Australia: The Land Where Time Began

A biography of the Australian continent 

Global Thermohaline Circulation

The thermohaline circulation is the circulation of the waters of the entire global ocean, that is a slow, churning current that is driven by variations in density caused by differences of temperature and salinity, rather than by wind as are most ocean currents. Located mainly in polar oceans, it involves surface water gaining salinity from the formation of ice, and during the process of freezing the salt is driven out of the ice and back to the sea. A slow flow of water is thus sucked in towards the poles from the tropics, which carries heat and salt. The shape of the continents and the Coriolis force associated with the rotation of the Earth and the shape of the continents modifies the flow.

The surface flow is a slow flow which mimics in some respects circulation that is wind-driven, though in this case the circulation would continue in the absence of wind. In the northern Indian Ocean and the North Pacific there are 2 broad areas where upwelling takes place. In these 2 areas deep water is brought to the surface and then slowly flows to the south and west. The Pacific water flows through the East Indies and into the Indian Ocean water which it joins to flow around the Cape of Good Hope then northwards to the tropical Atlantic. It gathers more water in the tropical Atlantic from the Gulf of Mexico then moves to the northeast across the North Atlantic, just as the Gulf Stream and North Atlantic Current do. The flow continues to the north towards the Arctic Ocean but then vanishes. Somewhere the water sinks. A slow flow of deep water can be detected moving southwards through the North Atlantic and South Atlantic, and when it reaches the Indian Ocean and the Pacific Ocean it completes a single full journey along a great conveyer belt after about 1,000 years. It is called the Great Ocean Conveyor Belt, though the flow is much more complex than this, as it is broken up into a series of cells. Overall it is an inexorable slow flow of water that is driven by the very fundamental driving forces of heating, cooling, evaporation and the rotation of the Earth.

The cogs driving the Great Ocean Conveyor are the forces of upwelling which brings deep water to the surface in the Indian Ocean and Pacific Ocean, and the downwelling forces causing downwelling of surface water. When considering the case of changes taking place in the polar regions these broad areas of upwelling are ignored while the driving forces of the more concentrated sinking are considered which occurs in the northern Atlantic. The question to be answered is where and why does it happen.

It has been found that it happens only in 2 locations that are surprisingly small in extent (Marshall & Schott, 1999). One is in the centre of the Labrador Sea, where the cold winter winds blowing off Labrador and Greenland chill the surface water. The density of the cooling water increases through the winter and eventually gains enough density to sink to great depths. The volume of the water that is sinking crucially depends on the air temperature during winter, and varies greatly from year to year. Wadhams says the other area of downwelling is more interesting as the process involves sea ice. This is a tiny area in the Greenland Sea, located at 75oN 0oW, and changes at this critical site affect the entire world.

Sources & Further reading

  1. Wadhams, P., 2016, A Farewell to Ice, Penguin Books Ltd


Author: M. H. Monroe
Last updated:  21/09/2016
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