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1 Living with the oceans. – A report on the state of the world's oceans

Great ocean currents

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The Coriolis force The Earth’s rotation causes all free linear motion on the Earth, such as air or water currents, to be ­diverted to one side. The diverting force is called the Coriolis force or Coriolis acceleration. It works in opposite directions in the northern and southern hemispheres. The Coriolis force is named after the French natural scientist Gaspare Gustave de Coriolis (1792 to 1843), who derived it mathematically.

1.9 >  Satellite photograph of the Gulf Stream and its eddies. Warm areas are red, cold areas are blue. © NASA

1.9 > Satellite photograph of the Gulf Stream and its eddies. Warm areas are red, cold areas are blue.

Eddies in the ocean – an important climate component

In addition to the large conveyor belt of thermohaline circulation, heat is also transported in the ocean by eddies, which are analogous to low-pressure systems in the atmosphere. But they are significantly smaller than the atmospheric low-pressure systems, which can often be several hundred kilometres wide. These mesoscale eddies form when water flows between regions with large ­density or temperature differences. They can be clearly recognized on satellite photographs. Investigations have shown that they not only occur at the ocean surface as, for example, in the North Atlantic area, but can also be located at great depths of thousands of metres, e.g. off the coast of Brazil. Because of their strong influence on the large-scale heat transport, these deep-sea eddies also play an important role in long-term climate processes.

Variable and dynamic – the influence of wind

Along with convection, winds also provide an important contribution in driving the ocean currents. In combination with the diverting force caused by the Earth’s rotation (Coriolis force) and the shape of the ocean basins, winds determine the characteristic patterns of the worldwide system of surface currents. Especially striking are the large gyres that extend across entire ocean basins, for example between America and Europe. These surface currents include the Gulf Stream in the Atlantic Ocean, which is driven both by wind and the thermohaline force, as well as the Kuroshio in the Pacific Ocean, whose intensity just decreases with depth. The Gulf Stream is a relatively fast current. Along the coast of North America it reaches a speed of around 3.6 kilometres per hour at the sea surface, which is a casual walking speed. It extends down to a depth of around 2000 metres, where the speed is around ten times slower because the influence of the wind is less and the density of the water is greater. Nevertheless, the wind can in fact have a direct influence down to great depths. Typical wind conditions can change for extended time periods. For example, the normally steady trade winds can blow from a different direction for months at a time, causing changes in the upwelling of water masses, and creating waves and currents in the ocean’s interior that resonate at depth for decades. These waves can also change the ocean temperature and thus also the regional climate. From satellites these waves are perceived as slowly moving ups and downs of the ocean surface.
Furthermore, in certain regions the prevailing winds cause persistent upwelling and downwelling motion. In some areas the winds drive surface waters away from the land masses, allowing cold water from greater depths to rise in its place. The surface-water temperatures in these areas are therefore especially low. Important upwelling regions are often found on the western margins of continents where the winds blow parallel to the coast (Chile, California, Namibia). In the southern hemisphere, for example, because of the Coriolis force, the water is pushed to the left away from the coast when the wind is blowing from the south. This produces a rolling motion in the water, whereby the water on the surface moves away from the coast and water rises to replace it from below. This upwelling water is usually rich in nutrients, which is why many upwelling regions are also abundant with fish.
1.10 > The world’s large ocean currents are also influenced by the prevailing winds. Warm ocean currents are red, and cold currents are shown in blue.
1.10 > The world’s large ocean currents are also influenced by the prevailing winds. Warm ocean currents are red, and cold currents are shown in blue. © maribus

1.11 > Heat exchange between the atmosphere and the sea surface (in watts per square metre) is very variable depending on the ocean region. Positive values indicate absorption of heat by the ocean, which is characteristic of the tropics, and negative values indicate a heat loss, which is typical for the northern latitudes. In the high arctic regions, however, heat loss is relatively low because the sea ice acts as an insulating layer and prevents heat escaping from the water. © maribus (after Barnier et al., 1994) 1.11 > Heat exchange between the atmosphere and the sea surface (in watts per square metre) is very variable depending on the ocean region. Positive values indicate absorption of heat by the ocean, which is characteristic of the tropics, and negative values indicate a heat loss, which is typical for the northern latitudes. In the high arctic regions, however, heat loss is relatively low because the sea ice acts as an insulating layer and prevents heat escaping from the water.

The ocean – a global storehouse for heat

In addition to huge masses of water, large ocean currents also transport enormous amounts of heat around the globe. Similar to the way the water tank in a heating system stores heat from the solar installation on the roof, the oceans are an immense heat reservoir that retains energy from the sun over a long time. The large ocean currents transport this heat for thousands of kilometres and, as illustrated by the example of the Gulf Stream, significantly influence the climate in many regions of the world. In the warm tropics and subtropics up to a latitude of around 30 degrees, more heat arrives at the Earth’s surface on a yearly average than it releases. In the higher latitudes, and extending to the poles, the opposite relationship exists. As a result the atmosphere and the oceans transport energy northward and southward from the equator to compensate for the imbalance. In some tropical regions, such as the eastern Pacific, the ocean gains more than 100 watts of heat per square metre, which is about what a hot-water tank produces to keep an apartment comfortable. In the higher latitudes the ocean releases heat. The areas of greatest heat loss are off the eastern coasts of North America and Asia and in parts of the Arctic, with values of up to 200 watts per square metre. In the North Atlantic and North Pacific regions the oceans release heat on an immense scale. The beneficiaries of this heat are those regions, including Europe, toward which the large current systems transport the warm water. The giant ocean currents transport a maximum amount of heat of just under three petawatts (quadrillion watts) to the north, which is around 600 times that produced by all the power stations worldwide. But the atmosphere also contributes to the energy balance between the tropics and the colder, higher latitudes. It transports an additional 2.5 to three petawatts of heat, resulting in a total northward transport of 5.5 to six petawatts. At European latitudes, heat transport in the atmosphere takes place through propagating low-pressure systems. In the Atlantic Ocean, however, the currents are more controlled and transport heat directly to the north. Here, warm water from the tropics flows northward far into the Arctic Ocean, where the water cools and releases heat into the environment. When it cools, the density increases. It sinks to greater depths and flows southward. The Atlantic current system transports enormous amounts of heat to the north through this thermohaline process and greatly exceeds the share transported by the wind-driven ocean circulation.
The Atlantic and Pacific Oceans each carry around one petawatt of heat northward from the tropics and sub­tropics. By comparison, the share moved by the Indian Ocean is negligible. In this system the Atlantic has a unique function among the world’s oceans. It is the only ocean basin that transports heat northward throughout its length, even in the southern hemisphere. Europeans all benefit from the northward trend, thanks to the Gulf Stream and the North Atlantic Current. The climate in the region of the North Atlantic is comparatively mild, especially in northwest Europe, including Germany. The winters in other regions at the same latitude are notably colder. In Canada, for example, the winter temperatures are around ten degrees Celsius lower than in Western Europe. But it is not the ocean circulation alone that causes the mild­climate. Air currents also contribute significantly to this phenomenon. The distribution of mountain ranges, particularly the Rocky Mountains, which run from north to south along the west coast of North America, together with the influence of the Coriolis force, causes the formation of very stable, large-scale vortices in the atmosphere called standing planetary waves. Such a vortex lies above the USA because the Rocky Mountains act as an obstacle to divert large air masses. As a consequence the winds are predominantly westerly over the Atlantic carrying relatively mild air to northwest Europe, and fend off the cold from the east. >
1.12 > Oceans contribute to the global transport of heat with different intensities. In the southern hemisphere, only the Atlantic transports heat to the north (positive values). The equator lies at zero degrees. The Atlantic and Pacific each carry around one petawatt of heat as far as 20 degrees north latitude. Further to the north, the Atlantic carries more than the Pacific. The Indian Ocean, on the other hand, makes a negligible contribution to northward heat transport.
1.12 > Oceans contribute to the global transport of heat with different intensities. In the southern hemisphere, only the Atlantic transports heat to the north (positive values). The equator lies at zero degrees. The Atlantic and Pacific each carry around one petawatt of heat as far as 20 degrees north latitude. Further to the north, the Atlantic carries more than the Pacific. The Indian Ocean, on the other hand, makes a negligible contribution to northward heat transport. © maribus (after Trenberth und Solomon, 1994)
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