The deep, salty currents that carry oxygen and nutrients to the ocean depths have been disappearing over the past few decades
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 offshore Antarctica became less salty as a result of
melting glaciers and more precipitation (both rain and snow),
researchers reported Sunday (March 2) 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.
The linchpins linking freshwater and cold currents are polynyas,
or natural holes within sea ice. These persistent regions of open water
form when upwellings of warm ocean water keep water temperatures above
freezing, or when winds drive sea ice away from the coast.
Polynyas are one of the main sources of Antarctica Bottom Water.
Polynyas act like natural refrigerators, letting frigid temperatures and
cold winds chill seawater and send it sinking down to the ocean bottom.
As the cold water sinks, warmer ocean water comes up to take its place,
maintaining the polynya's open water. [Album: Stunning Photos of Antarctic Ice]
But as Antarctica's ocean surface water has freshened, fewer polynyas
have appeared, the researchers found. That's because the fresher water
is less dense. Even if the water is very cold, it doesn't sink as
readily as saltier water, de Lavergne explained. The freshwater acts
like a lid, shutting down the ocean circulation that sends cold water to
the seafloor, and brings up warm water into the polynyas.
"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," de Lavergne told Live Science. "Mixing
them gets harder and harder."
In addition to warming and shrinking the Antarctic Bottom Water
currents, the reduction in polynyas could be trapping extra heat in the
Southern Ocean, de Lavergne said.
"If the warm waters aren't able to release their heat to the
atmosphere, then the heat is waiting in the deep ocean instead," he
said. "This could have slowed the rate of warming in the Southern
De Lavergne cautioned that the heat-storage effect is localized and not related to the so-called global warming "hiatus" — the recent slowdown in the rise of global surface temperatures.
"Our study is still a hypothesis," he added. "We say that climate
change is preventing convection from happening, but we do not know how
frequent it was in the past, so that's a big avenue for future
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