El Niño Unit (2.5 pts)

Transcription

1 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 1 Name: Section: El Niño Unit (2.5 pts) El Niño is a warm ocean current that flows along the Equator and towards the west coast of South America approximately every 4 to 7 years. It often reaches the coast in December, and is named El Niño after the baby Jesus whose birth is celebrated during December. The warm ocean water remains along the coast for about a year to 18 months on average. It pushes down cool, upwelling water, cutting off the supply of nutrients to phytoplankton. With less food, many animals like fish, birds, and marine mammals die, so fishermen s incomes plummet. In addition, more rain falls, leading to floods and mudslides. At the same time that El Niño affects the east side of the Pacific, the west side of the Pacific Ocean experiences a cooling and thus less rain. Thus, atmospheric pressure increases in the western Pacific and decreases in the eastern Pacific. This reversal of the normal conditions in the atmosphere is called the Southern Oscillation. During the last century, scientists began to recognize that that El Niño, the Southern Oscillation, and other changes across the entire Pacific Ocean (the world s largest ocean) were all linked together, and came to call the phenomenon ENSO (El Niño-Southern Oscillation). 1. What is an El Niño? A current that moves towards along the Equator. 2. How do El Niño conditions affect the temperature of the ocean on each side of the Pacific? In other words, do they get warmer or cooler? The water on the eastern side of Pacific Ocean at the Equator gets, and the water on the western side of the Pacific Ocean at the Equator gets. 3. How do El Niño conditions affect the climate on each side of the Pacific? In other words, do they get more clouds and rain or less? The eastern side of Pacific Ocean at the Equator gets clouds & rain, and the western side of the Pacific Ocean gets clouds & rain. 4. During El Niño conditions, is there more upwelling or less along the coast of the Americas? 5. During what time of year do El Niño conditions typically begin? 6. About how long do El Niño conditions typically last? 7. About how often do El Niño conditions happen?

2 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 2 Normal Conditions During normal conditions, the trade winds push ocean water near the Equator to the west. As the water travels near the Equator, it becomes warmer and warmer. Eventually it hits Asia, and begins to pile up, raising the sea surface on the western side of the ocean by about 3 feet. Gravity wants to push the water downhill towards the east, but the winds hold it in the west 1, so instead it flows downhill along the east coasts of Asia and Australia as western boundary currents 2. The trade Warm Northern Australia & Indonesia Land Warm winds also pull water away from the west coast of South America, and water rises up from below to replace it (upwelling). The warm water on the west side of the Pacific warms the air above it, lowering its density and causing it to rise. The air is moist owing to lots of evaporation from the warm ocean water, so as it rises and cools, the water vapor condenses, producing lots of clouds and rain. Cooler air from the east replaces the rising air, and is itself warmed, and rises. Air sinks on the cooler, east side of the Pacific 3, so clear skies with little rain are common there. Notice that the temperature difference between the two sides of the ocean creates winds which blow to the west. These winds cause warm water to pile up on the western side of the ocean, creating the temperature difference. Thus, the ocean water and winds reinforce one another; they are part of a positive feedback loop. Cold South America Land 1 In fact, some water does leak back along the Equator in the doldrums where the air is rising instead of pushing the water to the west. This current is call the Equatorial Countercurrent, because it flows along the Equator against the wind. Once the winds weaken, this current becomes the much stronger El Niño. 2 the Kuroshio and East Australian Current 3 The entire circulation is a convection cell.

3 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 3 El Niño Conditions During El Niño conditions, the trade winds weaken. This allows some warm water to flow downhill to the east, back across the Pacific. Since the water is no longer as warm in the west, the air above it does not get as warm and less air rises. Since less air rises, less air comes in from the west to replace the rising air. In other words, the winds become weaker. This allows more warm water to flow east, which weakens the winds, which allows more warm water to flow east, which weakens the winds, which allows you get the idea. As during normal conditions, there is a positive feedback loop in which the atmosphere and ocean keeping pushing one another to change more and more. 8. What causes winds? Winds occur when one location is warmer than another location. The warmer spot warms the air above it. This (A) the density of the air and thus causes it to (B). Cooler air from the cooler spot comes in to replace the rising air. Warm A: increases, decreases B: rise, sink If the air does not rise on the western side of the ocean, air will not move west to replace it. Northern Australia South & Indonesia America Land Warm Cold 9. Why to the winds near the Equator weaken or even reverse during El Niño conditions? The western side of the ocean is (C) during El Niño conditions and the eastern side of the ocean is (D) during El Niño conditions, so air on the western side of the ocean has a HIGHER density than normal (not as low) compared to the other side of the ocean. Therefore, (E) air rises on the western side of the ocean, and (E) air comes west to replace it. C: warmer, less warm D: cooler, less cool E: more, less Land

4 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 4 As the winds weaken, they can no longer push warm water away from the coast of South America, and the warm water of El Niño arrives to push down the cooler, upwelling water. (Why?) With no upwelling to fertilize the phytoplankton with nutrients, there is less food for animals, so many animals die. In addition, the warm water of El Niño warms the air and produces lots of evaporation. Thus, more moist air rises along the coast of South America, leading to more clouds and rain than usual. Once the warm water of El Niño hits the coast of South America, it flows north and south along the coast of the Americas, shutting down upwelling and increasing the amount of rainfall. 10. How and why does the amount of rainfall on each side of the ocean change during El Niño conditions? Water becomes warmer on the (A) side of the ocean. Warm water evaporates (B) easily than cold water, so there is (B) water vapor in the air available to become clouds & rain. Water becomes cooler (less warm) on the (A) side of the ocean, so there is (B) water vapor in the air available to become clouds & rain. A: eastern, western B: more, less 11. How and why is there less upwelling along the coast of the Americas during El Niño conditions? There is less upwelling along the coast of the Americas (on the eastern side of the ocean), because at the Equator, water is flowing (A), which is (B) the coast of the Americas. Upwelling occurs when winds and currents take water (B) a coast, and water rises up to replace the water that is taken away. A: east, west B: towards, away from

5 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 5 El Niño conditions mainly affect the Pacific Ocean at the Equator, changing water temperatures by up to 4 o C or so (that s 7 o F or more) in places. These changes then ripple outwards an affect other parts of the world. For example, the coast of California might typically change by 1 o C (2 o F) during El Niño conditions. This may not seem like much, but remember that ocean water s heat capacity is about 14 times greater than the heat capacity of the land. That means that the amount of heat needed to increase the temperature of ocean water by 2 o F would raise the temperature of the land by 28 o F! This is impressive, but it is not likely that all this additional heat will be transferred from the ocean to the land. As we have noted many times, the most important effect is the change in the amount of evaporation from the ocean and therefore the amount of water vapor that is available to become clouds and rain. During some El Niño years, warm water gets farther north than during others, so it not does always have the same effect on California. For example, during the 1976 El Niño, California had one of its worst droughts. The strongest and most reliable effects take place in the Pacific Ocean near the Equator. The changes that take place in the Pacific Ocean ripple outwards and can affect climate on the other side of the world. For example, El Niño shifts the position of the Jet Stream, a high-altitude air current ( wind ), with important implications for weather throughout the Northern Hemisphere. Scientists are not sure what causes normal conditions to shift to El Niño conditions. Most believe that a local change in the atmosphere kick-starts the feedback loop that eventually leads to full-blown El Niño conditions, because the atmosphere can change faster due to its lower density and heat capacity. Note: The gyres do not reverse during El Niño conditions. The trade winds weaken during El Niño conditions and sometimes do go backwards near the Equator, but the westerlies and momentum keep the gyres going in their normal direction. (This is a common misunderstanding, so I thought it was important to mention it.) 12. Does California always get more rain during El Niño conditions? 13. Do El Niño conditions only affect weather and climate near the Pacific Ocean, or do El Niño conditions change weather and climate in other parts of the world too? 14. Which are more likely to get El Niño conditions started, natural changes (fluctuations) in the atmosphere or ocean? 15. Does the California Current reverse and flow north during El Niño conditions?

6 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 6 Summary of Changes in the Ocean during typical El Niño conditions West Side of Pacific East Side of Pacific Answer the following questions about changes in the ocean during El Niño conditions. 16. Do the winds near the Equator get stronger or weaker? 17. a) Does the ocean current near the Equator begin to flow east or west? b) Sketch an arrow into the picture above showing the direction of the current. 18. Does the ocean current near the Equator begin to flow towards or away from the Americas? 19. Does the water on the eastern side of the ocean by the Americas get warmer or colder? 20. Does the water on the western side of the ocean by Asia get warmer or colder? 21. Is there more or less upwelling along the coast of the Americas? 22. Is there more or less life in the ocean near the coast of the Americas? 23. How does the climate of the west coast of the Americas change? In other words, does it get warmer or colder? Are there more clouds and rain, or less? 24. How does the climate of the east coast of the Asia change? In other words, does it get warmer or colder? Are there more clouds and rain, or less?

7 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 7 Effects of El Niño El-Niño-related changes in climate cost the state of California about $1 or $2 billion every time an El Niño occurs. By feeding data from the TAO array of ocean sensors (cost: about $10 million a year) into computer simulations of climate, we can now issue El Niño warnings with some degree of accuracy. Spending about $100 million on things like sandbags, clearing storm drains, giving appropriate training to disaster relief and reconstruction teams, and so on can cut the cost of an El Niño nearly in half and save lives. Clearly, this is a good investment: if there are two El Niño events each decade, we would spend about $300 million to prevent $1 billion or more in damage. The United States, Japan, and France maintain the array of ocean sensors. Climate vs. Weather In this class we focus on climate, not weather. Climate is the long-term average of weather conditions (what the weather is usually like). For example, Southern California has a warm, dry climate. This does not mean that it is always warm (we have our cooler days) or that it does not rain in Southern California; it means that our weather is warm most of the time and that rain is less common here than elsewhere. Another way to think about it: weather is what conditions are like a particular day, while climate is what conditions are typically like over a season or a year. El Niño is just one example of a number of large-scale climate phenomena that scientists are studying. La Niña conditions, for example, are the opposite of El Niño conditions. The trade winds become stronger and push more warm water to the west side of the Pacific. This leads to strong upwelling and abundant life on our side the ocean, but drought conditions on land. La Niña conditions tend to follow El Niño conditions, and then the atmosphere and ocean return to normal. Recently, scientists identified a variety of El Niño that is being called El Niño Modoki. (The Japanese word modoki means a similar but different thing. ). In these events, the warm water stops in the middle of the Pacific Ocean and the winds in the east Pacific get stronger, so more water is pushed away from the west coast of the United States. Thus, water along the west coast of the United States gets cooler than normal and the west coast gets less rain than normal. Other examples of El-Niño-like climate phenomena under investigation include the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), and and the Antarctic Circumpolar Wave (ACW). None are as well understood as El Niño, because they do not happen as often, so we have fewer well-studied examples. For example, the PDO flips from its positive phase to its negative phase once every 20 years or so, unlike El Niño conditions and normal conditions which alternate about once every 5 years or so. Expect to hear more about the PDO s effects on California within your lifetime. 25. Are El Niño conditions a change in climate or a change in weather?

8 T. James Noyes, El Camino College El Niño Unit (Topic 9B) page 8 This page was intentionally left blank.