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T. James Noyes, El Camino College Currents Unit (Topic 9A-1) – page 1

Name: Major Ocean Currents Unit (3.5 pts) Section:

Ocean Currents

An ocean 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 (via sails), being carried by an (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 . 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 . For example, the climate of much of the east 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 ), but they also can bring up nutrients from deep in the ocean, fertilizing (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. are an important cause of ocean currents in shallow coastal waters (like estuaries). differences can lead to convection cells composed of ocean currents. The is a kind of convection cell, and has an important impact on the climate of the . (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 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 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 , 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 . (70o is probably a better real-world estimate. It varies with 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 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. The subtropical gyre south of the Equator goes counterclockwise.

The only place without a gyre is the , also known as the Antarctic Ocean. In the Southern Ocean, the currents go all the way around the world. The Antarctic Circumpolar Current (or West Wind Drift) goes east around the continent of , and the East Wind Drift circles to the west closer to the coast of Antarctica. (Recall that winds and currents are often named for the direction that they come from, not the direction that they are going to, so the West Wind Drift goes east, and the East Wind Drift goes west.) Notice that the currents by Antarctica are going in the correct direction to form a clockwise gyre, just as we would expect from the pattern discussed above. (The dashed blue arrows in the lower right corner of the map diagram show what would happen IF land blocked the currents.) However, there is not land in the way to the currents to turn around and create a gyre. Again, near Antarctica the currents go straight east or west, and there is no gyre.

You will need to know these currents. T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 7

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? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 8

Why ocean water moves in gyres

Let’s examine the Northern . The trade winds push water west, away from the coast of . The water travels across the Pacific Ocean until it hits Asia, so it cannot go forward. It flows north along the coast of Asia. The water cannot stop after hitting the coast of Asia, because the trade winds continue to push more and more water west, and this incoming water pushes the water already at the coast out of the way and north along the coast of Asia. By the time the water flowing north reaches Japan, the winds have shifted. The push the water to the east, away from the coast of Japan and towards . (Recall that winds and currents are often 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 and across the Pacific by the trade winds.

In summary, the winds push water east or west across the ocean. The water flowing north or south along the coast is being moved out of the way by the water being pushed to the coast by the winds.

Green Arrows = Winds Blue Arrows = Ocean

13. What pushes ocean current A?

14. What pushes ocean current B?

15. What pushes ocean current C?

16. What pushes ocean current D? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 9

Major Currents near the Poles

Students often tell me that currents flow in gyres due to the Coriolis effect. It is true that the subtropical gyres all turn to the right north of the Equator and left south of the Equator. In fact, this can be a helpful way to remember which way the gyres go.

However, the Coriolis effect does NOT cause water to flow in gyres. The best evidence for this claim comes from the North Atlantic. The currents of the subpolar gyre flow west and east across the northern in the directions dictated by the winds. The land 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.)

Notice that there is no gyre in the Southern Ocean by Antarctica. This is because the winds can push water east or west without the water running into land. Thus, land must get in the way of ocean currents for water to turn north and south, and for gyre to form.

With these insights, we can now identify the conditions under which ocean currents will move in a gyre. The key factors needed to create gyres are (1) winds pushing water east at one latitude and west at another latitude, and (2) land in the way of the currents.

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)? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 10

Boundary Currents

The parts of the gyre that flow along the 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 of continents have western boundary currents next to them, and west coasts of continents have eastern boundary currents next to them! As you can imagine, this can lead to some confusion.

To help remember this, imagine a compass in the middle of an ocean as shown in the map below. Everything west of the compass is the west side of the ocean, and everything thing east of the compass is the east side of the ocean. This is the same reasoning that we use when labeling the west and east sides of a continent.

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.

T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 11

The first major ocean current to be measured and charted was the , the northward- flowing 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 , hence the name Gulf Stream. The Gulf Stream is a western , because it is on the western side of the Atlantic Ocean, so it is a fast, narrow, deep, warm current.

The other two boundary currents that I want you to know about are the and the Kuroshio. The California Current flows south along the coast of California. The California Current is a eastern boundary current, because it is on the eastern side of the Pacific Ocean, so it is a slow, wide, shallow, cold current.

The Kuroshio is a fast, warm water current along the east coast of Japan. Kuroi means black in Japanese, and shio means river, so Kuroshio means something like Black Stream. It is called the Black Stream, because warm water tends to have less life than cold water. It is the “lifeless river.” The Kuroshio is a western boundary current, because it is on the western side of the Pacific Ocean, so it is a fast, narrow, deep, warm current.

19. 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? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 12

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?

Western Intensification and Level

We say that western boundary currents of the subtropical gyres are “intensified,” because all of their characteristics (faster, narrower, deeper, and warmer) are more “extreme” than those of eastern boundary currents. The easiest of these characteristics to explain is . Think about where the currents are coming from. Western boundary currents come from the Equator, so they take warm water from the tropics towards the Poles. Eastern boundary currents flow away from the Poles, so they carry the colder water from near the Poles towards the Equator.

T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 13

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 map 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 (to rush north) to make room for all the water coming in behind it.

28. Where is the Coriolis effect stronger, near the Equator or near the Poles?

29. Where is 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)? T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 14

Further Comments about the Causes of Gyres

Technically the winds do not push the water directly east or west across the ocean. The trade winds and westerlies push the ocean water together in the North Pacific (the blue, dashed Ekman transport arrows in the map below). 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 .

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

There are other ways to explain why the water flows along the coasts than the one I gave previously. As more and more water is pushed into the coast by the winds, sea level rises along the coast. As we all know, water flows downhill, pulled down by gravity. It cannot flow downhill out into the ocean and away from the coast, because the winds are pushing water towards the coast, so the water piled up along the coast flows downhill in the only direction it can go: 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 the winds 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. As I pointed out earlier, 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 to move downhill (due to gravity) is balanced by the Coriolis effect. T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 15

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 ), but these concepts bring us well beyond the bounds of this introductory course.

Equatorial Counter Currents

Near the Equator, some water flows east, against the direction of most of the winds and currents in the tropics. The water flowing east near the Equator is called an equatorial countercurrent.

The water near the Equator is the warmest on the Earth. More water evaporates near the Equator, because the water is warmer. The warm water that evaporates warms the air. In addition, the warm water itself warms the air that it is in contact with. As we learned in Unit 8A, the warm air at the Equator rises due to it low density (compared to other air at the surface of the Earth). Air moves in from the north and south to replace the rising air. The air coming in from the side are the winds (the trade winds). The winds do not move directly north and south, but bend to the west due to the Coriolis effect. (The trade winds north of the Equator go southwest, and the trade winds south of the Equator go northwest.) The water near the Equator is pushed west by these winds.

However, in the region where the air is rising, the winds weaken, because the air starts moving upwards instead of horizontally. In other words, the air moves vertically instead of side to side. Where the air is rising, the air cannot push the water side-to-side and create currents. It is a “calm” area of the ocean known as the doldrums.

In the previous sections, we learned that the trade winds pile up water on the western sides of oceans. In the Pacific Ocean, the water is about 3 feet higher than on the western side of the ocean (by Asia) than on the eastern side of the ocean (by the Americas). In the region of weak winds where air is rising, there are no winds to push the water west and keep it piled up. In the region of weak winds (the doldrums), some of the water piled up flows to the east, pulled downhill by gravity. This flow of water to the east is the equatorial countercurrent.

Water remains piled up on the western side of the ocean, because the trade winds keep bringing more water west. Since water remains piled up on the western side of the ocean, the equatorial countercurrent keeps flowing, taking some of this water away to the east. T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 16

Meanders, Eddies, and Other Mesoscale Phenomena

At this point, we will end our discussion of ocean circulation patterns. I feel the need to emphasize that this is an introductory course so I have simplified matters. I have covered the most important large-scale, surface ocean currents. In this discussion, I have left out many ocean currents, including the Equatorial Undercurrent, the , the , the Oyashio, and more.

In addition, ocean currents are enormously complex. Ocean currents shift with time (“meander”), grow and shrink, speed up and slow down, twist in upon themselves and spin off rotating eddies, and so on. We do not have the time to go into the details of mesoscale phenomena like these in an introductory class, but I would like you to be aware that these things happen. You can see these complex details in the classic picture of water temperature and 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

A local example of such small scale features and their fluctuations is the behavior of the California Current near Los Angeles. There is a bend in the coastline of California at Santa Barbara County. The California Current goes more-or-less straight and detaches from the coast, T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 17 so the coast of southern California is not as close to the cold California Current as places farther north along the coast which are right next to the current. This is one reason our weather is significantly warmer. South of where the California Current leaves the coast, friction with the California Current tends to drag the water next to it south, and then water from farther south moves north along the coast to replace the water dragged away. This creates a small , a little counterclockwise gyre, in our local ocean. Sometime the water pulled north is warm water from farther south by Mexico, and sometimes it is well mixed with cold water that was once part of the California Current. Seasonal changes in the winds and changes in the winds associated with major weather patterns can alter the motion of our local eddy, shifting its location, speed, and size. The island and complex shape of the ocean floor also have a big influence on how the water near southern California reacts to the winds and California Current.

30. True or false? “Ocean currents change direction and speed over time (e.g., with the ), just like winds. They do not move in straight lines like arrows.”

T. James Noyes, El Camino College Ocean Currents Unit (Topic 9A-1) – page 18

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