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13: Deep Circulation in the Ocean

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    The direct forcing of the oceanic circulation by wind discussed in the last few chapters is strongest in the upper kilometer of the water column. Below a kilometer lie the vast water masses of the ocean extending to depths of 4–5 km. The water is everywhere cold, with a potential temperature less than 4\(^{\circ}\)C. The water mass is formed when cold, dense water sinks from the surface to great depths at high latitudes. It spreads out from these regions to fill the ocean basins. Deep mixing eventually pulls the water up through the thermocline over large areas of the ocean. It is this upward mixing that drives the deep circulation. The vast deep ocean is usually referred to as the abyss, and the circulation as the abyssal circulation.

    The densest water at the sea surface, water that is dense enough to sink to the bottom, is formed when frigid air blows across the ocean at high latitudes in winter in the Atlantic between Norway and Greenland and near Antarctica. The wind cools and evaporates water. If the wind is cold enough, sea ice forms, further increasing the salinity of the water because ice is fresher than sea water. Bottom water is produced only in these two regions. Cold, dense water is formed in the North Pacific, but it is not salty enough to sink to the bottom.

    At mid- and low latitudes, the density, even in winter, is sufficiently low that the water cannot sink more than a few hundred meters into the ocean. The only exception are some seas, such as the Mediterranean Sea, where evaporation is so great that the salinity of the water is sufficiently great for the water to sink to the bottom of these seas. If these seas can exchange water with the open ocean, the waters formed in winter in the seas mixes with the water in the open ocean and it spreads out along intermediate depths in the open ocean.


    This page titled 13: Deep Circulation in the Ocean is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert H. Stewart via source content that was edited to the style and standards of the LibreTexts platform.