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Transcript
Page 1 of 15 Properties of Seawater & Currents 1. Water & Climate: The ocean extends over 71% of the earth's surface. The ocean holds 98% of the 1.4 billion cubic kilometers of water on the planet, divided within three major basins (The Atlantic, Pacific and Indian Oceans). Seawater fills the basins separating the continents (Fig. 1) with an average depth of 3795 meters. The continental margins, extending from the seashore to around 2500 meters depth, cover 40.7% of the ocean (29% of Earth surface). The deep ocean covers about 59.3% of the ocean's surface (42% of Earth's surface). The extensive flat plains of the deep basins range 4000 to 5000 meters in depth. The mid-ocean ridge, marking the spreading of tectonic plates, is marked by mountains; the ridge crest is about 2500 m from the ocean surface. Page 2 of 15 The deepest ocean is found in the trenches where the plates are subducted, the Mariana Trench is 11,035 meters deep (compared to the 8848 meter height of Mount Everest). If the solid earth were made into a flat plain, the seawater would cover the entire earth to a depth of 2440 meters. If all of the water vapor in the atmosphere were converted to liquid it would cover the smoothed earth surface by about an 1-inch. Water has a very high heat capacity. This means that it holds a lot of energy. It takes a lot of energy to warm it up, but it also gives off that energy slowly. The upper 700 m hold the most heat. The lower levels of water also hold a lot of heat; much more than the land. The high capacity of water makes the ocean a powerful and stabilizing force of the Earth's climate system. The stored energy in the ocean helps to keep the land temperature moderate. One obvious consequence of the ocean's influence is the "marine effect", which acts to reduce the extreme differences between winter & summer and day & night air temperatures. Another is that the ocean circulation transfers a significant amount of heat from the equator to the Northern and Southern latitudes, helping the climate system to attain an approximately steady state condition. Page 3 of 15 2. Sea Surface Temperature: The temperature of the sea surface is high (27-30°C) near the equator, often the maximum value occurs a few degrees of latitude north of the equator. The temperature is low (at the sea water freezing point of -1.9°C due to the amount of salt in it) within the polar oceans. However, there are also changes of sea surface temperature with longitude (Northerly/Southerly directions). Warmer water moves poleward along the western sides of the ocean. The eastern tropical regions of each ocean are cooler than the western tropical margin. These are due to the movement of seawater in the horizontal (ocean currents) and vertical (upwelling/sinking) directions. Temperature and density of ocean water are inversely related: warm water means low density, cold water means higher density seawater. The salt content of the water also affects Ocean density. Page 4 of 15 3. Sea Surface Salinity: Sea water is about a 3.49% (or 35 ppt) salt solution, the rest is freshwater. The more salt, the denser the seawater. As the range of salt concentration in the ocean varies from about 3.2 to 3.8% (32 ppt - 38 ppt). Oceanographers refer to salt content as 'salinity', express salt concentration as parts per thousand. A region of excess evaporation, such as the subtropics tends to become salty, while the areas of excess rainfall become fresher. When seawater evaporates the salt remains behind, only the freshwater is transferred from the ocean to the atmosphere. That means that salinity is higher near the poles where water freezes. Sea ice formation also removes freshwater from the ocean, leaving behind a more saline solution. Along the shores of Antarctica this process produces dense water. This causes the water to sink and push the lower water out of the way starting a major deep water current that will travel around the globe. Page 5 of 15 Below the Sea Surface: Waters warmer than 10°C dominate the sea surface but do not extend much below 500 m within the ocean. Tropical and subtropical surface water is just a veneer of warmth over a cold ocean; typical deep ocean temperature vary from -1° to 3°C. The sharp drop off in temperature with depth is called the thermocline. The warm surface water is generally saltier than the cooler deep or polar waters. The halocline marks the drop of salinity with depth that accompanies the thermocline. The surface water warmth overrides the saltiness in governing density, so that the warm surface water regions coincide with buoyant (less dense) water. In polar regions buoyancy of the surface layer is mainly a consequence of the freshness of the surface water. Deep cold waters derive their properties at the sea surface during winter at high latitude. 4. Deep Water Masses: The deep Atlantic is relatively salty (34.9). This is due to the sinking of chilled saline surface water in the northern North Atlantic. The cooling makes the surface water dense, forcing it to sink, or convect into the deep ocean, and spread southward along the bottom. It is called North Atlantic Deep Water (NADW). In contrast the deep Pacific is lower in salinity (34.7), as it experiences no deep convection of cooled salty surface water, its surface layer is too fresh and buoyant to sink. Pacific deep water is due to the lower salinity water column of the southern ocean. Towards the sea floor, temperatures are near 0°C marking the presence of Antarctic Bottom Water (AABW) due to the very cold (-1.9°C; 34.65), dense water along the shores of Antarctica. At the base of the thermocline is the low salinity Antarctic Intermediate Water (AAIW) derived from sinking of cool (3° to 4°C), low salinity waters (34.4) from 50° 60°S marking the Antarctic Circumpolar Current and ocean polar front zone. Page 6 of 15 Ocean Currents and Climate Name: ___________________________________________ Date: _____________________________ There are two type of Ocean Currents: 1. Surface Currents -- Surface Circulation These waters make up about 10% of all the water in the ocean. These waters are the upper 400 meters of the ocean. 2. Deep Water Currents -- Thermohaline Circulation These waters make up the other 90% of the ocean. These waters move around the ocean basins by density driven forces and gravity. The density difference is caused by differences in temperatures and salinity. These deep waters sink into the deep ocean basins at high latitudes (top and bottom of the globe) where the temperatures are cold enough to cause the density to increase. Ocean Currents are influenced by two types of forces 1. Primary Forces--start the water moving The primary forces are: 1. Solar Heating 2. Winds 3. Gravity 4. Coriolis 2. Secondary Forces--influence where the currents flow Page 7 of 15 1. Surface Circulation Solar heating causes water to expand. Near the equator the water is about 8 centimeters higher than in middle latitudes. This causes a very slight slope and water wants to flow down the slope. Winds blowing on the surface of the ocean push the water. Friction creates a connection between the wind and the water's surface. A wind blowing for 10 hours across the ocean will cause the surface waters to flow at about 2% of the wind speed. Water will pile up in the direction the wind is blowing. Gravity will tend to pull the water down the "hill" or pile of water against the pressure gradient. The force of the Coriolis effect intervenes and causes the water to move to the right (in the northern hemisphere) around the mound of water. These large mounds of water and the flow around them are called Gyres. Gyres are large circular currents in each of the 5 main ocean basins. Page 8 of 15 Gyres The North Atlantic Gyre Note how the North Atlantic Gyre is separated into four distinct Currents: The North Equatorial Current, The Gulf Stream, The North Atlantic Current, and The Canary Current. But why doesn't the water spin towards the center of the ocean? Why does it flow around the hill in this circular motion. Remember the hill of water-- This hill is formed by the inward push of water through a process call Ekman Transport Remember the Coriolis Force moves objects to the right in the northern hemisphere (and the left in the Southern Hemisphere) Ekman Transport Wind blowing on the surface of the ocean has the greatest effect on the top layers of the water. However, for the lower layers of the ocean to move they must be pushed by the friction between the layers of water above. Consequently, the lower layer moves slower than the layer above. With each successive layer down in the water column the speed is reduce. This leads to the spiral affect seen in the above diagram. The net movement of water (averaged over the entire upper 330 meters of the ocean) is 90o to the right of the wind direction (in the northern hemisphere). Page 9 of 15 The effect of winds on the vertical movement of water Upwelling along the coast caused by Ekman transport of waters (waters move to the right of the wind). The waters moved offshore are replaced by waters from below. This brings cold, nutrient rich waters to the surface. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the abundance of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures (SST) . The increased plant life in upwelling regions results in high levels of animals production. Approximately 25% of the total global marine fish catches come from five upwellings that occupy only 5% of the total ocean area. These areas if the ocean have the greatest impact on nutrient-enriched waters and global fishery yields. This occurs along the coast of Eastern USA and Canada. Downwelling caused by Ekman transport onshore (movement of water to the right of the wind direction). Downwelling also occurs where the water density increases, causing the denser water to drop down to the ocean bottom Page 10 of 15 Investigative Activity #1: Currents and Marine Life Currents are powerful physical forces in the seas. They move water and heat around the globe, and help determine the chemical make-up of the water column. Currents also are a major factor in ocean ecosystems. Two types of current motion, upwelling and downwelling, strongly influence the distribution and abundance of marine life. Upwelling Currents play a huge role in marine productivity, through a process called upwelling. Sea life is concentrated in the sunlit waters near the surface, but most organic matter is far below, in deep waters and on the sea floor. When currents upwell, or flow up to the surface from beneath, they sweep vital nutrients back to where they're needed most. Nowhere is the link between ocean circulation and productivity more evident than around Antarctica. There, strong currents pump nitrogen and phosphate up from the deep sea to fuel vast blooms of algae and other plants. These plankton are eaten by swarms of shrimp-like crustaceans called krill. Because of upwelling nutrients, krill are abundant enough to feed the largest animals on earth, baleen whales, as well as myriad penguins, seals, and seabirds. In fact, despite the harsh conditions, the biomass of Antarctic krill is thought to be greater than that of any other animal on Earth. Downwelling The importance of upwelling to surface organisms is matched by the need of sea bottom life for downwelling, or the sinking of surface water. Surface water can be forced downward by the pressure of the “pile” of water that forms where currents converge or wind drives the sea against a coastline. But for bottom dwellers, the sinking of water caused by density changes is especially noteworthy. The global conveyer belt takes oxygen-rich surface water and flushes it through the deep sea. Without this renewal, the dissolved oxygen in bottom sediments and waters would quickly be used up by the decay of organic matter. Anaerobic bacteria would take over decomposition, leading to a build up of hydrogen sulfide. Few benthic animals would survive such toxic conditions. In the most extreme cases, a lack of downwelling may lead to mass extinctions. Paleontologists have suggested that 250 million years ago, deep circulation slowed nearly to a stop, and the ocean began to stagnate. Low oxygen, sulfide and methane-rich waters filled the ocean deeps and then spread onto the continental shelves, wiping out 95% of all marine species in the greatest extinction event in Earth history. Instructions: In this activity, you will explore the differences between upwelling and downwelling. Study the graphics and photographs illustrating upwelling and downwelling, then answer the questions about each process. Maps of the world’s major surface and deep currents are included as resources to help you understand where and how upwelling and downwelling occur. Page 11 of 15 Examine the following images related to Upwelling Page 12 of 15 Examine the following images related to Downwelling Resources Page 13 of 15 Questions Upwelling 1. Most primary productivity in the oceans occurs in surface waters, but most of the organic matter is at the bottom of the sea. Explain. Almost all food chains begin with photosynthesis, and photosynthesis occurs in sunlight. So marine plants will only live near the surface where light can penetrate, and of course that will keep herbivores, and their predators, up at the surface too. But once these organisms die, or shed scales, eggs, leaves, shells, or feces, they and their by-products will sink down toward the sea floor. 2. What causes upwelling? Upwelling occurs when surface currents move away from one another, or when winds push surface water away from the shore. This draws deeper water upward to replace the surface water. 3. Why is nutrient upwelling so powerful around Antarctica? The deep water in the global conveyer belt flows across the seafloor all the way from the North Atlantic before it reaches Antarctica, so it has a lot of time to pick up a lot of nutrients. There are a lot of surface currents flowing around the continent - the southern limbs of the Pacific, Atlantic, and Indian Ocean gyres and the Circumpolar and Sub polar Currents - to stir up the water. And there are likely to be strong polar winds blowing offshore, which would also drive upwelling. 4. What other conditions near the South Pole help stimulate lush plant growth? In summer, the sun shines non-stop, which would allow photosynthesis 24 hours a day. Downwelling 1. What causes downwelling? Down welling occurs when the water on the surface of the sea becomes denser than the water beneath it and so it sinks. Seawater gets denser when it gets colder or saltier. 2. Where does most downwelling occur? Most down welling happens at the poles. There, cold air chills the water. The water brought in by the surface gyres is pretty salty already, because it comes from the tropics, where evaporation increased salinity. And once it gets to high latitudes, the water becomes even more saline as ice forms and further concentrates sea salts. Page 14 of 15 Life on Earth nearly died out at the end of the Paleozoic Era 250 million years ago. At that time, all the continents had just come together to form a supercontinent called Pangaea, and a single superocean called Panthalassa. The global climate had warmed by several degrees, especially at the poles. 3. How do you think these conditions might have affected ocean circulation, and led to low oxygen levels in the sea? The arrangement of the continents would certainly make circulation patterns different than they are today. For example, it seems like on the surface, there wouldn't be so many gyres, since there was only one giant ocean basin. There might have been more east-west movement and perhaps fewer north-south currents, since there were fewer continental barriers to flows driven by the prevailing winds. If the air was warmer at the poles, then the water would be too. And without cold water and ice formation at high latitudes, there would be little down-welling and not much of a global conveyer belt in the deep sea. So all in all ocean circulation would probably have been much weaker than it is today. Oxygen in the deep sea would have been used up, and without strong down-welling, there was no way to replenish it. Critical Thinking Currents are important in marine ecosystems because they redistribute water, heat, nutrients, and oxygen about the ocean. At the same time, currents inevitably sweep over and carry off living organisms. Discuss how current flows might affect ocean organisms and species. Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers. Although many sea creatures are powerful, efficient swimmers, many others are ungainly or even immobile. For these animals, currents could offer a free ride. Corals and sponges, for instance, are attached to the bottom as adults, but when they reproduce, they release volumes of planktonic larvae into the water column. These tiny creatures are free to "go with the flow" and could be carried long distances. This could allow individuals to escape overpopulated areas with too much competition for resources, and allow the species to spread into and colonize new habitat. Once the juveniles settle down, currents will continue to bring other plankton and organic debris their way, providing a steady supply of food. Even the free-swimming animals can benefit from riding currents. Many marine vertebrates such as herring, eels, and turtles, hatch in rivers or close to the coast. Small and weak, the juveniles need currents to carry them to the open ocean feeding grounds where they grow and mature. Because currents tend to concentrate food resources in limited areas, such as by upwelling, they make it easier for predators to find prey. But a reliance on currents - for transportation and for maintaining water conditions - could be the downfall of individuals and populations. When currents shift, because of plate tectonics or climate change, ecosystems would be thrown into turmoil. The young could be carried into areas where they could not survive, and adults could be bathed in water whose temperature, salinity, or chemistry they cannot tolerate. Page 15 of 15 The Global Conveyor Belt Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation. In the Earth's polar regions ocean water gets very cold, forming sea ice. As a consequence the surrounding seawater gets saltier, because when sea ice forms, the salt is left behind. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deepocean currents driving the global conveyer belt. Thermohaline circulation drives a global-scale system of currents called the “global conveyor belt.” The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the Thermohaline circulation begins in the added salts, and sinks toward the ocean bottom. Surface water Earth's polar regions. When ocean water in moves in to replace the sinking water, thus creating a current. these areas gets very cold, sea ice forms. The surrounding seawater gets saltier, This deep water moves south, between the continents, past the increases in density and sinks. equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean. These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again. The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second). It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt. In addition, the conveyor moves an immense volume of water—more than 100 times the flow of the Amazon River (Ross, 1995). The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt as deep or bottom layers. The base of the world’s food chain depends on the cool, nutrient-rich waters that support the growth of algae and seaweed. The global conveyor belt is a strong, but easily disrupted process. Research suggests that the conveyor belt may be affected by climate change. If global warming results in increased rainfall in the North Atlantic, and the melting of glaciers and sea ice, the influx of warm freshwater onto the sea surface could block the formation of sea ice, disrupting the sinking of cold, salty water. This sequence of events could slow or even stop the conveyor belt, which could result in potentially drastic temperature changes in Europe. Currents: NOAA's National Ocean Service Education http://oceanservice.noaa.gov/education/tutorial_currents/welcome.html