Oceanography-Chapter 4 PDF

Movement of ocean waters Chapter 4: Currents Ocean water is in constant motion. There are three types of movement. Currents- a steady flow of ocean water caused primarily by wind patterns or differences in water density. Tides- the rise and fall of ocean water caused by the gravitational attraction...

Movement of ocean waters Chapter 4: Currents Ocean water is in constant motion. There are three types of movement. Currents- a steady flow of ocean water caused primarily by wind patterns or differences in water density. Tides- the rise and fall of ocean water caused by the gravitational attraction among the earth, moon, and the sun. Waves- the up and down movement of ocean water caused by pulses of energy that move through the water. Ocean Currents Ocean currents are a product of many factors. The primary factors are: wind differences in water density Insolation and the coriolis effect are the driving forces behind these factors. Insolation heats water unevenly. Cooling water to sink below warming water and warming water to rise above cooling water. The coriolis effect causes water currents to be deflected into a curved path. This means that the flow of currents is not exactly the same as the direction the wind is blowing. Ocean Currents Warm water flows away from the equator along the east coasts of continents and curves eastward. Cold water flows toward the equator along the west coasts of continents and curves westward. The result is what is known as the four great whirlpools. The Gulf Stream is a warm current that flows along the east coast of the United States. The Japan Current is a cold current that flows along the west coast of the United States. A warm, seasonal ocean current known as El Niño changes weather patterns in North and South America. There are two kinds of currents. Surface currents are caused mainly by wind. They can be in either warm or cold water They extend to depths of up to several hundred meters. Deep currents are caused mainly by differences in water density. Water density is affected by the salinity and the temperature of the water. An ocean current is continuous, directed movement of ocean water. Ocean currents are rivers of hot or cold water within the ocean. The currents are generated from the forces acting upon the water like the planet rotation, the wind, the temperature, salinity (hence isopycnal) differences and the gravitation of the moon. The depth contours, the shoreline and other currents influence the current's direction and strength. The meshing of all of these characteristics is what creates the great flow of the global conveyor belt which plays a dominant part in the climate of many of the Earth’s regions. Surface ocean currents are generally wind driven and develop their typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere because of the imposed wind stresses. In wind driven currents, the Ekman spiral effect results in the currents flowing at an angle to the driving winds. The areas of surface ocean currents move somewhat with the seasons; this is most notable in equatorial currents. Ekman transport, named for Vagn Walfrid Ekman, is the natural process by which wind causes movement of water near the ocean surface. Each layer of water in the ocean drags with it the layer beneath. Thus the movement of each layer of water is affected by the movement of the layer above, or below in the case of a frictional bottom boundary layer. It is obtained by vertically integrating the Ekman spiral. Because of the Coriolis effect, the ocean's surface movement is 45° to the right of direction of surface wind in the Northern Hemisphere, and 45° to the left in the Southern Hemisphere. The average movement of ocean water at all depths (and thus the Ekman transport) is 90° to the right of the wind in the Northern Hemisphere, and 90° to the left in the Southern Hemisphere. If such a current transports water away from a coast, it creates an upwelling of deep, nutrient-rich sea water. This has the effect of creating good fishing regions along coasts where this phenomenon occurs. Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation, also known as the ocean's conveyor belt, refers to the deep ocean density-driven ocean basin currents. These currents, which flow under the surface of the ocean and are thus hidden from immediate detection, are called submarine rivers. These are being researched by a fleet of underwater robots called Argo. Upwelling and downwelling areas in the oceans are areas where significant vertical movement of ocean water is observed. Ocean gyres A gyre is any manner of swirling vortex, particularly large-scale wind and ocean currents. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction which determine the circulation patterns from the wind curl (torque). The Earth's oceans have the following major gyres: North Atlantic Subpolar Gyre North Pacific Subpolar Gyre Contains the smaller Alaska Gyre North Atlantic Subtropical Gyre Gulf Stream, Labrador Current, East Greenland Current, North Atlantic Current, North Atlantic Equatorial Current North Pacific Subtropical Gyre also known as North Pacific Gyre This gyre comprises most of the northern Pacific Ocean. It is located between the equator and 50º N latitude and occupies an area of approximately ten million square miles (34 million km²).[clarify] The North Pacific Gyre has a clockwise circular pattern and comprises four prevailing ocean currents: the North Pacific Current to the north, the California Current to the east, the North Equatorial Current to the south, and the Kuroshio Current to the west. An accumulation of marine debris known as the "Great Pacific Garbage Patch" is collecting in the gyre. The Earth's oceans have the following major gyres: South Atlantic Subtropical Gyre Contains the smaller Brazil Current System South Pacific Subtropical Gyre Contains the smaller East Australian Current System Indian Ocean Subtropical Gyre (Southern Hemisphere) Contains the smaller Agulhas Current System Antarctic Circumpolar Current Weddell Sea Subpolar Gyre (Southern Ocean) Ross Sea Subpolar Gyre (Southern Ocean) Eddies are phenomena generated either by processes that destabilise alongslope currents (such as the Algerian or the Libyo-Egyptian Currents), or by the wind stress curl locally induced by orographic effects. They are characterised as cyclonic / anticyclonic, not constrained by the bathymetry and can progagate. Eddies are mesoscale (some 10s to a few 100s km) features that will be characterised as small (up to ~50 km), medium (50-150 km) or large (150-250 km). Eddies move in nearly circular patterns. Some of these loops would be roughly 100 to 200 km (or about 60 to 120 miles) in diameter. These loops are known as mesoscale eddies. These features are important because they are "hot " spots of intense biological and physical activity. If movement is in a counterclockwise direction in the northern hemisphere, it is called a cyclonic eddy. The center of the eddy is likely cooler and lower in height (by a few tens of centimeters) than the outer lying waters. On the other hand, if the rotation is clockwise, the feature is called an anti-cyclonic eddy and the center is warmer and higher (by a few tens of centimeters) than outer waters. The cyclonic eddy is called a cold-core eddy or ring and the anti-cyclonic eddy is called a warm-core eddy or ring. You can think of eddies as ocean weather. We often see atmospheric eddies in weather maps. Meteorologists call these features low and high pressure systems. The next time you watch your local weather, check out the circulation patterns. You’ll notice that the air circulation is counterclockwise (cyclonic) for northern hemispheric lows and clockwise (anti-cyclonic) for highs. Coriolis Effect The Coriolis effect is a curving of wind or water as it moves north or south. The curving is caused by the rotation of Earth. Earth rotates toward the East. Earth is shaped like a sphere (ball). The path of rotation at the equator is much longer than the path of rotation near the poles. Air and water at the equator is rotating faster than air and water near the poles. If this air or water moves away from the equator, its momentum causes it to curve ahead of the air or water into which it is moving. It curves to the east. Air and water near the poles is rotating slower than air and water at the equator. If this air or water moves toward the equator, its momentum causes it to curve behind the air or water into which it is moving. It curves to the west. El Nino El Niño is a warm ocean current that develops in December off the coast of Peru. This warm water on the surface prevents colder deep water from rising to the top with its nutrients that feed fish in those waters. Sometimes the El Niño lasts for weeks or even months. When this happens, it not only harms the fishing industry of Peru, but also alters weather patterns from northern South America all the way to the central United States. In strong El Niño years: The central United States receives more than normal rainfall. Atlantic hurricanes are weaker and less frequent. There is reduced arctic air moving into the United States. There is a more active storm pattern across the southern United States. It is incorrect to blame El Niño alone for bad weather. Many factors contribute to the weather patterns we experience. When El Niño appears, it is one of those factors. Four Great Whirlpools Water in the North Atlantic circulates in a clockwise pattern. It flows northward along the coast of the United States. It curves eastward toward northern Europe. It flows southward along the coast of Europe. It curves westward back toward the United States. Water in the North Pacific circulates in the same clockwise pattern. It flows northward along the coast of Asia. It curves eastward toward North America. It flows southward along the coast of North America. It curves westward back toward Asia. Water in the South Atlantic circulates in a counterclockwise pattern. It flows southward along the coast of South America. It curves eastward toward southern Africa. It flows northward along the coast of Africa. It curves westward back toward South America. Water in the South Pacific circulates in the same counterclockwise pattern. It flows southward along the coast of Australia. It curves eastward toward South America. It flows northward along the coast of South America. It curves westward back toward Australia. Insolation Insolation is a word made from 3 words. It stands for INcoming SOLar radiATION. This is the energy that Earth receives from the sun. It heats the land and water on Earth's surface. The surface radiates heat back into the air, heating it. Insolation is responsible for the circulation of water and air around our planet. This is the force that drives our weather. Parts of the Earth (near the equator) get much stronger insolation than other parts (near the poles). Areas in the middle latitudes (like the United States) receive strong insolation during summer and weak insolation in the winter. Air that is warmer rises because it is less dense. It spreads out above the colder, denser air which flows in underneath it. Water in the oceans behaves in much the same way. This motion of air and water carries moisture to dry areas, warms colder places and cools hot spots. Upwelling Downwelling What causes the ocean to circulate? Energy and matter are continually exchanged between the ocean and atmosphere, and these processes drive the ocean and atmospheric circulation. Evaporation, precipitation, plus heating and cooling bring about changes in the temperature and salinity of surface waters. Density changes that accompany changes in temperature and salinity can cause water to sink or rise in the ocean. Kinetic energy (energy of motion) is transferred from the wind (air in motion) to ocean depths of a few hundred meters. Winds are responsible for not only horizontal currents but also the vertical water motions within the surface layer (e.g., upwelling). To a large extent, horizontal movement of ocean surface waters mirrors the long-term average planetary circulation of the atmosphere. Three surface wind belts encircle each hemisphere: trade winds (equator to 30 degrees latitude), westerlies (30 to 60 degrees), and polar easterlies (60 to 90 degrees). The westerlies of middle latitudes and the trade winds of the tropics drive the most prominent features of ocean surface motion, large-scale roughly circular current systems elongated in the east-west direction known as gyres. Subtropical gyres are centered near 30 degrees latitude in the North and South Atlantic, the North and South Pacific, and the Indian Ocean. Gyres in the Northern and Southern Hemispheres are similar except that they rotate in opposite directions because the Coriolis effect acts in opposite directions in the two hemispheres. Ekman transport causes surface waters to move toward the central region of a subtropical gyre from all sides, producing a broad mound of water. Surface water begins flowing downhill. A balance develops between the Coriolis force and the force arising from the horizontal water pressure gradient such that surface currents flow parallel to the contours of elevation of sea level. This current is known as geostrophic flow. Surface water parcels flow outward and down slope from the center of the gyre. The Coriolis effect causes these parcels to shift direction to the right in the Northern Hemisphere (to the left in the Southern Hemisphere). Eventually, the outward-directed pressure gradient force balances the apparent force due to the Coriolis effect and the water parcels flow around the gyre and parallel to contours of elevation of sea level. The Gulf Stream The Gulf Stream is 80 kilometres (50 mi) to 150 kilometres (93 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep. The current velocity is fastest near the surface, with the maximum speed typically about 2.5 metres per second (5.6 mph). A river of sea water, called the Atlantic North Equatorial Current, flows westward off the coast of northern Africa. When this current interacts with the northeastern coast of South America, the current forks into two branches. One passes into the Caribbean Sea, while a second, the Antilles Current, flows north and east of the West Indies. These two branches rejoin north of the Straits of Florida. Consequently, the resulting Gulf Stream is a strong ocean current. It transports water at a rate of 30 million cubic meters per second (30 sverdrups) through the Florida Straits. After it passes Cape Hatteras, this rate increases to 80 million cubic meters per second. The volume of the Gulf Stream dwarfs all rivers that empty into the Atlantic combined, which barely total 0.6 million cubic meters per second. It is weaker, however, than the Antarctic Circumpolar Current. The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. At about 30°W, 40°N, it splits in two, with the northern stream crossing to northern Europe and the southern stream recirculating off West Africa. It is part of the North Atlantic Subtropical Gyre. The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean. The Gulf Stream is also a significant potential source of renewable power generation. Global oceanic conveyer belt and thermohaline circulation The global oceanic conveyer belt is a unifying concept that connects the ocean's surface and thermohaline (deep mass) circulation regimes, transporting heat and salt on a planetary scale. Thermohaline circulation, also called the Global Ocean Conveyor, moves water between the deep and surface ocean worldwide. The conveyor belt system can be thought of as beginning near Greenland and Iceland in the North Atlantic where dry, cold winds blowing from northern Canada chill surface waters. The combined chilling of surface waters, evaporation, and sea-ice formation produces cold, salty North Atlantic Deep Water (NADW). The newly formed NADW sinks and flows southward along the continental slope of North and South America toward Antarctica where the water mass then flows eastward around the Antarctic continent (in the Antarctic Circumpolar Current). There the NADW mixes with Antarctic bottom and deep waters (i.e., AABW and AADW). The resulting Common Water, also called Antarctic Circumpolar water, flows northward at depth into the three ocean basins (primarily the Pacific and Indian Oceans). A schematic showing the ocean "conveyor belt", where surface waters sink, enter deep water circulation, then resurface after slowly flowing through the deep ocean. Ocean’s vertical structure Except at high latitudes, the ocean is divided into three horizontal depth zones based on density: the mixed layer, pycnocline, and deep layer. At high latitudes, the pycnocline and mixed layer are absent. The biggest source of deep water is surface water that sinks in the North Atlantic Ocean. The Gulf Stream current brings highly saline water northward (salinity is high in mid-latitudes where evaporation is high and precipitation is low). Cold water from the north (Labrador and Irminger currents) meets the Gulf Stream and the water gets mixed. This cold, salty mixture can be dense enough to sink into the depths of the ocean. Thermohaline circulation Deep water circulation is driven by density differences. Density is in turn dependent on the temperature and salinity of the water sample. Because of this dependence, deep water circulation is sometimes referred to as thermohaline circulation. The term thermohaline circulation (THC) refers to the part of the largescale ocean circulation that is thought to be driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Thermohaline circulation is very slow. Once water sinks from the surface, it can spend a long time away from the surface until that water rises again to become part of surface circulation. Some scientists estimate that the residence time of deep water, the time spent away from the surface, can be about 200-500 years for the Atlantic Ocean and 1,000-2,000 years for the Pacific Ocean. Wind-driven surface currents (such as the Gulf Stream) head polewards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (transit time around 1600 years) upwell in the North Pacific. Extensive mixing takes place between the ocean basins, reducing differences between them and making the Earth's ocean a global system. The moving water masses transport both heat energy and matter (solids, dissolved substances and gases) around the globe. Hence the state of the circulation has a large impact on the climate of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. The Mediterranean is a sea of the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Europe, on the south by Africa, and on the east by Asia. It covers an approximate area of 2.5 million km² (965,000 sq mi), but its connection to the Atlantic (the Strait of Gibraltar) is only 14 km (9 mi) wide. Being nearly landlocked affects the Mediterranean Sea's properties; for instance, tides are very limited as a result of the narrow connection with the Atlantic Ocean. The Mediterranean is characterized and immediately recognized by its deep blue color. Evaporation greatly exceeds precipitation and river runoff in the Mediterranean, a fact that is central to the water circulation within the basin. Evaporation is especially high in its eastern half, causing the water level to decrease and salinity to increase eastward. This pressure gradient pushes relatively cool, lowsalinity water from the Atlantic across the basin; it warms and becomes saltier as it travels east, then sinks in the region of the Levant and circulates westward, to spill over the Strait of Gibraltar. Thus, seawater flow is eastward in the Strait's surface waters, and westward below; once in the Atlantic, this chemically-distinct "Mediterranean Intermediate Water" can persist thousands of kilometers away from its source.