Ocean currents
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In this section, we examine the characteristics of wind-driven surface-ocean currents, components of the huge gyres that dominate the central regions of the open ocean. Western boundary currents are the strongest segments of these gyres and often spawn warm- and cold-core eddies known as rings. In coastal and equatorial regions, coupling of wind and surface waters can cause upwelling and downwelling. GyresThe long-term average pattern of ocean surface currents is plotted in the map above. Some currents are relatively warm whereas others are cold. Winds associated with a passing storm system can disturb the ocean surface and cause the actual flow of ocean currents locally to deviate temporarily from long-term average patterns. Surface currents within gyres vary considerably in strength, width, and depth. The northeastward flowing Gulf Stream of the northwestern Atlantic and the Kuroshio Current of the northwestern Pacific are the swiftest surface currents with velocities averaging 3 to 4 km per hr (1.8 to 2.5 mph). Those currents are also relatively deep and narrow, usually measuring no more than 50 to 75 km (30 to 45 mi) across. On the eastern arms of these gyres, the southward flowing Canary and California Currents, respectively, are hundreds of kilometers wide and rarely flow at more than 1 km per hr (0.6 mph). The westward flowing South Equatorial Current links the two subtropical gyres of the Atlantic Ocean. The eastward projection of Brazil splits the South Equatorial Current into two segments. The segment flowing southward forms the western arm of the South Atlantic gyre (the Brazil Current, a western boundary current). The segment flowing northward merges with the North Equatorial Current, which then splits into two currents that rejoin as they exit the Gulf of Mexico between Florida and Cuba to become the Florida Current. This current becomes the Gulf Stream that flows northeasterly and passes Cape Hatteras, NC. In that region, the current speed may be as great as 9 km per hr (5.5 mph). Near Chesapeake Bay, the amount of water transported in the Gulf Stream exceeds 90 million cubic m per sec; the volume of water transported falls to about 40 million cubic m per sec by the time the current reaches southern Newfoundland. (For comparison purposes, 90 million cubic m per sec is equivalent to about 4500 times the discharge of the Mississippi River-enough to fill the Lake Superior basin in about 1.5 days.) The North Atlantic subtropical gyre includes the Gulf Stream, which becomes the North Atlantic Current at about 40 degrees N and 45 degrees W and flows easterly across the North Atlantic. The cold waters of the Labrador Current flow southeastward between Canada and Greenland while the East Greenland Current flows southwestward between Greenland and Iceland. Farther east, the North Atlantic Current splits into the Norwegian Current (which flows northeasterly between Iceland and Europe along the coast of Norway) and the Canary Current (which flows southward along the west coast of Spain, Portugal, and North Africa). The Canary Current merges with the North Atlantic Equatorial Current, thus completing the North Atlantic subtropical gyre. |
Like their Northern Hemisphere counterparts, currents in the South Pacific and South Atlantic are narrowest and flow most rapidly along their western margins but are broad and slow along their eastern margins. The Indian Ocean gyre varies more than the others in response to seasonal reversal of monsoon winds. In summer, moist surface winds blow from sea to land whereas in winter, dry surface winds blow from land to sea. Above the central regions of the ocean basins are broad expanses of generally light winds or calm air associated with the semi-permanent subtropical high-pressure systems centered near 30 degrees latitude. These massive fair-weather systems are semi-permanent in that they persist throughout the year but undergo seasonal shifts in relative strength and location (following the sun). Persistent fair weather and high temperatures enhance the rate of evaporation in these regions of the subtropical ocean resulting in surface seawater with a salinity significantly higher than average. An example is the vast Sargasso Sea which lies under the Bermuda-Azores subtropical high in the North Atlantic and features an average surface water salinity of 36.5 to 37.0 psi. In addition to the South Pacific and South Atlantic Ocean gyres, prevailing winds generate the Antarctic Circumpolar Current. At about 60 degrees S, this current also makes up, at least in part, the southern edges of the gyres in the Atlantic, Pacific, and Indian Oceans. This easterly-flowing current encircles the Antarctic continent rather than rotating as a basin-centered gyre and features the ocean's greatest water flow. Such a globe-circling current is possible only in the Southern Ocean; elsewhere, continents interrupt east-west currents. The Drake Passage, the relatively narrow strait between Cape Horn (the southern tip of South America) and the Antarctic Peninsula, deflects some waters from the Antarctic Circumpolar Current to form a portion of the Peru Current that flows northward along the west coast of South America. Sub-polar gyres, smaller than their subtropical counterparts, occur at high latitudes of the Northern Hemisphere; they are the Alaska gyre in the far North Pacific and the gyre south of Greenland in the far North Atlantic. The counterclockwise surface winds in the Aleutian and Icelandic sub-polar lows drive the sub-polar gyres. (The Aleutian low and Icelandic low are persistent features of the atmosphere's planetary-scale circulation.) Hence, viewed from above, the rotation in the sub-polar gyres is opposite that of the Northern Hemisphere subtropical gyres. Ekman transport moves surface waters away from the central region of the sub-polar gyres. The thinner surface layer permits more nutrient-rich waters from deeper in the ocean to move upward into the photic zone, thereby increasing productivity in these regions. Adapted from DataStreme Ocean and |
