Oceanography 10, T. James Noyes, El Camino College 09A-1. Ocean Currents

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1 Oceanography 10, T. James Noyes, El Camino College 09A-1 INTRODUCTION TO OCEAN CURRENTS Ocean Currents What is an ocean current? Why do we care about 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 1. 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). National Oceanic and Atmospheric Administration / Department of Commerce Most organisms start life as plankton. They are carried from place to place by ocean currents. NOAA / Department of Commerce 1 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. Oceanography 10, T. James Noyes, El Camino College 09A-1

2 Oceanography 10, T. James Noyes, El Camino College 09A-2 What causes ocean currents? Tides are an important cause of ocean currents in shallow coastal waters (like estuaries). Ocean currents can be created in several Density differences can lead to convection cells different ways, but most ocean currents at the in the ocean, causing thermohaline circulation. 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. 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 I think you get the idea. Thus, water ends up going in different directions at different depths 2. 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). Ekman showed mathematically how most of the water flows approximately 90 o to the side of the wind 3 owing to the Coriolis effect, so oceanographers often refer to the overall motion of the water as the Ekman transport. Wind surface water deeper water even deeper water "Ekman Spiral" Ekman Transport (Overall Water Direction) Wind even deeper water surface water deeper water 2 At a certain depth, it actually goes in the opposite direction of the wind! 3 70 o is probably a better real-world estimate Oceanography 10, T. James Noyes, El Camino College 09A-2

3 Oceanography 10, T. James Noyes, El Camino College 09A-3 even deeper water deeper water Wind surface water Ekman Transport (Overall Water Direction) National Oceanic and Atmospheric Administration / Department of Commerce 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. Oceanography 10, T. James Noyes, El Camino College 09A-3

4 Oceanography 10, T. James Noyes, El Camino College 09A-4 MAJOR OCEAN CURRENTS Overall Ocean Circulation Pattern Examine the figure below showing the large-scale ocean circulation pattern. The dominant ocean current pattern in most oceans is a gyre. A gyre is a group of ocean currents moving in a huge, horizontal loop ( circle ). The ocean has 5 subtropical gyres 4 (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 5 around the continent of Antarctica, and the East Wind Drift circles to the west closer to the coast of Antarctica. 60 o N Subpolar Subtropical (Clockwise) Equator Equator Subtropical (Counterclockwise) 60 o S You will need to memorize these currents. Notice that in the northern hemisphere the red arrows turn to the right as they go around the gyre, resulting in a clockwise circulation. In the southern hemisphere, the red arrows turn to the left, resulting in a counterclockwise circulation. Although the direction of these currents are consistent with the Coriolis effect (and this is a good way to remember the directions), the subpolar gyre (blue arrows) demonstrates that the Coriolis effect does not cause the currents to move in these directions, because the subpolar gyre goes counterclockwise (turns to the left), the opposite of the subtropical gyres of the northern hemisphere. Notice that the currents by Antarctica (blue arrows) do something similar. If the currents turned to their left to make a gyre (they do not, but the dotted arrows show what they would do if they did), then the gyre would go clockwise, opposite to the subtropical gyres of the southern hemisphere. 4 Subtropical means below the tropics. In this case, it means the part of the world that is next to the tropics (it is not as hot as the tropics, the temperature in the subtropics is below the temperature in the tropics.) Similarly, subpolar refers to the parts of the world next to the Poles (not as cold as the Poles). 5 Recall that winds and currents are named for the direction that they come from, not the direction that they are going to, so the West Wind Drift comes from the west and goes to the east. Oceanography 10, T. James Noyes, El Camino College 09A-4

5 Oceanography 10, T. James Noyes, El Camino College 09A-5 Ocean currents have had important impacts on history. Columbus was a good enough sailor to know to get into the trade winds (and the southern part of the North Atlantic gyre) so that he could sail as quickly as possibly across the Atlantic. This is why he landed first on Caribbean islands, not North America. Spanish ships with goods from East Asia (e.g., starting in the Philippines) went up to Japan, across the Pacific, and down the coast to Mexico. Goods were then carried across the narrow parts of Central America, where they and stolen treasure from the Americas were loaded onto ships heading back to Europe. The ships first went up the east coast of North America, and then turned east toward Spain. The Spanish used the winds and currents to travel faster and safely over the oceans, but this made them vulnerable to pirates who knew the best routes as well. Later, ships would use the convenient winds and currents to bring slaves from West Africa to the Caribbean and the Americas. 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 6. 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 7, away from the coast of Japan and towards California. 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 8. 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. 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 6 The trade winds and westerlies push the ocean water together. 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. 7 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. 8 Some water also goes north and loops around to join the eastward current again after traveling up to Alaska, like a small subpolar gyre. Oceanography 10, T. James Noyes, El Camino College 09A-5

6 Oceanography 10, T. James Noyes, El Camino College 09A-6 There are at least 2 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. 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 the directions of the winds and 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 9. 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 particularly on quizzes and exams so watch out! Kuroshio East Coast WBC California Current EBC West Coast East Coast Gulf Stream WBC 9 The southern tip of South American, Cape Horn, does get in the way. (If it were a bit closer to Antarctica, a huge gyre stretching across the southern parts of all 3 oceans would form.) All the water flowing west and east by Antarctica suddenly finds that it has to squeeze through the narrow gap between South America and Antarctica. To do so, it accelerates (moves faster) so that it all can get through, pushed by the water behind. This creates the fastest of the major ocean currents in the world. There is also a tremendous amount of variability, as currents have to move up and over shallow underwater plateaus, and the west and east wind drifts rub against one another, creating huge eddies. Add to this the strong winds which create huge waves (over this very large fetch), and you have one of the most dangerous bodies of water in the ocean. After Magellan (who led the first expedition to sail all the way around the world) sailed out of these waters, he called the ocean that they arrived in the Pacific, because it was much more peaceful than where they had been. Ironically, the Pacific Ocean is actually more violent that the Atlantic and Indian Oceans. Oceanography 10, T. James Noyes, El Camino College 09A-6

7 Oceanography 10, T. James Noyes, El Camino College 09A-7 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. East Coast Western Boundary Current Eastern Boundary Current Ocean West Coast Although many sailors were already aware of the Gulf Stream, Benjamin Franklin was the first person to accurately measure the Gulf Stream. He could tell whether his ship was in it or not, because the waters of the Gulf Stream are unusually warm. (Western boundary currents are warm because they come from the Equator where the water is warm. Similarly, eastern boundary currents carry cooler water from closer to the Poles.) Not only did he conduct measurements himself, but he was postmaster for the American colonies, so he asked the captains of the ships who carried mail between the England and the colonies how long it took them to cross the Atlantic and about the route they took. By painstakingly piecing all of this information together, he produced the first accurate map of the Gulf Stream, which helped ships cross the ocean much faster in both directions. (Going against a current slows you down, so sometimes it is faster to take a longer route that avoids the current.) This helped the British send troops over here during the American Revolution. Fortunately, Ben Franklin famous in Europe as a scientist for his work on the Gulf Stream, electricity 10, stoves, and much more used his status to convince the French king to intervene on the side of the United States. The French 10 Benjamin Franklin did not discover electricity. He used his famous kite experiment to demonstrate that lightning is a form of electricity, and then used that knowledge to construct the first lightning rod. As you know, metals conduct electricity. A lightning rod gives the electricity of the lightning bolt an easy path to the ground; otherwise it has to go through the building (which is easier than going through the air and is why lightning tends to strike tall objects: don t get caught in out in the open during a thunderstorm!). This was a big problem back then, because, upon occasion, lightning would strike a building with lots of gunpowder with explosive consequences. In fact, churches were the most commonly struck buildings, because they tended to be the tallest buildings around. (It was considered inappropriate to build your home taller than God s home.) This confused a number of people: why would God want to destroy churches? Worse yet, people would go up into steeple to ring the bell (thought to ward off lighting), so lots of bell-ringers were hurt and killed. Interestingly, some people argued against using lightning rods, because they thought that they the defied God s will. Ben Franklin, among others, argued that these people shouldn t run into their houses during rainstorms, because that defied God s attempt send rain down upon them. Oceanography 10, T. James Noyes, El Camino College 09A-7

8 Oceanography 10, T. James Noyes, El Camino College 09A-8 provided weapons, troops, and crucially their navy which kept the British navy from rescuing their soldiers at Yorktown 11. This allowed George Washington s army to capture the largest British army in the colonies and force the British the world s superpower at the time to surrender. (Notice that science and the ocean actually played a crucial role in American independence. Neat, huh?) 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 11 The French king was upset about losing Canada to the British during the recent French and Indian War (also known as the Seven Years War ). The British increased taxes on the American colonists to pay for the war (after all, the British army defended the colonies during the war) which ended up being one of the causes of the American Revolution ( no taxation without representation ). The French were also bankrupted by the French and Indian war, but Franklin convinced the French king that with a little money, he could have his revenge upon the British. Oceanography 10, T. James Noyes, El Camino College 09A-8

9 Oceanography 10, T. James Noyes, El Camino College 09A-9 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 on the right. CURRENTS AND CLIMATE Temperature of Ocean Water. Red = Warm, Blue = Cold. The Gulf Stream is the wiggly red-orange feature extending up into the green water. SeaWiFS / NASA / NOAA Warm currents typically warm the air above them, and cold currents typically cool the air above them. (Try to keep up ) In addition, warm water evaporates more easily than cold water, making the air above more humid. As we saw in the last section on the atmosphere (8A), if this air rises, it will cool and its moisture will condense into rain. Thus, the southeast coast of the United States is more humid and gets more rain than the southwest coast of the United States, because they live next to the warm Gulf Stream while we live by the cool California Current. Warm water and cold water do not make the local climate warm or cold; they make it warmer or colder and more humid or less humid than it would otherwise be. So, the cold water of the California Current does not make California a cold place, but it keeps California from being even warmer and more humid. I don t know about you, but I dislike humidity and think California is warm enough already. Thank you, California Current! If you look at a map of the world, you may notice that Boston, Massachusetts, is at the same latitude as Spain (In other words, both places are the same distance from the Equator.). However, they clearly do not have the same climate: as the Pilgrims could tell you, Massachusetts is colder than Northern Canada Boston Ireland & England Spain Oceanography 10, T. James Noyes, El Camino College 09A-9

10 Oceanography 10, T. James Noyes, El Camino College 09A-10 Spain. Similarly, England is at the same latitude as Northern Canada. (England is not a particularly warm place, but it is a lot warmer than Northern Canada.) These differences in climate are related to the ocean currents. The warm Gulf Stream begins to leave the east coast near North Carolina, so it never reaches Boston. Instead, the Gulf Stream crosses the Atlantic Ocean 12, aiming for Spain. The water cools as it crosses the ocean, giving up its heat to the cooler, northern air. It then hits the coast of Europe and Africa. Some of the water heads south down the coast of Africa, but the rest turns north and becomes part of the subpolar gyre. This unusually warm water (for this latitude) passes the coast of England and Ireland. It makes the air more humid which leads to larger amounts of rain 13 (thus, Ireland is the Emerald Isle, very green). Then, the water spends time closer to the Poles, becomes very cold, and comes back down the east coast of Greenland and Canada (it is called the Labrador Current ). It cools New England (including Boston), and its cool water mixes with the warm waters of the Gulf Stream, producing turbulence and mixing that stirs up nutrients and leads to one of the world s most abundant fisheries (well, it used to be until we wiped most of the fish out). The cool water also carries ice bergs to the south with it and helps keep them from melting, which led to the Titanic s fateful encounter with an iceberg on its maiden voyage from Europe. Eastern boundary currents like the California Current and the Canary Current (off the coast of west Africa) start out cold, but become warmer as they move towards the Equator. The water eventually leaves the coast and travels westward across the ocean, becoming warmer and warmer the longer it spends near the Equator. By the time the water reaches the west side of the ocean, it has become the warmest water in the world and begins going up the east coasts as western boundary currents. In the Atlantic Ocean, some of this warm water is trapped in the Gulf of Mexico. Florida is surrounded by the warm water of the Gulf on one side and the Gulf Stream on the other, making it very warm and humid. It often rains every afternoon during the summer. By the afternoon, the Sun has warmed the water even more, increasing the amount of evaporation. The warm, moist air rises and cools, producing huge clouds and rain. This warm water and the Gulf Stream also help fuel hurricanes, keeping them powerful and sometimes even giving them a boost before they strike land. A final thought: The motion of the ocean (lots of fun to say aloud) keeps the Poles from becoming too cold and the Equator from becoming too hot, making them both nicer places to live. As we ve seen the gyres carry warm water towards the Poles in western boundary currents and cold water towards the Equator in eastern boundary currents. 12 After the Gulf Stream leaves the coast, we call it the Gulf Stream Extension or the North Atlantic Current. 13 It also keeps the atmosphere warm enough that it does not snow too much. Oceanography 10, T. James Noyes, El Camino College 09A-10

11 Oceanography 10, T. James Noyes, El Camino College 09A-11 UPWELLING AND DOWNWELLING: OCEAN CURRENTS AND OCEAN LIFE Upwelling occurs when ocean currents bring water upwards from down deep, and downwelling occurs when ocean currents push surface water downwards. (Rocket science, huh? Try to keep up. ) The upwelling water is cold (it comes from down deep), so it does not want to rise owing to its higher density; ocean currents have to force the water to rise. Downwelling occurs where winds cause ocean currents to come together (to converge ). The water has nowhere to go, so it starts piling up, raising the sea surface. Eventually, the weight of the hill of water starts pushing the water below it downwards (causes downwelling ). Hill Notice that the sea surface goes up where there is downwelling, and goes down where there in upwelling. Watch out! Remember that the change in the sea surface CAUSES upwelling or downwelling. In other words, deeper water is REACTING to changes at the surface. "Gap" Downwelling Upwelling Upwelling occurs where winds cause ocean current to move apart (to diverge ). This creates a hole or gap in the surface of the ocean. Eventually, it becomes so deep that water from below starts rising up 14 to replace the water that is pushed away by the winds. Upwelling water is colder and saltier than surface water, making it possible for oceanographers to detect where and when upwelling occurs. It is also rich in nutrients 15, so upwelling causes the growth of lots of phytoplankton (who need the nutrients to carry out photosynthesis and for their shells) who become food for zooplankton. Upwelling is common in many places throughout the world. I would like you to know about 3 of the most important upwelling zones: along the west coasts of the continents = on the east sides of the oceans (in the subtropics) at the Equator in the middle of the Southern (Antarctic) Ocean Water sinks on either side of the gap owing to the higher pressure (more water = more weight pressing down). The water that is already down deep has to go somewhere, so water rises where the pressure is lower. 15 A pet peeve of mine is when students imply that water is nutrient-rich because it is cold. This is simply not true; cooling water does not add nutrients to it. Deep water, not cold water, tends to have more nutrients, because there are no phytoplankton down deep using them up, and new nutrients are added by bacteria decomposing ( breaking down ) the dead, decaying matter sinking down from the surface. 16 Water also upwells at similar latitudes in the Northern Hemisphere, but I do not consider it to be one of the major upwelling zones. Oceanography 10, T. James Noyes, El Camino College 09A-11

12 Oceanography 10, T. James Noyes, El Camino College 09A-12 In the satellite image below, dark blue & purple colors indicate that very few phytoplankton are present while red & orange & yellow colors indicate that huge amounts of phytoplankton are present. Light blue & light green colors indicate that lots of phytoplankton are present. As you can see, phytoplankton are very abundant near the coasts, because of all the nutrients that are washed off the land. Notice, though, that there is more life along the west coasts than east coasts owing to upwelling. For example, compare the west coast of Africa to the east coast of the Americas across the Atlantic Ocean. Similarly, the light blue tongue across the Equator shows that there is more life at the Equator, also owing to upwelling. (Notice that upwelling is strongest along the west coasts near the Equator.) Red / Orange = Tremendous Amounts of Phytoplankton Light Blue / Light Green = Large Amounts of Phytoplankton Dark Blue / Purple = Few Phytoplankton The satellite image clearly shows that life is more abundant in the colder waters near the Poles than in the warmer waters closer to the Equator. This often confuses my students, because they (correctly) reason that warmer water is warm due to more sunlight, and sunlight is needed for photosynthesis. What they forget, though, is that warm water has a lower density, so phytoplankton sink more easily in warm water, and the more they sink, the harder it is for them to get sunlight. More importantly, if the surface water is too warm, waves have difficulty stirring up nutrient-rich water from down deep. (The deeper water is cold and dense, so when the waves try to bring the water up, it immediately sinks back down.) The upwelling in the middle of the Southern (Antarctic) Ocean adds even more nutrients to the surface water, which produces more life than any other place in the open ocean. Oceanography 10, T. James Noyes, El Camino College 09A-12

13 Oceanography 10, T. James Noyes, El Camino College 09A-13 Now let s examine the wind patterns that lead to upwelling in the places that we have discussed. Coastal winds tend to blow south along the coast of California. They push water to their right (westward), away from the coast, opening up a gap in the surface of the ocean. Cooler, nutrient-rich water then rises to fill in the gap. At the Equator, the winds push water away from the Equator, because the Coriolis effect causes the northern trade winds to push water north and the southern trade winds to push water south. This creates a gap in the ocean surface, and water rises up to fill in the gap. In the Southern Ocean, the water moves to the left of the winds, also creating a gap in the middle of the ocean. Note, however, that water is also pushed into the coast of Antarctica, causing downwelling. Downwelling also occurs in the centers of the subtropical gyres, because the winds are pushing the water together (see example F on page 3), which is why there is very little life in the center of the gyres (dark blue areas on the previous page). California The Equator Land Equator Upwelling Upwelling California "Gap" Land Upwelling Upwelling can occur for reasons other than those discussed so far. For example, an ocean current can run into an underwater hill (a seamount ) and be forced up and over it. We do not have time and space here to discuss all the causes of upwelling, but please be aware that other causes exist. Upwelling Downwelling Map-View Pictures Winds = green solid arrows Ekman Transport = dashed arrows Southern (Antarctic) Ocean Antarctica Antarctica "Gap" seamount Upwelling Downwelling Oceanography 10, T. James Noyes, El Camino College 09A-13

14 Oceanography 10, T. James Noyes, El Camino College 09A-14 GEOSTROPHIC CURRENTS, DOWNWELLING, AND THE CAUSE OF THE GYRES 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 and downwelling (e.g., figure on page 5). 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. WINDS AND CURRENTS ALONG THE COAST OF CALIFORNIA In this section, I want to review the winds and currents along the coast of California, something that I think all of my students should be familiar with by the end of the course. The westerlies push water to the east across the Pacific Ocean. By the time it reaches the coast of California, this water has cooled down, and it turns to the south, becoming the California Current. This water helps cool the climate of California (it does not make the climate warm, but keeps it from being even warmer than it is now) and helps the reduce the amount of evaporation, making the coastal air unusually dry for a coastal environment. The current begins to leave the coast near southern California where the trade winds begin to push it westward across the ocean. This allows warmer water to sneak northward up the coast to southern California, where it meets and mixes with the cooler water from the north, producing turbulence which stirs up nutrients and ultimately leads to more fish. Southern California has a very complex and constantly shifting ocean environment. Coastal winds tend to blow south along the coast of California 17, particularly during the spring and summer. This pushes water (including the California Current) away from coast, and cold, nutrient-rich water from down deep come up to replace it ( upwelling ). The nutrients fertilize algae like phytoplankton, allowing them to carry out photosynthesis and grow. The phytoplankton become food for zooplankton who in turn are food for fish. Thus, upwelling creates abundant life along the coast of California. During the winter, coastal winds tend to blow northward along the coast. These winds can create the Davidson Current, warm water flowing northward along the coast of California, and shut down coastal upwelling. (The California Current does not stop or reverse; it is pushed away from the coast.) Winds = Green Arrows with Tails Ekman Transport = Purple Dashed Arrows Ocean Currents = Solid Arrows, No Tails The California Current begins to leave the coast near Santa Barbara. 17 The land is warmer than the ocean, so the warmer air over the land rises, and the cooler air from the ocean comes in to replace it. However, the Coriolis effect and coastal mountains cause this air to bend towards the south along the coast. Oceanography 10, T. James Noyes, El Camino College 09A-14