Deep ocean circulation

Whereas above the permanent thermocline the ocean circulation is mainly wind driven, the deep ocean circulation is powered by density gradients due to salinity and temperature differences. These differences are mostly produced by surface processes, which feed cold, saline water to the deep ocean basins in compensation for the deep water delivered to the surface by upwelling. Although upwelling occurs chiefly in narrow coastal locations, subsidence takes place largely in two broad ocean regions – the northern North Atlantic and the Antarctic Weddell Sea.

In the North Atlantic, particularly in winter, heating and evaporation produce warm, saline water which flows northwards both in the near-surface (Gulf Stream-North Atlantic Current and at intermediate depths of around 800 m. In the Norwegian and Greenland Seas, its density is enhanced by further evaporation due to high winds, by the formation of sea ice, which expells brine during ice growth, and by cooling. Exposed to evap­oration and to the chill high-latitude air masses, the surface water cools from about 10 to 2°C, releasing immense amounts of heat to the atmosphere, supplementing solar insolation there by some 25-30 per cent and heating Western Europe. The resulting dense high-latitude water, equiva­lent in volume to about twenty times the combined discharge of all the world's rivers, sinks to the bottom of the North Atlantic and fuels a south­ward-flowing density (thermohaline) current, which forms part of a global deep-water conveyor belt. This broad, slow and diffuse How, occurring at depths of greater than 1,500 m, is augmented in the South Atlantic/circum-Antarctic/ Weddell Sea region by more cold, saline, dense subsiding water. The conveyor belt then flows east­wards under the Coriolis influence, turning north into the Indian and, especially, the Pacific Ocean. The time taken for the conveyor belt circulation to move from the North Atlantic to the North Pacific has been estimated at 500-1,000 years. In the Pacific and Indian Oceans, a decrease of salinity due to water mixing causes the conveyor belt to rise and to form a less deep return flow to the Atlantic, the whole global circulation occupying some 1,500 years or so. An important aspect of this conveyor belt flow is that the western Pacific Ocean contains a deep source of warm summer water (29°C). This heat differential with the eastern Pacific assists the high phase Walker circulation.

The thermal significance of the conveyor belt implies that any change in it may promote climatic changes, which may be apparent at time scales of several hundred or thousand years. It has been suggested, however, that any impediment to the rise of deep conveyor belt water might cause ocean surface temperatures to drop by 6°C within 30 years at latitudes of 60°N. Changes to the conveyor belt circulation might be caused by lowering the salinity of the surface water of the North Atlantic by increased precipitation, ice melting, or fresh water inflow. However, the complex mechanisms and consequences of the deep ocean conveyor belt are still only imperfectly understood.

Упражнение 9.

Ответьте на следующие вопросы, исходя из информации
в тексте:

1. Что является основными факторами, обусловливающими глубоководную циркуляцию в океане?

2. Каковы основные крупномасштабные особенности глубоководной циркуляции в океане?

3. Чем могут быть вызваны изменения в глубоководной циркуляции, и к каким последствиям они могут привести?

Упражнение 10.

Выпишите 10 ключевых слов из текста в упражнении 8.

Упражнение 11.

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Дата добавления: 2014-12-27