Antarctic Circumpolar Current

THE ANTARCTIC CIRCuMPoLAR CuRRENT (ACC), also known as the West Wind Drift, is the only current that flows completely around the globe, unimpeded by continents. Famous explorers have often referenced the ACC in their navigational logs, including Edmond Halley (the first to note the ACC in a 1699-1700 voyage), James Cook, James Clark Ross, Sir Francis Drake, James Weddell. The ACC is notably the roughest sea crossing for navigators, particularly the 497 mi. (800 km.) wide Drake Passage extending around Cape Horn and the Antarctic Peninsula. The role of the ACC as "mixer of the deep oceans" also has a significant impact on global climate.

The ACC, as the name implies, flows around the continent of Antarctica in an eastward direction driven by westerly winds through the Atlantic, Indian, and Pacific Oceans. The ACC is as deep as 6,562-13,123 ft. (2,000-4,000 m.) and as wide as 1,243 mi. (2,000 km.), accounting for the vast transport of waters despite its relatively slow eastward current. It is estimated that some of the seawater of the ACC travels the entire circumference of the globe (24,900 mi.) in a mere eight years. For comparison, the ACC carries 150 times more water around Antarctica than flows through all of the world's rivers combined.

While flow of the ACC is not blocked by any landmasses, it is severely constrained by them. The borders of the ACC are further defined by convergence fronts with significant temperature and salinity variability. The greatest temperature change is north of the ACC in the Subtropical Convergence (Front), where the average sea surface temperature decreases from 54 degrees F (12 degrees C) to 45-46 degrees F (7-8 degrees C) in the ACC and salinity decreases from 34.9 or greater to 34.6 or less.

The southern boundary of the ACC is defined by the westward flowing Antarctic Coastal Current with a surface temperature around 30 degrees F (minus 1 degree C). Mean ACC temperature ranges from 41-30 degrees F (5 to minus 1 degree C). Climate change and ocean warming will likely have a significant effect on the ACC, because of this typically narrow temperature range. Any otherwise small increases in sea surface temperatures may induce dramatic effects on the system.

effect on the climate

The ocean water temperatures of the ACC and the southern oceans play a critical role in the climate of the rest of the planet. Scientists have found that the ACC controls the climate of Earth in three ways. First, the ACC connects the world's major oceans (Atlantic, Pacific, and Indian), resulting in the redistribution of temperature and salinity. Changes in sea surface water temperatures can have severe impacts on regional weather patterns. Second, the vertical circulation of the waters of the ACC renews the deep waters of the world's oceans. This occurs as the waters freeze during the Antarctic winter and warm during the summer. The cooling of the surface waters during freezing increases the density of the water, causing it to sink, and dragging life-sustaining physical (heat and nutrients) and chemical (gases) resources from the surface to the depths, up to 2.5-3 mi. (4 or 5 km.) below the surface).

And, third, due to the vast size of the ACC, it contributes significantly to gas exchange with the atmosphere. It is estimated that the oceans contain 50 times the levels of carbon as the atmosphere on average, and the oceans act as a carbon sink, drawing carbon dioxide (CO2) out of the atmosphere. CO2is produced by the burning of fossil fuels and deforestation and has been identified as a greenhouse gas. The ocean removes approximately half of the 6-7 billion tons of carbon that are released annually into the atmosphere.

mixer of the deep oceans

The ACC is referred to as the ocean's mixer. The center of the current is the Circumpolar Deep Water. This water is a mixture of deep water from all of the world's oceans. The upper areas of this water are oxygen poor, where deep waters have been brought toward the surface by vertical circulation. The deeper waters are typically very saline, with primary sources in the Mediterranean Ocean (via exchange with the Atlantic). In the ACC, these "extreme waters" mix with waters from all of the oceans, including waters that are more oxygen rich and less saline, concomitantly forming a more uniform composite. This mixture is then redistributed to all of the world's oceans by the ACC, driven by wind, replenishing the deep waters and increasing overall ocean productivity.

Regionally, shifts in the ACC relocating warmer and colder waters can cause severe weather changes in those areas. While few landmasses are directly impacted by ACC contributions to weather patterns, these currents are carried far north, directly affecting some of the world's most populated areas. Further, organisms within the ACC are dependent on the stability of currents and temperatures in the ACC surface waters. Included in this are microorganisms, algae, phytoplankton, zooplankton, fish, and migratory species, including whales.

Warming of sea surface temperatures beyond the narrow range of 30-41 degrees F (minus 1 to 5 degrees) C will affect primary productivity (phytoplankton growth rates) in addition to survival, growth, and reproduction of all food web species. Researchers are most interested in the interaction of the ACC with the ice system of Antarctica (drift ice and the Antarctic Ice Sheet) and the global atmosphere. Knowing more about the transport of temperature and salinity by these waters will provide greater insight into climate modeling and predicting future climate conditions on Earth.

sEE ALso: Antarctic Ice Sheets; Intertropical Convergence Zone; Modeling of Ocean Circulation; Oceanography; Sea Ice; Sea Water, Composition of; Thermohaline Circulation; Wind-Driven Circulation; Winds, Westerlies.

BIBLIoGRAPHY. Australian Commonwealth Scientific and Research Organization (CSIRO), Southern Ocean and Antarctic Circumpolar Current (CSIRO, 2005); R.H. Stewart, Introduction to Physical Oceanography (Open Source Textbook, 2005).

Sandra Connelly Rochester Institute of Technology

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