The Antarctic circumpolar current

At high latitudes, the Southern Hemisphere has a qualitatively different geometry than the Northern Hemisphere: the other continents do not connect to Antarctica, and so the oceans are free to circulate all the way around the globe, forming the Antarctic Circumpolar Current (ACC). The flow is predominantly zonal (east-west) and in that sense resembles that of the atmosphere. An enormous transport of water is sustained over great lengths, comparable to that of the great gyres. The strength of ocean currents is traditionally measured in sverdrups (Sv), which are named after Harald Sver-drup, a famous Norwegian oceanographer working in the first half of the twentieth century. A sverdrup was originally defined as a volume transport of 106 m3 s-1 of water, although nowadays it is also commonly used as a measure of mass flow, namely 109 kg s-1 of water or 1 million metric tons of fluid per second. (The two definitions are not exactly equivalent because the density of water is not exactly 1,000 kg m-3, but in any case, the sverdrup is usually used in an informal, approximate way.)

The ACC has an average flow rate of about 120 Sv and goes up to 150 Sv in places. In comparison, the Gulf Stream varies from about 30 Sv off the coast of Florida, increasing in strength as it flows northward and water joins it from the Atlantic, eventually peaking at about 150 Sv after it leaves the coast near Cape Hatteras in North Carolina. To give some perspective, the flow of the mighty Amazon River is about 0.2 Sv, and the flow of all the world's rivers into the ocean totals about 1 Sv, whereas the westerly winds in the atmosphere carry up to 500 Sv of air.

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