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A shift from briny to fresh in Antarctica's ocean waters in recent decades could explain the shutdown of the Southern Ocean's coldest, deepest currents, a new study finds.
The cold currents, called the Antarctic Bottom Water, are chilly, salty rivers that flow from the underwater edge of the Antarctic continent north toward the equator, keeping to the bottom of the seafloor. The currents carry oxygen, carbon and nutrients down to the deepest parts of the ocean. Previous studies have found this deep, dense water is disappearing, though researchers aren't sure if the shrinkage is part of a long-term trend linked to global warming, or a natural cycle.
The new study suggests that Antarctica's changing climate is to blame for the shrinking Antarctica Bottom Water. In the past 60 years, the ocean surface off Antarctica's shore became less salty as a result of melting glaciers and more precipitation, both rain and snow, researchers reported Sunday in the journal Nature Climate Change. This growing freshwater layer is the key link in a chain that prevents the cold-water currents from forming, the study finds.
"Deep ocean waters only mix directly to the surface in a few small regions of the global ocean, so this has effectively shut one of the main conduits for deep-ocean heat to escape," said Casimir de Lavergne, an oceanographer at McGill University in Montreal.
De Lavergne said the freshwater acts like a lid, shutting down the ocean circulation that sends cold water to the seafloor and brings warm water up to the mixing regions.
"What we suggest is, the change in salinity of the surface water makes them so light that even very strong cooling is not sufficient to make them dense enough to sink," he told LiveScience. "Mixing them gets harder and harder."