Ocean Currents

Recall that the oceans are vertically stratified according to density (a function of salinity and temperature) and cold, salty water is denser than warmer and fresher water. The sinking of these cold, dense layers propels a global convection and circulation system. Convection connects two distinctly different components of the ocean: the

near­surface layers that are in contact with the variable atmospheric fields of wind, radiation and precipitation, and the deep regions of the ocean.

At the surface, currents, temperature and salin­

ity fluctuate on a scale of weeks to months but at greater depths the environmental conditions change over time scales of decades or centuries.

The freezing of water in the Polar Regions plays a central role in driving the global circulation sys­

tem. This is called the thermohaline circulation system because it is influenced by both the tem­

perature and salinity characteristics of the ocean water (Fig. 1.24). When water freezes to ice, it only contains about five tenths of a per cent salt and this increases the salinity of the surrounding ocean water and thus its density. In the region around Iceland (in the Greenland and Labrador Seas) these cold waters of the Atlantic sink down as a kind of

Fig. 1.22 Satellite images show that ocean currents can have complicated patterns. The depicted Gulf Stream is a strong ocean current that carries warm water from the sunny tropics to higher latitudes. This current extends from the Gulf of Mexico up the East Coast of the United States (Florida current) and departs from North America south of the Chesapeake Bay and heads across the Atlantic to the British Isles. The water within the Gulf Stream moves at a stately pace of four miles per hour. Even though the current cools as the water travels thousands of miles northward, it remains warm enough to moderate Northern European climate. The coldest waters are shown as a purple colour, with blue, green, yellow, and red representing progressively warmer water. Temperatures range from 7 to 22 degrees Celsius. (Photo credit: The sea surface temperature image was created at the University of Miami using the 11­ and 12­micron bands, by Bob Evans, Peter Minnett, and co­workers for NASA’s Visible Earth, http://visibleearth.nasa.gov/view_rec.php?id=215).

Wind

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Surface current Watermovement in a surface current

© 2011 Hans van der Baan / Ingeborg Scheffers

elevator to the deep. This water mass produced by convection in the Arctic is called North Atlantic Deep Water (NADW). A similar convection cell exists in the Antarctic regions which produces the Antarctic Bottom Water (AABW) that flows across the ocean floor halfway around the globe into the North Atlantic. Because of their temperature and higher salinity the water masses produced here sink all the way to the sea floor and form the bot­

tom layers, upon which flows the slightly warmer Arctic deep water. Both deep water currents move

very slowly (at around one to three kilometres per day, due to their high density) along the abyssal plains towards the equator, where the temperature at 6000–7000 m depth is not higher than 2–3°C.

In the near surface layers of the ocean a return flow of warm water occurs in the global conveyor belt of thermohaline circulation. The warm upper water from the east Pacific Ocean crosses north of Australia and through the Indian Ocean south of Africa and moves northwards to feed the Gulf Stream from Mexico to NW Europe. The amount of

Fig. 1.23 The “Ekman Spiral”, shows a complex system of water movements. Because of the Coriolis effect, a constant turning of the water mass (spiralling clock­wise in the Northern Hemisphere) can be observed.

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Warm and cold water ocean currents

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equator © 2011 Hans van der Baan / Ingeborg Scheffers

(January conditions based on 30-year record, according U.S Navy Oceanographic Office) cold, saline deep water warm surface water Antarctic bottom water convection areas

Warm and cold water ocean currents

© 2011 Hans van der Baan / Ingeborg Scheffers

Fig. 1.24 All ocean currents are connected by the so called “conveyor belt”, which is an important factor in driving Earth’s climate.

Fig. 1.25 A simplified picture of surface ocean currents.

water involved in the thermohaline circulation sys­

tem is immense with a volume of around 400.000 cubic kilometres, which is equivalent to about one third of the total water in the ocean. This is enough water to fill a swimming pool 400 kilometres long, 100 kilometres wide, and ten kilometres deep.

On average, the oceanic conveyor belt transports about 20 million cubic metres of water per second past a given location, which is almost 5000 times the amount that flows over Niagara Falls in North America (World Ocean Review 2010). In general, circulation and exchange of the water masses is very slow and in the order of several hundred years for the complete cycle.

Wind that flows across the sea creates a fric­

tion between the air and the surface of the ocean.

As a consequence the surface winds, in combi­

nation with the Coriolis Effect and the shape of the ocean basins, drag the water slowly forward, creating a current of water as broad as the air current, but rarely more than 50–100 m deep.

Consequently, the surface circulation pattern of the oceans is widely adapted to the global wind pattern (Fig. 1.25) and shows a distinctive pattern:

These patterns curve to the right – clockwise – in the Northern Hemisphere and to the left – coun­

ter clockwise – in the Southern Hemisphere. Each major ocean current in both hemispheres is part of a large subcircular current system called a gyre.

Five gyres exist in the world’s oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean. Simplified we can say that on both sides of the equator warm, westward-flowing currents, the North and South equatorial currents, occur as the trade winds blow towards the west on either side of the equator, dragging the surface ocean water along with them. Sandwiched in between them and flow­

ing eastward along the equator is the Equatorial Counter current which is associated with the dol­

drums, a belt of light and variable winds. When the North and South equatorial currents encounter landmasses along the western edge of the ocean basin they are deflected poleward. These western boundary currents flow parallel to the coastline towards the poles and transport enormous amounts of heat into the higher latitudes, significantly influ­

encing the climate in many regions of the world.

In the Atlantic Basin, this current is called the Gulf Stream, a relatively fast current flowing along the coast of North America towards Europe.

It reaches a speed of around 3.6 kilometres per

hour at the sea surface, which is a casual walking speed! Europeans all benefit from the Gulf Stream as the climate in the region of the North Atlantic is comparatively mild, especially in northwest Europe whereas the winters in other regions at the same latitude are notably colder.