T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 1

Name: Currents, Climate, and Life (2.5 pts) Section:

Currents and Climate

Warm currents typically warm the air above them, and cold currents typically cool the air above them. In addition, warm water evaporates more easily than cold water, making the air above more humid. (Humid air is air with lots of water vapor in it.) As we saw in the last Unit on the atmosphere (Unit 8A), if air rises, it will cool and water vapor in the air 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 or cold water along a coast do not make the local climate “warm” or “cold.” Warm water along a coast makes the local climate warmer and more humid than it would otherwise be. Cold water along a coast makes the local climate cooler and 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. So, I say “Thank you, California Current!” (That being said, some more rain would not hurt.) T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 2

As we have seen in the previous Unit (Unit 9A-1) the gyres carry warm water towards the Poles in western boundary currents (WBCs) and cold water towards the Equator in eastern boundary currents (EBCs). Thus, the motion of the ocean keeps the Poles from becoming too cold and the Equator from becoming too hot, making them both nicer places to live.

Note that once the cold water of eastern boundary currents reaches the tropics, it turns to the west and spends a long time traveling across the ocean, soaking up more and more sunlight and getting warmer and warmer during its journey. Once the water reaches the west side of the ocean and the east coast of a continent, it has become quite warm. This warm water is then taken towards the Poles by western boundary currents. Eventually the water turns east and goes back across the ocean. The warm water loses more and more heat as it travels towards the Poles and back east across the ocean. By the time the water reaches the east side of the ocean and the west coast of a continent, it is quite cold. The water is taken towards the Equator by an eastern boundary current, and the cycle repeats.

1. How does a warm current affect the climate along a nearby coast? Does it make the climate warmer or cooler? Does it make the climate more humid or less humid?

2. How does a cold current affect the climate along a nearby coast? Does it make the climate warmer or cooler? Does is make the climate more humid or less humid?

3. Overall, do ocean currents warm or cool the Equator? Do ocean currents warm or cool the Poles?

T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 3

Upwelling and Downwelling

Upwelling water is water that rises up to the surface due to the motion of currents at the surface of the ocean.

Downwelling water is water that moves down from the surface of the ocean due to the motion of surface currents.

Downwelling occurs where winds cause ocean currents to come together (converge) or run into land. 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 “the hill” downwards and thus causes downwelling.

Upwelling occurs where winds cause ocean currents to move apart (diverge) or pull water away from the land. This creates a “hole” or “gap” in the surface of the ocean. Eventually, the “gap” becomes so deep that water from below starts rising up to replace the water that is pushed away by the winds. (Remember that winds only create currents at the surface of the ocean. The currents do not reach down very deep into the ocean. Eventually, a depth is reached that remains unaffected by the winds. It is this deeper water that moves up to replace the water taken away by the surface currents, to fill in the hole at the surface of the ocean produced by the winds.)

Notice that the sea surface goes up where there is downwelling, and goes down where there is upwelling. Remember that the change in the sea surface CAUSES upwelling or downwelling. In other words, deeper water is REACTING to changes at the surface.

Upwelling water is cold, since it is water from deeper in the ocean. Cold water does not “want” to rise, because it has a higher density than the water at the surface. Ocean currents have to force the water to rise by taking away the surface water and creating a hole in the surface of the ocean. If the ocean currents stop or change direction, the cold upwelled water will sink back down again. T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 4

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 the time and space here to discuss all the causes of upwelling. Please be aware that other causes exist.

4. What is downwelling?

5. Does downwelling happen where sea level goes up or down?

6. What causes the sea level to change and produce downwelling?

7. Why does the change in sea level cause downwelling?

8. What is upwelling?

9. Does upwelling happen where sea level goes up or down?

10. What causes the sea level to change and produce upwelling?

11. Why does the change in sea level cause upwelling? T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 5

Effects of Upwelling

Since the water comes up from below the surface, upwelling water is colder and saltier than surface water, making it possible for oceanographers to detect where and when upwelling occurs. Upwelled water is also rich in nutrients, so upwelling causes the growth of lots of phytoplankton (who need the nutrients to carry out photosynthesis and for their shells). Of course, the phytoplankton become food for zooplankton. The zooplankton are in turn food for small fish, and the small fish are eaten by larger fish. Thus, animals are attracted to upwelling zones to feed. We have learned that there is typically more life near the coast than in the middle of the ocean, far from land. Upwelling zones are one of the major exception to this pattern. Upwelling zones can occur in the middle of the ocean, far from land, and support lots of living things.

Upwelling benefits phytoplankton in other ways. The upward moving water pushes the phytoplankton up towards the Sun. In addition, the upwelled water is both cold and salty, and therefore has a high density. High density water provides more support. High density water pushes phytoplankton upwards, helping them get more sunlight for photosynthesis.

By the way, deeper water tends to have more nutrients than surface water, because there are no phytoplankton down deep absorbing them, and new nutrients are added by bacteria decomposing (“breaking down”) dead, decaying matter and wastes sinking down from the surface.

Wave Mixing versus Upwelling

Waves do NOT cause upwelling. Wave orbitals DO make water move up and down. Waves make cold water come up from below, and push warmer surface water down. But then the waves bring the cold water back down and warm water back up, because the waves’ orbitals make the water move quickly in a circle. In fact, the waves stir or mix the warmer surface water with cooler water from below, deepening the surface mixed layer of the ocean and cooling it. This is rather different from upwelling which happens when the warmer surface water is pushed away and the water below comes up to replace it. Like upwelling, wave mixing does bring up nutrients, but upwelling reaches down much deeper than wave orbitals and brings up more nutrients. T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 6

Upwelling Zones

Upwelling is common in many places throughout the world. Upwelling occurs along most coasts at some point, often during specific seasons. 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 Ocean (Antarctic Ocean) In these regions, there is a lot of upwelling ever year.

12. Does upwelling make the water at the surface of the ocean warmer or colder?

13. Does upwelling make the water at the surface of the ocean fresher (less salty) or saltier?

14. Does upwelling increase or decrease the amount of nutrients at the surface of the ocean?

15. Why does the water below the surface typically have more nutrients than the water at the surface?

16. True or false? “Fish are more abundant in upwelling zones.”

17. True or false? “Upwelling is typically more common along the west coasts of continents.”

18. True or false? “The warm water of the Equator prevents upwelling along the Equator.”

19. True or false? “Upwelling occurs in the middle of both the Arctic Ocean and the Antarctic Ocean.” T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 7

Explanation of Upwelling along the Coast of California

You will be asked to examine maps showing winds and land, and identify likely regions of upwelling and downwelling. Let us look at a few real-world examples.

Along the coast of California, coastal winds tend to blow to the south during spring and summer. Water gets pushed to the right of the wind in the northern hemisphere due to the Coriolis effect. West is to the right of the direction of the wind (south), so the Ekman transport is west. In other words, the coastal winds push water away from the coast. This happens all along the coast, so water cannot come in from the sides to replace the water taken away from the coast. Eventually, the drop in sea level along the coast is so deep that water from below comes up to replace the water being taken away. In short, upwelling occurs along the coast of California.

In the map diagram above, the green arrow shows the direction of the wind, and the red arrows show the direction that the water is pushed by the winds (the Ekman transport). T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 8

Explanation of Upwelling at the Equator

The trade winds north of the Equator are called the northeast trades. The northeast trades blow towards the southwest. (Remember: winds are often named for where they come from, not where they are going to.) In the northern hemisphere, water is pushed to the right of the wind. Northwest is to the right of southwest, so the northeast trades push water northwest. In other words, Ekman transport is northwest.

In the map diagram above, green arrows show the direction of the winds, and blue arrows show the direction that the water is pushed by the winds (the Ekman transport).

The trade winds south of the Equator are called the southeast trades. The southeast trades blow towards the northwest. In the southern hemisphere, water is pushed to the left of the wind. Southwest is to the left of northwest, so the southeast trades push water southwest. In other words, Ekman transport is southwest.

Let us now put these two deductions together. North of the Equator, water is pushed northwest by the winds. South of the Equator, water is pushed southwest by the winds. Therefore, water is being pushed away from the Equator. Sea level drops more and more at the Equator as more and more water is taken away. Eventually, the gap in the surface of the ocean is large enough that water starts coming up from below to start filling in the gap, to replace the water that is being taken away by the winds. T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 9

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 the water 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 reduce the amount of evaporation, making the coastal air unusually dry for a coastal environment. This also means that there is less water vapor in the air that can become rain, and that California gets less rain than it otherwise would.

The California Current begins to leave the coast near southern California (Santa Barbara) 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 particularly during the spring and summer. (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.) The coastal winds push water (including the California Current) away from the coast, and cold, nutrient-rich water from down deep comes 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 Winds = Green Arrows with Tails the coast of California. Ekman Transport = Purple Dashed Arrows Ocean Currents = Solid Blue Arrows, No Tails 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 a little bit.) T. James Noyes, El Camino College Currents, Climate, and Life (Topic 9A-2) – page 10

20. Does the California Current carry water to the north or south?

21. Is the California Current a warm current or a cold current?

22. How does the California Current affect the climate of California?

23. Does upwelling occur along the coast of California?

24. What is the Davidson Current?