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Research /
Oceans General Characteristics of the World's Oceans As mentioned in the introduction, the ocean's floor is not level. This is due in part to currents, waves, atmospheric pressure differences, and variations in gravity. Currents in the ocean, or water masses in motion, allow for the circulation of water and its contents. The driving force of currents is the frictional drag of winds, along with the interplay of the Coriolis force, density differences caused by temperature and salinity, the configuration of the continents and ocean floor, and astronomical forces, like the moon. Currents can be broken down into two forms of circulation. One is wind-driven circulation and the other is thermohaline circulation. Wind-driven circulation is the horizontal movement of the upper waters set in motion by moving air masses. It is fast and plays a major role in the transporting of the surplus heat from the tropics to the cooler high latitudes. Thermohaline circulation, on the other hand, is the slow moving water mass of the deep ocean. It is a vertical movement produced at the surface in high latitudes by temperature and salinity, which create high density masses. These high density masses then sink and spread beneath the surface waters resulting in deep-water movement or circulation. The image below shows the characteristic currents of the ocean.
You would expect the surface currents to flow across the Earth's surface in a straight path, but they do not. Instead they deflect because of the Coriolis force, which is due to the Earth's rotation. This deflection then forms circulation systems known as gyres. Gyres typically appear towards the western side of each ocean basin and have slightly stonger currents. The reason for this is western intensification. As trade winds sweep across the ocean surface, they push the water near the equator in a westward direction through concentrated channels. When the water reaches its western margins it actually piles up to about 15 cm and then spills northward and southward in strong currents, flowing through tight channels (called western boundary currents). In adition to the equatorial western current, an equatorial countercurrent flowing eastward through the full extent of the Pacific, Atlantic, and Indian oceans, is generated. This strong current flows either alongside or just below the westward current. Examples of the western intensification are the Gulf Stream and the Kuroshio in the northern hemisphere. In the Gulf Stream, water can move 70-240 km in a day, although completing the entire gyre in which it lies may take about a year. If you were to imagine taking a cross-sectional view of the ocean, it would look like the graph above. The diagram illustrates the general ocean currents as you go deeper into the sea. The y-axis is depth in meters and the x-axis is latitude. The black area indicates average bathymetry over longitude. The arrows indicate the direction in which the water mass is moving. Usually, off the Pacific coasts of North and South America and the subtropical and midlatitude west coast of Africa, surface water moves across the coast developing an upwelling current. This rise of cool and nutrient-rich water from great depths replaces the vacating water. While in other regions like the western end of an equatorial current or along the margins of Antarctica, accumulated water is pushed downward in a downwelling current. Additionally, significant mixing curents are generated along the ocean floor carrying heat energy and salinity and travel the full extent of the ocean basins. If these upwellings in the Indian Ocean and North Pacific were to be interconnected with the downwellings in the North Atlanic, they would form a vast conveyor belt that could take up to a 1000 years to traverse. The diagram above simulates the general ocean currents of the Atlantic Ocean as you go deeper into the water. The y-axis is depth in meters and the x-axis is latitude. The black area indicates average bathymetry over longitude. The arrows indicate the direction in which the water mass is moving. The World Ocean Circulation Experiment (WOCE) which began in 1990 is a five-year research program which was established to gain more information about oceans; specifically on their contribution to heat flow. This information can be further applied in creating more accurate models of decadal climate change. According to WOCE, it is said that oceans are approximately just as responsible for the flow of heat from the tropics to higher latitudes in the Northern Hemisphere as the atmosphere.
http://icp.giss.nasa.gov/research/oceans/oceanchars/currents.html Last updated: 1999:03:05 |