These are circulations in a vertical plane due to differences of density caused by horizontal variations of temperature and salinity. The reason for the circulations is that any parcel of water in the ocean rises when it is less dense than the surroundings, and subsides when it is more dense, like parcels of air in the atmosphere (Section 7.3). For instance, sea-ice consists of water alone, so that the previously dissolved salt is rejected into the water beneath, making it heavy and thus subside. This water from just beneath the ice is near freezing point (at -2°C) and therefore more dense than any other water, so that it sinks to the bottom of the ocean. This is called deep-water formation, and is most common in the Weddell Sea in Antarctica (Chapter 16). It occurs only occasionally and briefly, usually triggered by an outbreak of strong, cold winds off Antarctica. The process creates Antarctic Bottom Water, which spreads out along the ocean floor of the entire globe (Figure11.18 ), and may resurface in some area of upwelling, several thousands of years later.
A similar subsidence occurs off south-east Greenland, creating a layer called the 'Circumpolar Deep Water', which lies just above the Bottom Water, because it is slightly less dense. The subsidence carries carbon dioxide in surface water down, to be sequestered in the much larger volume of the ocean depths (Figure 1.3). So it is an important factor in determining how much carbon dioxide remains in the atmosphere, which governs future climate change (Note 2.L).
Deep-water formation is an example of a thermohaline circulation, a circulation driven by density differences caused by temperature and/ or salinity differences (Note 11.B). Such currents are often discontinuous and fairly small. An example is the occasional flow of water from the Red Sea, with a salinity as high as 40%o (due to little precipitation but evaporation at about 10 mm/day), into the Indian Ocean, where it forms a density current just like an atmospheric density current (Note 8.C).
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