Ocean Circulation 1:The Dynamic Ocean

How do ocean currents affect the global climate?

They make temperatures more moderate in both tropical and temperate regions.

Density Currents

Density currents are vertical currents of ocean water that result from density differences among water masses. Denser water sinks and slowly spreads out beneath the surface. An increase in seawater density can be caused by a decrease in temperature or an increase in salinity. Processes that increase the salinity of water include evaporation and the formation of sea ice. Processes that decrease the salinity of water include precipitation, runoff from land, icebergs melting, and sea ice melting. Density changes due to salinity variations are important in very high latitudes, where water temperature remains low and relatively constant.

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Why does upwelling increase the productivity of ocean waters?

It lifts nitrates, phosphates, and other dissolved nutrients to the surface.

Antartica

Near Antarctica, surface conditions create the highest density water in the world. This cold, salty water slowly sinks to the seafloor, where it moves throughout the ocean basins in slow currents. After sinking from the surface of the ocean, deep waters will not reappear at the surface for an average of 500 to 2000 years.

Ocean Currents

Ocean currents also play a major role in maintaining Earth's heat balance. They do this by transferring heat from the tropics, where there is an excess of heat, to the polar regions, where less heat exists. Ocean water movement accounts for about a quarter of this heat transport. Winds transport the remaining three-quarters.

How does upwelling affect the productivity of many regions of the ocean, including waters off the coasts of California and Peru?

Upwelling increases productivity by lifting nutrients to the surface.

A Conveyor Belt

A simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean through the Indian and Pacific oceans and back again. Warm water in the ocean's upper layers flows toward the poles. When the water reaches the poles, its temperature drops, and salinity increases, making it denser. Because the water is dense, it sinks and moves toward the equator. It returns to the equator as cold, deep water that eventually upwells to complete the circuit. As this "conveyor belt" moves around the globe, it influences the global climate by converting warm water to cold water and releasing heat to the atmosphere.

The Coriolis Effect

Although wind is the force that generates surface currents, other factors also influence the movement of ocean waters. The most significant of these is the Coriolis effect. The Coriolis effect is the deflection of currents away from their original course as a result of Earth's rotation. Because of Earth's rotation, currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As a consequence, gyres flow in opposite directions in the two different hemispheres.

Coastal Upwelling

Coastal upwelling occurs in these areas when winds blow toward the equator and parallel to the coast. Coastal winds combined with the Coriolis effect cause surface water to move away from shore. As the surface layer moves away from the coast, it is replaced by water that "upwells" from below the surface. This slow, upward movement of water from depths of 50 to 300 meters brings water that is cooler than the original surface water and results in lower surface water temperatures near the shore.

Cold Currents

Cold currents begin in cold, high-latitude regions. As cold water currents travel toward the equator, they help moderate the warm temperatures of adjacent land areas. Such is the case for the Benguela Current along western Africa, the Peru Current along the west coast of South America, and the California Current.

How do deep-ocean currents move differently than surface currents?

Deep-ocean currents experience mostly vertical movement, whereas surface currents experience mostly horizontal movement.

Evaporation

Density currents can also result from increased salinity of ocean water due to evaporation. In the Mediterranean Sea, conditions exist that lead to the formation of a dense water mass at the surface that sinks and eventually flows into the Atlantic Ocean. Climate conditions in the eastern Mediterranean include a dry northwest wind and sunny days. These conditions lead to an annual excess of evaporation compared with the amount of precipitation. When seawater evaporates, salt is left behind, and the salinity of the remaining water increases.

What determines the direction of gyres in both the Northern Hemisphere and Southern Hemisphere?

Earth's rotation

Gyres

Huge circular-moving current systems dominate the surfaces of the oceans. These large whirls of water within an ocean basin are called gyres (gyros = a circle). There are five main ocean gyres: the North Pacific Gyre, the South Pacific Gyre, the North Atlantic Gyre, the South Atlantic Gyre, and the Indian Ocean Gyre.

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Upwelling

In addition to producing surface currents, winds can also cause vertical water movements. Upwelling is the rising of cold water from deeper layers to replace warmer surface water. Upwelling is a common wind-induced vertical movement. One type of upwelling, called coastal upwelling, is most characteristic along the west coasts of continents, most notably along California, western South America, and West Africa.

Deep-Ocean Circulation

In contrast to the largely horizontal movements of surface currents, deep-ocean circulation has a significant vertical component. It accounts for the thorough mixing of deep-water masses.

High Latitudes

Most water involved in deep-ocean density currents begins in high latitudes at the surface. These deep ocean currents are driven by differences in water density. Deep ocean currents occur far below the surface of the oceans. Water can become denser with a decrease in temperature or an increase in salinity. At the Earth's poles, the air chills the water molecules at the water surface, causing the molecules to slow down and move closer together. This decrease in volume makes the water denser.

Surface Circulation

Ocean currents are masses of ocean water that flow from one place to another. The amount of water can be large or small. Ocean currents can be at the surface or deep below. The creation of these currents can be simple or complex. In all cases, however, the currents that are generated involve water masses in motion.

Ocean Currents and Climate

Ocean currents have an important effect on climates. When currents from low-latitude regions move into higher latitudes, they transfer heat from warmer to cooler areas on Earth.

High Latitudes 2

Seawater at the poles can also get cold enough to freeze. As seawater freezes, the dissolved solids get left behind, making the water below the ice saltier and, therefore, denser. Water can also become saltier in warmer areas of the globe due to evaporation. The denser water sinks to the ocean floor and forms deep ocean currents. This process is called thermohaline (thermo = heat, haline = salt) circulation. After this water sinks, it is removed from the physical processes that increased its density in the first place. Its temperature and salinity remain largely unchanged during the time it is in the deep ocean. Because of this, oceanographers can track the movements of density currents in the deep ocean. By knowing the temperature, salinity, and density of a water mass, scientists are able to map the slow circulation of the water mass through the ocean.

Surface Currents

Surface currents are movements of water that flow horizontally in the upper part of the ocean's surface. Surface currents develop from friction between the ocean and the wind that blows across its surface. Some of these currents do not last long, and they affect only small areas. Such water movements are responses to local or seasonal influences. Other surface currents are more permanent and extend over large portions of the oceans. These major horizontal movements of surface waters are closely related to the general circulation pattern of the atmosphere.

Salinity

The surface waters of the eastern Mediterranean Sea have a salinity of about 38 parts per thousand. In the winter months, this water flows out of the Mediterranean Sea into the Atlantic Ocean. At 38 parts per thousand, this water is denser than the Atlantic Ocean surface water at 35 parts per thousand, so it sinks. This Mediterranean water mass can be tracked as far south as Antarctica.

Great Britain and Newfoundland are located directly opposite of each other across the Atlantic Ocean. Why do the two places have different climates?

The Gulf Stream brings warm water to Great Britain.

The Gulf Stream

The Gulf Stream, a warm water current shown in the figure, is an excellent example of this phenomenon. The Gulf Stream brings warm water from the equator up to the North Atlantic Current, which is an extension of the Gulf Stream. This current allows Great Britain and much of northwestern Europe to be warmer during the winter than one would expect for their latitudes, which are similar to the latitudes of Alaska and Newfoundland. The prevailing westerly winds carry this warming effect far inland. For example, Berlin, Germany (52 degrees north latitude), has an average January temperature similar to that experienced at New York City, which lies 12 degrees latitude farther south. The effects of these warm ocean currents are felt mostly in the middle latitudes in winter. In contrast, the influence of cold currents is most felt in the tropics or during summer months in the middle latitudes.

What happens as currents move from low-latitude regions to higher latitude regions?

The currents transfer heat.

What causes the friction that fuels surface currents?

The friction between the ocean and the wind that blows across the surface of the ocean causes surface currents to form.

How can density currents form in warm waters, such as the Mediterranean Sea?

The salinity of surface water increases from evaporation.

Upwelling Nutrients

Upwelling brings greater concentrations of dissolved nutrients, such as nitrates and phosphates, to the ocean surface. These nutrient-enriched waters from below promote the growth of microscopic plankton, which in turn support extensive populations of fish and other marine organisms.

How do ocean currents affect climate?

Warm water currents that come from equatorial regions transfer heat to cooler areas of Earth. Coldwater currents from the poles moderate warm temperatures of adjacent land areas.

How do surface currents develop?

Winds blow water across the surface.

At high latitudes, which process helps form a deep-ocean density current?

sea ice forming