The Ocean Current Conveyor Belt

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Ocean waters are constantly on the move. How they move influences climate and living conditions for plants and animals, even on land. Currents flow in complex patterns affected by the wind, the water's salinity and heat content, bottom topography, and the earth's rotation.

Seawater circulates on a global scale, taking thousands of years to complete a circuit of the globe. Water moves around the world in a continuous three-dimensional current (Figure 1). The currents disperse energy around our planet and influence the Earth’s climate.

Figure 1. Earth's global currents.

How does the converyor belt work?

Deep water forms when sea water entering polar regions cools or freezes, becoming saltier and denser. Colder or saltier water tends to sink.

A global "conveyor belt" is set in motion when deep water forms in the North Atlantic, sinks, moves south, and circulates around Antarctica, and then moves northward to the Indian, Pacific, and Atlantic basins. It can take one thousand years for water from the North Atlantic to find its way into the North Pacific.

Warm surface currents invariably flow from the tropics to the higher latitudes, driven mainly by winds, as well as the Earth's rotation.

Figure 2. The wind patterns on Earth.

Recent findings have suggested that the ocean current conveyor belt can be switched on and off. Where is the switch? It is thought that it is somewhere in the North Atlantic and the Norwegian Sea. One concern is that some human activities may be a factor in switching the belt off. This could lead to Earth experiencing a mini ice-age.

Thermohaline circulation

As mentioned above, temperature [thermo] differences will influence water density. The colder the water, the denser it becomes. Warm water is less dense and will float on colder, denser water.

Sea water is salty [haline] but not all sea water has the same saltiness or density. There is a trade-off between the effects of the water temperature and salinity on water density.

Currently, the thermo part of this interaction appears to be dominant. In the Norwegian Sea cold, salty water (very dense), sinks to great depths and pushes cold water along the ocean floor towards the equator. The deep water gradually warms up on this journey and rises towards the surface. At the same time, the warm surface water from the equator is forced northwards towards the pole and cools as it flows north.

Global warming

Some scientists think that global warming is making the haline force stronger than the temperature effect. If this is true the consequence may be a much colder climate in northern Europe. Initial warming of the air will cause more evaporation and rainfall. Added to this will be the melting of polar ice caps, adding more fresh water to the oceans. Because fresh water is less dense than salt water, it will form a layer over the warmer, saltier water reducing the heat transfer from the water to the atmosphere. This might switch off the global current conveyor belt and stop the water transferring heat energy to northern Europe. It could also influence other areas of the global ocean.

Figure 3. Cross section through the Atlantic ocean from the North Pole (left) to the South Pole (right). Warm salty water is dense and sinks to the bottom as it moves from north to south. Cold fresh water is also dense and sinks to the bottom as it moves from the South Pole northward. At the surface the water the density of the water is modified by evaporation, rain, and heat.

In the cartoon above, zones of high evaporation (the desert band) and zones of high rainfall (the temperate zone) are shown. These influence the salinity of the surface water. But ice does too. In winter it freezes, drawing fresh water from the sea, while leaving saltier water behind. In the summer, the reverse happens. Whether salty or fresh, the water is very cold and dense enough to sink to the deepest layers of the ocean.

Guided by mid-ocean ridges and continental margins, the deep waters do not swirl around by Coriolis forces resulting from the Earth’s rotation, but flow in one direction. At the South Pole, the North Pole water re-surfaces, rich in nutrients. But some of the water is cooled further and becomes Antarctic bottom water. This water sinks to the bottom of both the Pacific, Indian and Atlantic Oceans, fills the deepest parts of their basins and ultimately flows northward in all of these oceans.