Dense Antarctic water returning to the Atlantic

The supply of dense Antarctic water from the bottom of the ocean to the Atlantic has declined in recent years. However, a new study explains for the first time how this has stabilised since 2014, and slightly recovered. This is due to the variability in upstream dense waters, and has implications for the global climate.

This research has been published in the journal Nature Climate Change, was led by the British Antarctic Survey, and includes authors from the NOC.

The observations presented in the paper shed light on the links between the deep ocean near Antarctica and the large-scale global ocean circulation, which prevents rapid surface climate change by trapping a large portion of human-produced carbon and warming in the deep ocean. The reduced density of deep water, caused by warmer temperatures and increased ice melt, is associated with a weakening of this ocean circulation, with implications for global climate.

NOC scientist, and author of the research Dr Brian King, said “The research highlights the importance of sustained ship-based observing at the highest standards, to reveal small climate signals over long periods of time. The Southern Ocean is a crucial region in which to observe these signals, but it also presents ocean observers with some of the greatest technical challenges, requiring specialist facilities such as the new research vessel RRS Sir David Attenborough. The NOC has been working with UK colleagues on sustained campaigns of deep-ocean observing in the Southern Ocean since 1993. Most recently under the NERC’s ORCHESTRA research program.”

Seawater that has been cooled around Antarctica sinks to the bottom of the ocean forming Antarctic Bottom Water, which can be found across the deepest parts of most of the World Ocean. The volumes of the denser subsets of this water mass have been decreasing in the Scotia Sea, a key pathway from the Weddell Sea toward the rest of the global ocean over the last decades.

Using ship-based measurements of temperature and salinity (saltiness) spanning thirty years, from British, German, and US research cruises from 1989–2018, researchers have tracked the changes in the Scotia Sea, and attribute them to changes happening upstream in the Weddell Sea, closer to Antarctica, possibly due to changes in winds, sea ice formation, or glacial ice melt on the continental margins.

Lead author Dr Povl Abrahamsen, oceanographer at British Antarctic Survey, says:

“The deep oceans have been warming across much of the world for decades, so we were surprised to suddenly see this trend reversing and stabilising in the Scotia Sea. While we do not know if this marks a reversal in the observed trends or merely a hiatus in the decline, this calls for better understanding of the ocean processes in the formation regions of these water masses nearer Antarctica”.

Co-author Dr Kurt Polzin, oceanographer at Woods Hole Oceanographic Institution, says:

“This region, the Scotia Sea, is unique in that it hosts several different physical mechanisms which launder dense water to make it lighter within a relatively small basin (the Southern Scotia Sea). This small basin relative to a relatively large volume transport enables researchers to assess changes in water mass production ultimately coming from the Antarctic Shelves on a biennial basis, compared to decadal time scales from other sections.”

Co-author Dr Andrew Meijers, Polar Oceans deputy science leader at British Antarctic Survey, says:

“This is an exciting result that has emerged from a valuable long-term monitoring effort in the South Atlantic. This is the first time that anyone has observed significant changes in the downward trend of these deep water volumes and shows that even the very deep ocean can change on quite rapid timescales. The surprising recovery suggests that we should be making a much greater effort in observing and understanding the regions around the Antarctic where these waters are formed, so that we can predict how they may respond to ongoing climatic shifts.”

Co-author Prof. Alberto Naveira Garabato from the University of Southampton, says:

“Our findings suggest that the widespread changes in climate that are affecting Antarctica and the surrounding Southern Ocean may hold surprising twists with important global implications. We must continue to raise our game in observing and modelling the Southern Ocean climate system in order to be able to predict these major events in the mid-term future.”

The study makes use of the long time-series of observations in the Drake Passage sustained by the NOC over more than 25 years. This is in addition to data collected by the World Ocean Circulation Experiment, and measurements coordinated by the Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP).

The research was carried out as part of the UK Natural Environment Research Council (NERC)-funded projects Dynamics of the Orkney Passage Outflow (DynOPO) and Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA), with additional support from projects funded by the US National Science Foundation and National Oceanic and Atmospheric Administration.