Southern Ocean circulation reversed

A major reversal in ocean circulation has been detected in the Southern Ocean for the first time, with key climate implications. Using Earth observation satellites, a study led by the National Oceanographic Center (NOC, United Kingdom) and published in PNAS, shows that, since 2016, a sustained increase in surface salinity has been detected in the region between the polar and subpolar gyres of the Antarctic Ocean. This change in water composition suggests that the deep ocean circulation in the Southern Hemisphere — known as the SMOC — is not only being altered, but has reversed. That is, instead of sinking into the depths, surface water is being replaced by deep water masses rising to the surface, bringing with them heat and carbon dioxide (CO₂) that had been trapped for centuries.

The surface waters around Antarctica are growing saltier, even as sea ice is diminishing rapidly. Since 2015, Antarctica has lost an area of sea ice comparable in size to Greenland – one of the fastest environmental changes on the planet in recent decades. Typically, melting ice reduces the salinity of surface waters, creating conditions that then help ice to regenerate. But there has been a sudden rise in surface salinity south of 50° S latitude, implying that the process involved in ice melt freshening surface waters is more intricate. This change was completely unexpected. Since the early 1980s, surface waters had been getting fresher and colder, helping sea ice expand.

Saltier surface waters alter the dynamics of the ocean. Normally, the cold, fresh surface water sits on top of warmer, saltier water deep below, since buoyancy at these latitudes is largely controlled by salinity. This layering, traps heat in the ocean depths, keeping surface waters cool and helping sea ice to form. But now the saltier surface waters are enabling heat from deeper down to rise more readily. This upward flow of warmer water melts sea ice from beneath, making it significantly harder for ice to reform. If this trend of higher salinity and reduced ice persists, it could lead to lasting changes in the Southern Ocean, with consequences for the rest of the world.

The implications are far-reaching as changes in this remote region can disrupt global ocean currents, affect climate patterns, and alter ecosystems far beyond the Antarctic. We might be closer to passing a tipping point than expected and we have potentially entered a new state defined by persistent sea ice decline, sustained by a newly discovered feedback loop.

The loss of Antarctic sea ice has far-reaching global consequences. As the ice melts, more heat stored in the ocean is released into the atmosphere, fuelling more intense storms and accelerating climate change. This, in turn, contributes to extreme heatwaves on land and further melting of the Antarctic ice sheet – leading to rising sea levels around the world. Diminished sea ice also threatens vital habitats for penguins and other species that depend on the ice for survival.

Antarctica is no longer the stable, frozen continent it was once thought to be. It is undergoing rapid and unexpected changes that current climate models didn’t predict. Until recently, these models assumed that a warming climate would lead to increased precipitation and ice melt, freshening surface waters and helping to maintain relatively stable sea ice levels. That assumption no longer holds true. These new findings reveal a rise in surface water salinity, a breakdown in the ocean’s layered structure, and a much faster decline in sea ice than previously anticipated.

This is a true reversal of ocean circulation in the Southern Hemisphere, never seen before. While the world is debating the potential collapse of the AMOC in the North Atlantic, the SMOC is not just weakening, but has reversed, with probable unprecedented global climate impacts.

The consequences of this reversal are already becoming visible. The upwelling of deep, warm, CO₂-rich waters could in the long term double current atmospheric CO₂ concentrations by releasing carbon that has been stored in the deep ocean for centuries, potentially with catastrophic consequences for the global climate.

Until now, the Southern Ocean region was virtually inaccessible to satellites due to its low temperatures and the complex, ever-changing dynamics of sea ice. A new processor has provided surface salinity data of unprecedented quality in this region, providing a coherent explanation for the rapid Antarctic sea ice loss that had puzzled the scientific community.

The study redefines the role of the Southern Ocean in the global climate system. The SMOC plays an essential role in regulating the planet’s heat and carbon. Its disruption could trigger cascading effects on other circulation systems such as the AMOC, with potential consequences for the climate in Europe and other regions. The planet is signalling that we are crossing critical thresholds, visible in the Southern Ocean.

SOURCES: https://www.icm.csic.es/en/news/major-reversal-ocean-circulation-detect…
https://www.esa.int/Applications/Observing_the_Earth/FutureEO/SMOS/Anta…