In the cold depths along the sea floor, Antarctic Bottom Waters are part of a global circulatory system, supplying oxygen-, carbon- and nutrient-rich waters to the world’s oceans. Over the last decade, scientists have been monitoring changes in these waters. But a new study from the Woods Hole Oceanographic Institution (WHOI) suggests these changes are themselves shifting in unexpected ways, with potentially significant consequences for the ocean and climate.
In a paper published January 25 in Science Advances, a team led by WHOI oceanographers Viviane Menezes and Alison Macdonald report that Antarctic Bottom Water (AABW) has freshened at a surprising rate between 2007 and 2016—a shift that could alter ocean circulation and ultimately contribute to rising sea levels.
“If you change the circulation, you change everything in the ocean,” said Menezes, a WHOI postdoctoral investigator and the study’s lead author. Ocean circulation drives the movement of warm and cold waters around the world, so it is essential to storing and regulating heat and plays a key role in Earth’s temperature and climate. “But we don’t have the whole story yet. We have some new pieces, but we don’t have the entire puzzle.”
The puzzle itself isn’t new: past studies suggest that AABW has been undergoing significant changes for decades. Since the 1990s, an international program of repeat surveys has periodically sampled certain ocean basins around the world to track the circulation and conditions at these spots over time. Along one string of sites, or “stations,” that stretches from Antarctica to the southern Indian Ocean, researchers have tracked the conditions of AABW—a layer of profoundly cold water less than 0°C (it stays liquid because of its salt content, or salinity) that moves through the abyssal ocean, mixing with warmer waters as it circulates around the globe in the Southern Ocean and northward into all three of the major ocean basins.
The AABW forms along the Antarctic ice shelves, where strong winds cool open areas of water, called polynyas, until some of the water freezes. The salt in the water doesn’t freeze, however, so the unfrozen seawater around the ice becomes saltier. The salt makes the water denser, causing it to sink to the ocean bottom.
Continue reading at Woods Hole Oceanographic Institution.
Photo via Woods Hole Oceanographic Institution.