Ocean Currents Unit (4 pts)

Transcription

1 Name: Section: Ocean Currents Unit (Topic 9A-1) page 1 Ocean Currents Unit (4 pts) Ocean Currents An ocean current is like a river in the ocean: water is flowing traveling from place to place. 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 (e.g., oil = money), so knowledge of ocean currents is still very important. (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 (e.g., the east and west coasts of the United States are quite different) Marine biologists are interested in ocean currents for several reasons. Not only do they transport organisms particularly their larvae (babies who are plankton) from place to place, 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? 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

2 Ocean Currents Unit (Topic 9A-1) page 2 waters (like estuaries). Density differences can lead to convection cells in the ocean, causing thermohaline circulation. 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 rotation of the Earth beneath it (i.e., the Coriolis effect). This surface water pushes the water below it, but the water below it tends to bend off to the side owing to the Coriolis effect. The subsurface water pushes the water beneath it, but the deeper water tends to bend off to the side, etc. 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 direction. (90% of an iceberg in beneath the surface like a cube of ice floating in your drink so they 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. Wind surface water deeper water even deeper water "Ekman Spiral" Ekman Transport (Overall Water Direction) Wind even deeper water surface water deeper water 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.

3 Ocean Currents Unit (Topic 9A-1) page 3 Ekman Transport: the overall direction water is pushed by the wind Ekman showed mathematically how most of the water flows approximately 90 o 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. (70 o is probably a better real-world estimate.) even deeper water deeper water Wind surface water Ekman Transport (Overall Water Direction) The figure below shows how the water (dashed blue arrows) moves in response to various winds (solid green arrows) in both northern and southern hemispheres. Notice that winds can push water together or apart, and towards land or away from land. This will have important implications later on. 7. What is Ekman transport? 8. What is the direction of Ekman transport (the overall motion of the water) for the winds in the map below? Put an arrow in each picture, and write its direction (north, northeast, east, southeast, south, southwest, west, northwest) next to it. N Northern Hemisphere Southern Hemisphere A B C D Land

4 Ocean Currents Unit (Topic 9A-1) page 4 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). The only place without a gyre is the Southern (or Antarctic) Ocean. Here, the currents go all the way around the world. The Antarctic Circumpolar Current (or West Wind Drift) goes east around the continent of Antarctica, and the East Wind Drift circles to the west closer to the coast of Antarctica. Recall that winds and currents are named for the direction that they come from, not the direction that they are going to. 60 o N Subpolar Subtropical (Clockwise) Equator Equator Subtropical (Counterclockwise) 60 o S You will need to memorize these currents. Identify the pattern by answering the questions below. 9. Does the currents between Greenland and northern Europe (the northernmost gyre) go clockwise (turn to their right) or counterclockwise (turn to their left)? 10. Do the currents just north of Equator go clockwise (turn to their right) or counterclockwise (turn to their left)? 11. Do the currents just south of Equator go clockwise (turn to their right) or counterclockwise (turn to their left)? 12. What is different about the currents between Antarctica and continents north of it?

5 Ocean Currents Unit (Topic 9A-1) page 5 What causes ocean water to move in gyres? Let s examine the Northern Pacific Ocean. The trade winds push water west, away from the coast of North America. The water travels across the Pacific Ocean until it hits Asia, so it cannot go forward. It flows north along the coast Asia; it cannot stop at the coast of the Asia, because the trade winds continue to push more water west, and this incoming water pushes the water out the way and along the coast of Asia. By the time the water flowing north reaches Japan, the winds have shifted. The westerlies push the water to the east, away from the coast of Japan and towards California. (Recall that the winds and currents are named for the direction that they come from, not the direction that they are going to, so the westerlies come from the west and go to the east.) When the water reaches California, it is forced to stop or turn by the land. The winds continue pushing more and more water towards the coast of California, and this water pushes the water already along the coast out of the way and down the coast to the south. This water begins to leave the coast near the bend in the coast of California (Point Concepcion not far from Santa Barbara), and is pushed west again, away from the coast by the trade winds. B C D A Green Arrows (Arrows with Tails) = Winds Blue Arrows (Dashed Arrows) = Direction Water is Pushed by the Wind Purple Arrows (Solid Arrows, No Tails) = Actual Motion of the Water 13. What pushes ocean current A? 14. What pushes ocean current B? 15. What pushes ocean current C? 16. What pushes ocean current D?

6 Ocean Currents Unit (Topic 9A-1) page 6 Further Comments about the Causes of Gyres Overall, the trade winds and westerlies push the ocean water together in the North Pacific (the blue, dashed Ekman transport arrows). The currents cannot go north and south into one another (they are in one another s way), but they can slide west and east, respectively, at these latitudes. There are other ways to explain why the water flows along the coasts. As more and more water is pushed into the coast by the winds, sea level rises along the coast (it really does!). As we all know, water flows downhill (pulled down by gravity). It cannot flow downhill back into the ocean, because the winds are pushing water towards the coast, so it flows downhill in the only direction it can: along the coast. Just as water piles up when winds push water into the coast, winds create a hole or gap in the surface of the ocean where they push water away from the coast. Water further up the coast will flow down the coast ( downhill ) to fill in the gap. Professional oceanographers have a more detailed and complicated understanding of the causes of the gyres. Notice that the winds try to push the water together in the center of the oceans, causing sea level to rise. Gravity pulls the water downhill, away from the center of the gyre, but the water turns to the side under the influence of the Coriolis effect and ends up going around the hill in a circle instead of away from the hill. A current is said to be in geostrophic balance when the pressure to move downhill (due to gravity) is balanced by the Coriolis effect. There is yet another way to understand the cause of the gyres involving the conservation of angular momentum or more precisely, the conservation of potential vorticity but these concepts bring us well beyond the bounds of this course. Notice that the currents of the subpolar gyre flow west and east across the northern Atlantic Ocean in the directions dictated by the winds. The land forces them to turn, creating a counterclockwise gyre. The Coriolis effect is not needed to explain the motion of the subpolar gyre, and in fact would make the gyre go in the other direction, so the Coriolis effect cannot be one of the most important factors that create gyres. (The Coriolis effect does affect the currents, but it does not create the gyres.) The key factors are (i) the directions of the winds and (ii) the presence of land in the way. Where in the world are their no large gyres? In the Southern Ocean, where there are no continents in the way to force currents to turn. 17. True or false? The direction of the gyres is determined by the Coriolis effect. In other words, in the northern hemisphere all currents in gyres turn to their right (go clockwise). 18. Why isn t there a gyre in the Southern Ocean (the ocean next to Antarctica)?

7 Ocean Currents Unit (Topic 9A-1) page 7 Boundary Currents The parts of the gyre that flow along the coasts of the continents are called boundary currents. In other words, the boundary currents flow along the edges or boundaries of the ocean. There are two kinds of boundary currents: eastern boundary currents (EBCs) and western boundary currents (WBCs). Just as the west coast is on the west side of the continent and the east coast is on the east side of the continent, western boundary currents are found on the western sides of the oceans and eastern boundary currents are found on the eastern sides of the oceans. This sounds simple enough, until you realize that this means that the east coasts have western boundary currents next to them, and west coasts have eastern boundary currents next to them! As you can imagine, this can lead to some confusion. Kuroshio East Coast In this class, I focus on the boundary currents of the subtropical gyres. Their western boundary currents are faster, narrower, deeper, and warmer than the eastern boundary currents. (Or, if you prefer, their eastern boundary currents are slower, wider, shallower, and colder than their western boundary currents.) The first major ocean current to be measured and charted was the Gulf Stream, the northward-flowing, warm current off the east coast of the United States. As we noted earlier, a current is like a river (a stream) and it comes from the Gulf of Mexico, hence the name Gulf Stream. The other two boundary currents that I want you to know the properties of are the California Current and the Kuroshio. ( What is the name of the current along the coast of California? Don t you wish that I would ask this on an exam?) The California Current is a slow, cold water current that flows south along the coast of California. The Kuroshio, like the Gulf Stream, is a fast, warm water current along the east coast of Japan. Kuroi means black in Japanese, and shio means river, so Kuroshio means Black Stream. It is called the Black Stream because warm water tends to have less life than cold water; it is the lifeless river. WBC East Coast Western Boundary Current California Current EBC West Coast Eastern Boundary Current Ocean East Coast Gulf Stream WBC West Coast

8 Ocean Currents Unit (Topic 9A-1) page What is a boundary current? 20. Are western boundary currents found next to the west coasts of continents or the east coasts of continents? 21. Which is faster, a western boundary current or an eastern boundary current? 22. Which is deeper, a western boundary current or an eastern boundary current? 23. Which is wider, a western boundary current or an eastern boundary current? 24. Which is warmer, a western boundary current or an eastern boundary current? 25. What is the name of the boundary current off the east coast of the United States? 26. What is the name of the boundary current off the east coast of Asia? 27. Is the California Current a western boundary current or an eastern boundary current?

9 Ocean Currents Unit (Topic 9A-1) page 9 Western Intensification and Sea Level We say that western boundary currents are intensified, because all of their characteristics (faster, narrower, deeper, warmer) are more extreme than those of eastern boundary currents. The easiest of these characteristics to explain is temperature (think about where the currents come from). The other characteristics have to do with the Earth s rotation (the Coriolis effect). Perhaps the easiest way to explain western intensification is to think about how the Coriolis effect alters currents as they travel from one side of the ocean to the other side of the ocean. As the eastward-flowing current in the figure below travels across the North Pacific ocean towards California, it naturally bends to its right (south) since the Coriolis effect is stronger near the Poles. By the time it reaches the coast, it has already pretty much turned, so it gently flows down the coast. On the other hand, the westward-flowing current near the Equator hardly turns at all, and it runs into the land all at once. An enormous amount of water builds up along the coast (raising sea surface about 3 feet!), creating a pile of warm water that we call the Pacific Warm Pool. All this water has to flow north at the same time, so it has to speed up rush north to make room for all the water coming in behind it. Northern Australia & Indonesia South America Land Warm Land Cold 28. Where is the Coriolis effect stronger, near the Equator or near the Poles? 29. Where is sea level higher at the Equator, on the west side of the Pacific Ocean (by Asia and Australia) or on the east side of the Pacific Ocean (by South America)?

10 Ocean Currents Unit (Topic 9A-1) page 10 Meanders, Eddies, and Other Mesoscale Phenomena At this point, we will end our discussion of ocean circulation patterns. I have covered the most important large-scale, surface ocean currents. In reality, ocean currents are enormously complex: they shift with time ( meander ), grow and shrink, speed up and slow down, twist in upon themselves and spin off rotating eddies, etc. We do not have the time to go into the details of meso-scale phenomena like these, but I would like you to be aware that the subject exists. You can see these complex details in the classic picture of the Gulf Stream below. Temperature of Ocean Water Red = Warm, Blue = Cold. The Gulf Stream is the wiggly red-orange feature extending up into the green water. Courtesy of SeaWiFS / NASA / NOAA 30. True or false? Ocean currents change direction and speed over time (e.g., with the seasons), just like winds. They do not move in straight lines like arrows.