The Southern Ocean, a key part of the global climate system, is undergoing a remarkable and abrupt change. After decades of expected warming of the polar Southern Ocean (south of 50°S), in response to climate warming and contributed to the expansion of Antarctic sea ice – scientists have now revealed a dramatic reversal. Since 2015, satellite observations show a significant increase in surface salinity across the circumpolar Southern Ocean., which matches dramatic decline in Antarctic sea ice coverThis shift, with multiple record sea ice minima in both summer and winter, suggests a potential transition of the Southern Ocean to a new state characterized by permanently reduced sea ice coverage.

Prior to 2015, in situ measurements were consistent with a fresher upper ocean, leading to increased stratification of the upper ocean. Strong stratification, where cold, fresh surface waters overlie warmer, saltier deep waters, prevented the upward transport of subsurface heat to the surface, thereby helping to maintain extensive sea ice cover and limiting the formation of open ocean polynyas. However, this trend has suddenly reversed after 2015: a period of salinization has occurred. In response to the increase in surface salinity, upper ocean stratification weakened. Satellite data on sea surface salinity (SSS) effectively capture the sharp increase in 2015-2016 and subsequent persistent high valuesThe strongest salinity anomalies were observed in the upper 100 to 200 meters of the water column. The correlation between satellite-derived SSS and sea ice extent is strong and negative (R = -0.62), which further strengthens their connection.

The physical mechanisms behind these changes are key to understanding the new state of the Southern Ocean. Sea ice changes are closely linked to upper ocean stratification, which at these latitudes is primarily driven by salinity. As surface waters become saltier, the density difference between the surface and deeper layers decreases, weakening stratification. This weakening of stratification allows vertical mixing to more easily transport heat from deeper, warmer layers upward, either by melting sea ice from below or by limiting its growth. In addition, increased salinity led to a weakening of stratification and subsequently facilitated the reappearance of the Maud Rise polynya in the Weddell Sea, a phenomenon last observed in the mid-1970s. This contrasts sharply with previous decades, when sweetening and the absence of large wormwoods were prevalent.

The key contribution of this research is a demonstration that satellites can now monitor these changes in real time. The study combined a new satellite-derived regional SSS product, satellite-derived sea ice extent, and in situ hydrographic profiles from Argo floats. The SSS product consists of a time series of level 3 maps from 2011 to 2023, generated using the Soil Moisture and Ocean Salinity satellite. Sea ice extent is also derived from satellite data. These technologies provide unprecedented insight into the dynamics of the Southern Ocean. Continuous satellite observations of surface salinity will be essential to determine whether Antarctic sea ice is undergoing a long-term shift to permanently low cover..

This rapid shift since 2015 is also surprising given previous expectations. Anthropogenic forcing was widely expected to lead to surface saltation and increased stratification in the polar oceans. Model studies predicted a saltification of the Southern Ocean due to intensified transport of freshwater polar waters, increased precipitation, and increased melting of Antarctic ice. However The rapid changes observed over the past decade 1) are inconsistent with the prevailing expectation of anthropogenically driven sweetening and 2) are unprecedented in the satellite record.This suggests that current understanding and observations may be insufficient to accurately predict future changesOngoing satellite missions and in situ monitoring are now more critical than ever to track and understand the drivers of recent and future changes in the ice-ocean system, including atmospheric forcing, ocean dynamics, and ice-ocean-atmosphere feedbacks. Spring

The study is published in the journal Proceedings of the National Academy of Sciences .

Glossary of key terms

• Anthropogenic forcing: An impact on the climate system or the environment that results from human activity, such as the release of greenhouse gases.
• Argo buoys: An international network of robotic profiling floats that measure ocean temperature and salinity from the surface to a depth of 2,000 meters, providing extensive in situ data on the state of the ocean.
• Circumpolar: Circumpolar; in the context of the Southern Ocean, refers to phenomena that occur all the way around Antarctica.
• Net density: Water density adjusted for pressure effects, allowing comparison of the densities of different water masses without the influence of depth. This is key to understanding ocean stratification.
• Deep Convection: The process by which denser surface water sinks to greater depths in the ocean, leading to vertical mixing and transfer of heat and matter.
• Upper ocean stratification: Stratification of the upper ocean based on density differences (usually caused by differences in temperature and salinity). Strong stratification resists vertical mixing.
• Maud Rise polynya: A historically observed area of open water (polynya) in the Weddell Sea ice associated with the upwelling of warmer waters. Its reappearance is an indicator of altered ocean conditions.
• Sea ice: Ice that forms directly from seawater when it freezes. It floats on the ocean surface and plays a key role in global climate by reflecting sunlight and affecting the exchange of heat between the ocean and the atmosphere.
• Freshening: The process of decreasing the salinity of seawater, often due to the influx of freshwater from melting glaciers, ice sheets, or increased precipitation.
• Open-ocean polynyas: Areas of open water that form in an ice-covered sea far from the coast, as opposed to coastal polynyas. They are often associated with convective processes.
• Pictocline: A layer of water in the ocean where density increases rapidly with depth. It acts as a barrier to vertical mixing.
• Polar regions of the Southern Ocean: The area of the Southern Ocean south of 50°S, characterized by low temperatures, the presence of sea ice, and a key role in global ocean circulation.
• Regime shift: A sudden and lasting change in the structure and/or function of an ecosystem, often leading to a new stable state. In this context, it refers to the sea ice system in the Southern Ocean.
• Salinification: The process of increasing the salinity of seawater, often due to a decrease in freshwater inflow, evaporation, or specific ocean dynamics.
• Salinity: The amount of salt dissolved in water, usually measured in practical salinity units (pss). It is a key factor affecting the density of seawater.
• Ice-ocean feedback: The process by which changes in ice cover (e.g. melting or formation) affect ocean conditions (e.g. temperature, salinity, circulation), which in turn affect the ice itself, creating a self-reinforcing or self-regulating cycle.
• Sea ice loss: Reduction in the extent and/or volume of sea ice.
• Vertical mixing: The movement of water up and down in the ocean, which transfers heat, nutrients, and other substances between different depths. It is influenced by stratification.
• Weddell Sea: A large sea in the Southern Ocean, located east of the Antarctic Peninsula, known for its sea ice and deep convection processes.