T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 1 Name: Major Ocean Currents Unit (3.5 pts) Section: Ocean Currents An ocean current is like a river in the ocean: water is flowing (travelling) from place to place. This is different from ocean waves. Recall that we have learned that water moves in a circular orbital beneath waves. Since the water moves in a small loop, the water in a wave does not travel anywhere but more-or-less wiggles in place. Thus, you should talk about currents, not waves, if you wish to discuss water pushing ships and other things across the ocean. Discuss currents if you want to talk about water pushing against ships and other things, slowing them down, Historically, ocean currents have been very important for transportation. When crossing the ocean in a ship powered by the wind (via sails), being carried by an ocean current (or avoiding a current going the opposite direction) could save a ship more than a week of travel time. Modern ships are powerful enough to go against most ocean currents, but doing so costs time and fuel, so knowledge of ocean currents is still very important. (Time is money, and fuel costs money too.) About 40% of all the goods imported into the United States (worth $200 billion) come through the ports of Los Angeles and Long Beach. Port activity contributes $39 billion in wages and taxes to the local economy, and is related to about 800,000 local jobs.) In addition, ocean currents are studied because they carry things in the water from place to place in the ocean, like ocean pollution. Knowledge of the local ocean currents, for example, can help us determine where sewage is leaking into the ocean or predict how far away the pollution from a leaking sewage pipe will affect the shoreline. Oil companies need to study ocean currents to prepare emergency plans in case an oil spill occurs. Ocean currents also carry warm and cold water from place to place, and can have a significant impact on a region’s climate. For example, the climate of much of the east coast of the United States is quite different from the climate of the west coast of the United States, because a lot of the east coast is next to a warm current, while the west coast is next to a cold current. Marine biologists are interested in ocean currents for several reasons. Not only do they transport organisms from place to place, particularly their larvae (babies who are plankton), but they also can bring up nutrients from deep in the ocean, fertilizing phytoplankton (who are the foundation of the food chain). 1. What is an ocean current? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 2 2. Why do we study ocean currents? How can knowledge of ocean currents lead to practical benefits? What causes ocean currents? Ocean currents can be created in several different ways, but most ocean currents at the surface of the ocean are created by the wind pushing the surface of the water. Waves can be an important part of this process: The wind causes waves to grow and break, causing water to surge forward and become an ocean current. Tides are an important cause of ocean currents in shallow coastal waters (like estuaries). Density differences can lead to convection cells composed of ocean currents. The thermohaline circulation is a kind of convection cell, and has an important impact on the climate of the Earth. (We will study the thermohaline circulation in Unit 12-2.) Oddly, the major ocean currents do NOT go in the same direction as the wind. At first, the water does go in the same direction as the wind, but the water tends to bend off to the side owing to the rotation of the Earth beneath it (the Coriolis effect). The surface water pushes the water below it, but the water below it tends to bend off to the side owing to the Coriolis effect. This water pushes the water beneath it, and the deeper water tends to bend off to the side of its push, and so on. (See Unit 8A-1 on winds for a review of the Coriolis effect.) Thus, water ends up going in different directions at different depths. Oceanographers refer to this current pattern as the Ekman Spiral, named for the oceanographer who first explained what was happening. Arctic explorers were the first to point out that ocean currents go to the side of the wind (to the right of the wind in the northern hemisphere) by observing icebergs floating in this T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 3 direction. (90% of an iceberg in beneath the surface, like a cube of ice floating in your drink. So, icebergs are mainly pushed by the water, not the wind.) 3. What causes (pushes) most ocean currents? 4. True or false? “Currents go in the same direction as the wind.” 5. Why don’t currents typically go in the same direction as the wind pushing them? 6. True or false? “The surface current may be going in a different direction than the current below it.” Ekman Transport: the direction most water is pushed by the wind Ekman showed mathematically how most of the water flows approximately 90o to the side of the wind owing to the Coriolis effect, so oceanographers often refer to the overall motion of the water as the Ekman transport. (70o is probably a better real-world estimate. It varies with latitude since the Coriolis effect is stronger near the Poles, and weaker near the Equator.) As the diagram above indicates, the water at different depths is going in different directions. Like most oceanographers, we will simplify matters by discussing the Ekman transport. In the example above, the wind (big green arrow) is going east in the northern hemisphere. In this case the Ekman transport (big pink arrow) is to the south. In other words, most of the water pushed by this wind will try to go south. T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 4 The diagram below shows how the water (dashed blue arrows) moves in response to various winds (solid green arrows) in both northern and southern hemispheres. Recall that objects turn to the right of their direction of motion in the northern hemisphere and the left of their direction of motion in the southern hemisphere. Notice that winds can push water together or apart, and towards land or away from land. This will have important implications later on. You will be given maps with winds like those shown above and be asked determine the direction of the Ekman transport (dashed blue arrows). To get questions like these correct every time, rotate or turn the map so that the wind (the green arrow) is pointing away from you. Then your “right” and the wind’s “right” will be the same, and your “left” and the wind’s “left” will be the same. If it is the northern hemisphere, draw an arrow representing the Ekman transport (the water) going to the right of the green arrow. If it is the southern hemisphere, draw an arrow representing the Ekman transport (the water) going to the left of the green arrow. Here is another strategy for determining the direction of Ekman transport: Make copies of the index cards on the right. Now, take your index card or piece of paper and rotate it so that the green arrow points in the same direction as the wind or current you are given. (Check to make sure you are using the card for the correct hemisphere. The hemisphere should be written somewhere on the map or near the edge of the map containing the wind arrow.) The black arrow on your index card shows you the direction the water will go relative to T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 5 the wind. For example, the picture on the left below shows me holding the northern hemisphere index card next to the wind going southwest, and the black arrow on the card tells me that the Ekman transport will be northwest. In other words, the wind will push most water to the northwest. The picture on the right above shows me holding a southern hemisphere index card next to each wind. For each wind in a map, draw Ekman transport arrows. The water is moving in different directions in different places in the map, because the winds are moving in different directions in different places. 7. What is Ekman transport? 8. What is the direction of Ekman transport (the overall motion of the water) for the winds in each of the maps below? Put an arrow in each picture, and write its direction (north, northeast, east, southeast, south, southwest, west, northwest) next to it. T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 6 Overall Ocean Circulation Pattern Examine the map below showing the large-scale ocean circulation. The dominant current pattern in most oceans is a gyre. A gyre is a group of ocean currents moving in a huge, horizontal loop that goes north in some places and south in other places. The ocean has 5 subtropical gyres (red arrows by the Equator) and one subpolar gyre (blue arrows by northern Europe). Notice how the gyres alternate direction of rotation. The subpolar gyre goes counterclockwise. The subtropical gyre north of the Equator goes clockwise.