VIEWS: 9 PAGES: 5 POSTED ON: 8/19/2011
Ocean Surface Currents http://www.sfos.uaf.edu/msl111/notes/cur.html#1 1. Ocean currents are primarily driven by winds, and so the global patterns of ocean currents correspond to wind patterns. However, the ocean (particularly patterns of sea surface temperature) also influence winds. So, the atmosphere and ocean are coupled, and both must be considered in order to understand surface ocean circulation. 2.Winds are due to convection of the atmosphere. The sun warms the land or ocean surface. The earth's surface warms the air. The air becomes less dense, and rises. The air cools at higher levels in the atmosphere, becomes more dense, and sinks. There is a general pattern of rising air at the equator, where the earth's surface is very warm. Cooling of this warm, moist air leads to condensation of water vapor. This results in very high rainfall in equatorial regions. Also note that much of the heat carried with the rising air is latent heat of vaporization, which is released when the water vapor condenses. Winds are horizontal flows of air between zones of upward and downward motion. Convection of the atmosphere transports heat (much of it as latent heat) from the equator toward the poles. 3. The rotation of the Earth results in three large convection cells in each hemisphere, extending from roughly 0 to 30°, 30-60°, and 60-90°. Rising air (low atmospheric pressure) dominates at 0° (actually about 5° N) and at 60° N and S latitude. These are zones of high precipitation. Sinking air (high atmospheric pressure) dominates at 30° N and S latitude and at the poles. These are zones of low precipitation. You might expect that winds would blow toward the north and south. Actually, global winds blow partly to the east and west. 4. Global wind patterns are influenced by the Earth's rotation; this influence is called the Coriolis Effect. The atmosphere and ocean are only weakly connected to the Earth's surface. So, the atmosphere and ocean are not compelled to rotate at the same speed as the Earth. Observations of winds (and ocean currents) are usually conducted from a fixed point on the earth. Such points are always rotating about the Earth's axis. In the absence of outside forces (like friction), moving objects will continue to travel in a straight line. But, this motion will not appear to be a straight line if the observer is moving, as observers are on the rotating Earth. It will appear to the observer that some force has acted on the moving object to change its direction. But, no force has actually been applied. So, although the Coriolis Effect is sometimes called the Coriolis Force, that is not really correct. 5. The Coriolis Effect deflects moving objects that are subject to little or no friction with the earth's surface. Moving objects are deflected to the right of their path of travel in the northern hemisphere. Moving objects are deflected to the left of their path of travel in the southern hemisphere. The Coriolis Effect increases with increasing latitude. It is zero at the equator. This is because the circumference of the Earth changes more with increasing latitude as you move closer to the poles. The Coriolis Effect increases with increasing speed (velocity). The Coriolis Effect occurs because an object that appears to be stationary to an observer on the Earth is actually rotating with the Earth's surface. If that object is not connected to the Earth's surface, it retains that rotation when it moves. But, the earth's surface in the object's new locations is not rotating at the same speed, because the rotational velocity of the earth's surface varies with latitude. So, to the fixed observer, comparing the motion to fixed points on the earth's surface, it appears that the object is not moving in a straight line. 6. The Coriolis effect modifies atmospheric circulation so that: Winds from 0° to 30° and from 60° to 90° latitude blow partly from the east (and so are called easterlies). Surface winds from 0 to 30° and from 60 to 90° also blow toward the equator. The winds in the 0 to 30° N latitude band are called the northeast trade winds (blowing from the north and east toward the equator). The winds in the 0 to 30° S latitude belt are called the southeast trade winds. Winds from 30° to 60° latitude blow partly from the west (and are called westerlies). These winds also tend to blow away from the equator (southwesterlies in the northern hemisphere and northwesterlies in the southern hemisphere). 7. The differences in seasonal heating of the land and the ocean produce changes in the global wind patterns, as well as local winds that differ from the global average winds. In summer, the land heats faster and is warmer than the sea. So, air rises over land and sinks over the ocean, resulting in winds that blow from ocean to land. Often this pattern results in high precipitation over land. This condition is called wet monsoon in the tropics. In winter, the land is colder than the sea. Air sinks over land and rises over the ocean, resulting in high pressure over land, winds blowing from land to sea, and, often, little rainfall. This condition is called dry monsoon in the tropics. 8. Hurricanes, typhoons, and cyclones are violent wind systems that draw their energy from the warm surface ocean. Warm, moist air rises. Energy in this air mass comes both from heat and the latent heat of vaporization of water. Air flows horizontally along the Earth's surface, into the low pressure area created by the rising air. The Coriolis Effect deflects that air to the right in the northern hemisphere. That causes the air mass to spin counterclockwise (cyclonic circulation) in the northern hemisphere. Ordinary storms form similarly; hurricanes are unusually strong because of the greater energy available from very warm ocean surfaces in the tropics. 9. Wind causes movement of ocean water, because of friction between the moving air and the sea surface. Water speed can be up to about 2% of wind speed. However, the sea surface does not move in the same direction as the wind; it is deflected by the Coriolis Effect. Motion of the sea surface is 45° to the right of the wind in the northern hemisphere (left of the wind in the southern hemisphere), theoretically. Observed deflections are often less. For example, Fridthof Nansen observed ice drift 20°-40° to the right of the wind when his ship was frozen in polar ice 1893-96. Friction between successive layers also causes subsurface water to move in response to the wind. The transfer of energy between layers is not 100% efficient, so deeper layers move progressively slower. At <100 m, wind-driven motion ceases. Motion of each successive layer is deflected by the Coriolis effect. So, the direction of water motion rotates to the right in the northern hemisphere (left in the southern hemisphere) This pattern of water motion is called the Ekman Spiral. Net transport of water due to winds is theoretically 90° to the right of the wind direction in the northern hemisphere. This Ekman Transport is the summed-up water movement for all of the layers in the Ekman Spiral. 10. Global wind patterns and Ekman Transport cause the global patterns of sea surface currents. The trade winds near the equator (from the northeast and southeast) cause surface currents that flow parallel to the equator, from east to west. (Equatorial currents.) Net wind-driven transport of water is toward the northwest and southwest. This results in divergence of the sea surface. Water must move upwards at the equator to replace water that is transported by the winds. This upward movement is called upwelling. Deeper waters are generally rich in nutrients. Thus, upwelling areas tend to be highly productive. The accumulation of biogenic sediments near the Equator reflects this high productivity. Westerly winds from 30° to 60° latitude result in surface currents which flow to the west. Net transport of water is toward 30° latitude. Note that the tropical trade winds also transport water toward 30°. This results in convergence of surface waters. Water in zones of convergence tends to flow downwards; this motion is called downwelling. Because downwelling tends to move nutrient-rich waters farther from the surface, such areas have low productivity. The effects of polar easterly winds are complicated by land masses. 11. Ekman transport also causes coastal upwelling. When a northerly wind blows parallel to a coast in the northern hemisphere, water transport (90° to the right of the wind) is away from shore. Because of the coast, surface water can't replace the water that is transported away. Instead, water upwells from depths of up to 300 m. This water is rich in nutrients, causing coastal upwelling areas to be highly productive. Coastal upwelling occurs seasonally along the western margins of the Americas and Africa and in the Arabian Sea. The westerlies also cause upwelling in some areas around the Antarctic continent. A particularly productive coastal upwelling region occurs along the coast of Peru. During some years, weakening of the trade winds causes warmer, nutrient poor water at depths < 300 m in this area. Upwelling in this case does not supply nutrients, productivity is low, and massive die-offs of sea birds and failure of the fisheries occurs. This event is called El Nino. El Nino reflects global changes in the atmosphere and ocean, which cause other unusual weather patterns, including: Heavy rainfall and flooding in the western U.S. and western South America. Droughts in Indonesia and Africa. Warmer-than-normal conditions in the Gulf of Alaska and Bering Sea (normally lag behind El Nino in the tropics). 12. The surface currents of the ocean form gyres, or circular patterns of water movement. Gyres result from Ekman Transport of water, the force of gravity, and the Coriolis Effect. Convergence of water toward 30° latitude is caused by the trade winds and the westerlies. This causes the sea surface to be slightly higher in elevation near this latitude. Gravity acts to make this water flow "downhill". But the Coriolis Effect deflects the water to the right in the northern hemisphere (left in the southern hemisphere). In the gyres, the Coriolis effect and the force of gravity are balanced. Under this condition, the flow of water is circular, around the center of the elevated region of ocean. Circulation is clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere. Westward intensification of currents results from the Coriolis Effect and from the continents blocking the generally westward flow of water in the tropics. Currents on the western side of ocean basins, called western boundary currents, tend to be unusually deep, narrow, and swift. Examples are the Gulf Stream and the Kuroshio. These are important because they transport heat to higher latitudes and thus decrease climatic differences between the tropics and high latitudes. 13. Surface ocean currents often form meanders and eddies. So, current flow is much more irregular than the expression "river in the sea" would suggest. Eddies form from meanders in currents. Their circulation direction depends on the direction of the meander and the current flow. For example, a meander to the right in a northward-flowing current, the Gulf Stream, results in a counter-clockwise eddy with a cold "core", called a cold-core ring. A meander to the left in the Gulf Stream forms a clockwise eddy with a warm core. Such rings often trap and transport animals to unusual locations, e.g., tropical fishes off Long Island are brought by warm core rings. What are the major wind patterns over the earth's surface, and why do they occur? Easterlies in the tropics (northeasterly trade winds north of the equator and southeasterly trade winds south of the equator) Westerlies in mid-latitudes. Easterlies in polar regions. These patterns are due to the convection of the atmosphere, caused by solar heating of the Earth's surface, and the Coriolis Effect which results from the Earth's rotation. What is the Coriolis Effect? The Coriolis Effect is an apparent deflection of moving bodies, which are not strongly attached to the Earth's surface, to the right of their path of travel in the northern hemisphere and to the left in the southern hemisphere. The Coriolis effect is due to the Earth's rotation; an observer on the Earth moves relative to the moving body, so even though the object is travelling in a straight line, it's path appears to curve. The Coriolis Effect increases with increasing latitude and with increasing speed of the moving body. What is Ekman Transport, and how is it related to surface ocean currents? Ekman transport is the movement of water due to winds blowing across the sea surface, which is 90° to the right of the wind direction in the northern hemisphere and 90° to the left of the wind direction in the southern hemisphere because of the Coriolis Effect. What are the global patterns of ocean surface currents, and why do they occur? Ocean surface currents are arranged in roughly circular patterns (gyres). There are two large gyres, one to the north and and one to the south of the equator in the Atlantic and Pacific, and one gyre in the Indian Ocean. These gyres flow clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. There are smaller counterclockwise gyres in the northernmost parts of the Pacific and Atlantic, and a continuous circumpolar current, from west to east, surrounding Antarctica. Currents are more intense on the western side of ocean basins, due to the Coriolis Effect and the presence of the continental boundaries. The gyre circulation is a result of the global wind patterns, which result in the elevation of the sea surface in the centers of gyres, and a balance between the force of gravity (which acts to make the sea surface level) and the Coriolis Effect.
"Ocean Surface Currents"