Scientists discover an abrupt reversal in Antarctic Intermediate Water trends after 2016 with the ocean layer becoming saltier denser and deeper as sea ice declines.
A major shift in a vast ocean water layer across the Southern Hemisphere has raised concerns about the long term impacts of climate change on global ocean circulation, heat storage and carbon absorption. Researchers have found that Antarctic Intermediate Water (AAIW) which plays a crucial role in transporting heat, freshwater, oxygen and carbon, has rapidly reversed its long term trends.
The study led by researchers from the University of Melbourne and the Indian Institute of Science (IISc), found that the water layer became saltier denser and deeper between 2016 and 2024 following decades of freshening. The findings were published in the journal Nature Climate Change.
Between 1995 and 2015, seasonal Antarctic sea ice melt supplied freshwater to the ocean, helping maintain the AAIW’s fresher characteristics. However after 2016, Antarctic sea ice declined sharply reducing freshwater input and contributing to the sudden change in the water layer.
The international research team analysed observations from the global Argo network of robotic ocean monitoring floats alongside satellite measurements and atmospheric datasets. According to the researchers, this is the first direct observational evidence of the reversal in AAIW properties.
Lead researcher Associate Professor Bishakhdatta Gayen said a comparable transition may have occurred around 130,000 years ago, near the end of the last interglacial period. However, that earlier change unfolded over thousands of years whereas the recent shift happened within roughly two decades.
Implications for heat and carbon storage
Oceans absorb more than 90% of the excess heat accumulated in the climate system from human activities and take up a substantial share of human produced carbon dioxide. By storing and redistributing heat and carbon, they help moderate atmospheric warming.
Researchers warn that changes in AAIW could affect how surface waters transport heat and carbon into the ocean interior. If the water layer’s capacity to absorb and store these materials changes, climate projections could underestimate deep ocean warming and regional sea level rise.
However, the researchers stressed that the scale and timing of these potential effects remain uncertain.
Potential consequences for global ocean circulation
As the altered water mass moves northwards, it travels through the Atlantic, Indian and Pacific oceans. Because AAIW forms part of the upper branch of the global ocean overturning circulation sustained changes could eventually influence the Atlantic Meridional Overturning Circulation a major system of currents associated with climate patterns in the North Atlantic.
The changes could also affect atmospheric circulation, weather patterns and storm tracks across the Southern Hemisphere with consequences potentially extending worldwide.
The findings highlight the importance of continued ocean monitoring and improved climate modelling. Scientists say understanding how rapidly changing Antarctic conditions influence the deep ocean will be critical to predicting future warming, sea level changes and the wider consequences of climate change.