From Eos: “Notorious Ocean Current Is Far Stronger Than Previously Thought”

Eos news bloc

Eos

12.27.16
Emily Underwood

The Antarctic Circumpolar Current is the only ocean current to circle the planet and the largest wind-driven current on Earth. It’s also 30% more powerful than scientists realized.

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An ocean circulation model shows the Antarctic Circumpolar Current swirling around Antarctica, with slow-moving water in blue and warmer colors indicating faster speeds (red represents speeds above 1 mile per hour). But how much water is really flowing through the current? Recent fieldwork provides unexpected results. Credit: M. Mazloff, MIT; Source: San Diego Supercomputer Center, UC San Diego

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SDSC’s GORDON supercomputer

Notorious among sailors for its strength and the rough seas it creates, the Antarctic Circumpolar Current (ACC) is the largest wind-driven current on Earth and the only ocean current to travel all the way around the planet. Now, researchers have found that the current transports 30% more water than previously thought. The revised estimate is an important update for scientists studying how the world’s oceans will respond to a warming climate.

The ACC transports massive amounts of water between the Atlantic, Indian, and Pacific oceans in an eastward loop. Just how much water has long been uncertain, however, because of the difficulty and expense of accurately measuring its flow.

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A working day aboard research vessel and ice breaker N. B. Palmer. All hands concentrate as a current- and pressure-recording inverted echo sounder (CPIES) is deployed off the working deck into the Antarctic Circumpolar Current to begin its 4-year measurement mission at the seafloor in Drake Passage. Credit: T. Chereskin

For the new study, Donohue et al. installed gauges along the bottom of Drake Passage, spanning an 800-kilometer passage between Cape Horn and the South Shetland Islands of Antarctica. Housed in glass spheres and spaced between 30 and 60 kilometers apart along a line near the seafloor, the gauges included pressure sensors, floating current meters attached by 50-meter tethers, and instruments that measure acoustic travel time from the seafloor to the sea surface.

The classic estimate used for the ACC’s transport is 134 sverdrups (Sv). One sverdrup is equivalent to 1 million cubic meters per second. Using 4 years of data collection from 2007 to 2011, the researchers found that the transport rate was 30% higher than historical estimates, around 173.3 Sv. Although it’s possible that stronger winds in the Southern Ocean over the past few decades may have caused the increase, satellite-based studies showing that transport has remained fairly steady during this time suggest that improved measurement tools, not increased wind, are responsible for the discrepancy. (Geophysical Research Letters, doi:10.1002/2016GL070319, 2016)

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