. Current climate models often lack interactive ice sheets, exhibit biases in the Southern Ocean, and have variable cloud feedbacks. While higher-resolution ocean models offer better insights, biases in reanalyzed wind data persist, and ice sheet models have their own limitations.
Nevertheless, existing models can still yield valuable insights, and this new data will serve as a critical benchmark for future modeling efforts. A comprehensive understanding and more accurate predictions will likely emerge from synthesizing observations with a range of modeling approaches, a point the scientific community is increasingly recognizing.
References
- G.A. Schmidt, A. Romanou, L.A. Roach, K.D. Mankoff, Q. Li, C.D. Rye, M. Kelley, J.C. Marshall, and J.J.M. Busecke, “Anomalous Meltwater From Ice Sheets and Ice Shelves Is a Historical Forcing”, Geophysical Research Letters, vol. 50, 2023.
- A. Silvano, A. Narayanan, R. Catany, E. Olmedo, V. González‐Gambau, A. Turiel, R. Sabia, M.R. Mazloff, T. Spira, F.A. Haumann, and A.C. Naveira Garabato, “Rising surface salinity and declining sea ice: A new Southern Ocean state revealed by satellites”, Proceedings of the National Academy of Sciences, vol. 122, 2025.
