Southern Ocean water mass reverses decades-long freshening trend, study finds

A deep-reaching current that forms in the Southern Ocean and spreads through much of the world's oceans has abruptly reversed a decades-long trend, according to a study published Wednesday in Nature Climate Change. Antarctic Intermediate Water, a layer that carries heat, carbon and nutrients northward at depths of roughly 600 to 1,000 meters, had been growing fresher for decades as Antarctic sea ice expanded and melted each summer. Since 2016, researchers found, it has been growing saltier, denser and deeper instead.

The water mass forms where cold, ice-laden surface water sinks in the Southern Ocean before spreading beneath the surface into the Pacific, Atlantic and Indian Oceans. Climate scientists treat it as one of the planet's main channels for moving heat and dissolved carbon out of the atmosphere and into the deep ocean, a process that has helped slow the pace of surface warming even as oceans have absorbed more than 90 percent of the extra heat trapped by human-driven warming.

Two decades, not millennia

The research team, led by Kaushik Mishra and Bishakhdatta Gayen of the Indian Institute of Science together with Alberto Naveira Garabato of the University of Southampton, drew on roughly 4,000 autonomous Argo floats that drift through the ocean measuring temperature and salinity, supplemented by satellite and atmospheric data. Before 2016, they found, the volume of meltwater entering the Southern Ocean each summer was increasing by about 14 milli-Sverdrups a decade and was already equivalent to roughly three times the Amazon River's annual flow. After 2016, as Antarctic sea ice shrank to record lows, that meltwater contribution began falling by about 36 milli-Sverdrups a decade, and the core of Antarctic Intermediate Water started salting at a rate of roughly 0.0048 grams per kilogram per decade, the University of Melbourne said in a statement accompanying the paper.

The researchers describe their work as the first direct observational evidence that this water mass has switched behavior. They note that the only previous comparable shift occurred roughly 130,000 years ago, at the end of the last interglacial period, and unfolded over thousands of years rather than two decades.

"That transition took thousands of years, not two decades," said Bishakhdatta Gayen, a co-author on the study.

What it could mean

The study does not pinpoint a single cause, but the authors link the reversal to the sharp decline in Antarctic sea ice extent recorded since 2016, which has reduced the amount of fresh meltwater reaching the surface ocean each summer. A saltier, denser Antarctic Intermediate Water sinks to greater depths, which the researchers say could change how far down the ocean carries excess heat and carbon, with knock-on effects for ocean stratification in ocean basins far from Antarctica.

The authors caution that the observational record is still short relative to the ocean processes involved, and that longer-term monitoring through the Argo float network will be needed to determine whether the trend continues or proves temporary. They say the next step is to examine how the shift is already showing up in water masses downstream of the Southern Ocean.