A new decades-long study, based on long-term ocean measurements collected by ships and robotic floating devices, shows that a warm mass called “circumpolar deep water” has expanded and shifted toward the Antarctic continental shelf over the past 20 years. Ice shelves play an important role in holding back Antarctica’s inland ice sheets and glaciers, which collectively hold enough freshwater to raise sea level by about 58 meters.
It’s the first time that scientists have observed the shift in deep-ocean heat throughout the Southern Ocean. Previously, they didn’t had enough ocean observations to detect the warming trend.
Previous observations of the Southern Ocean were limited to transects recorded by ships roughly once a decade. To fill the gaps in the record, the researchers, including scientists from the Scripps Institution of Oceanography and UCLA, supplemented the ship measurements with publicly available data collected by a global array of autonomous floats, which drift through the upper ocean. These so-called Argo floats provide continuous snapshots of the ocean, but the program hasn’t been running as long as ships have been collecting detailed hydrographic sections.
Using machine learning, the researchers took the Argo float data and combined it with long-term patterns drawn from ships measurements to build a new record capturing detailed monthly snapshots over the last four decades, allowing them to uncover the shift in warm waters.
The Southern Ocean plays a key role in regulating global heat and carbon storage, so changes in heat distribution here have wider implications for the global climate system. For example, in the frigid waters around the poles, extremely cold, dense water forms and sinks to the deep ocean. As the water sinks, it draws down heat, carbon and nutrients, setting in motion a global “conveyor belt” of currents, including the Atlantic Meridional Overturning Circulation (AMOC), which shuttles water around the Atlantic.
Climate models indicate that warmer air temperatures and added freshwater from ice melt are reducing the formation of this dense water in the North Atlantic, potentially leading to a weakening of the AMOC. Similar changes have also recently been forecast for the Southern Ocean.
“We can now see this scenario is already emerging in the observations,” said Lanham. “This isn’t just a possible future scenario suggested by models; it’s something that is happening now, bringing wider implications for how carbon, nutrients and heat are cycled through the global ocean.”
Data courtesy of University of Cambridge