El Niño / Southern Oscillation

Size: px

Start display at page:

Download "El Niño / Southern Oscillation"

Shauna White
6 years ago
Views:

1 El Niño / Southern Oscillation Student Packet 2 Use contents of this packet as you feel appropriate. You are free to copy and use any of the material in this lesson plan. Packet Contents Introduction on Climate Variability Concentrating on El Niño / Southern Oscillation (ENSO) can be copied to hand out to students ENSO Worksheet -What is ENSO? ENSO Worksheet - ENSO Phases Defined ENSO Worksheet - Historical Perspective ENSO Worksheet - Other Phenomena Affecting Climate Variability Approximate Locations of the Niño Regions in the Pacific Ocean to be used to make a transparency

2 Introduction on Climate Variability Concentrating on El Niño / Southern Oscillation (ENSO) What is ENSO? El Niño / Southern Oscillation (ENSO) refers to the ocean-atmosphere system in the tropical Pacific Ocean. Changes in this system have important consequences for seasonal climate variability worldwide. ENSO as a physical occurrence is a proven fact; as real as snowstorms and thunderstorms. Scientists, however, currently do not completely understand the way ENSO works. An important fact of weather and climate is its variability. Although definitions vary, one useful definition is climate refers to weather conditions occurring over a period of two weeks or longer. Weather is what is experienced over a time period of less than two weeks. Different time periods are used to define variability. Time periods range from daily weather differences, to seasonal differences, to multi-year or longer time periods. An example of a longer timeperiod variability would be two winters in a row with warmer than normal temperatures. Another example would be a colder than normal winter followed by a winter of normal temperatures followed by a winter of warmer than normal temperatures. Such year-to-year variations in the climate patterns are associated with changes in the wind, air pressure, storm tracks, and jet streams that encompass areas larger than that of the region you are interested in. Year-to-year changes in climate variability are linked to the ENSO phenomena. Knowledge of ENSO conditions provides the basis for most current forecasts of climate conditions more than one month into the future. Unfortunately, terms used to describe the changes that occur in the ocean-atmosphere system in the Pacific Ocean basin are not standardized. However, ENSO is becoming the term that scientists are using to describe both changes in Pacific sea surface temperatures and air pressures differences relative to a long-term average. The acronym, ENSO, is composed of El Niño- Southern Oscillation, where El Niño is the oceanic component and the Southern Oscillation is the atmospheric component of the phenomenon. El Niño is sometimes used interchangeably with ENSO because the term El Niño is often used in the popular media. In the Learning Modules, ENSO refers to general changes in the oceanatmosphere system, whereas El Niño will be used to refer to a specific change. ENSO Phases Defined In April 2005, twenty-six nations of the World Meteorological Organization s Regional Association IV agreed on a definition for ENSO phases. The definitions for the three phases, El Niño, La Niña, and Neutral are as follows. El Niño is characterized by positive (warmer) sea surface temperatures change from normal in the Niño 3.4 region (see definition below) greater than or equal to 0.5 degrees Celsius averaged over three consecutive months. La Niña is characterized by negative (cooler) sea surface temperatures change from normal in the Niño 3.4 region (see definition below) greater than or equal to 0.5 degrees Celsius averaged over three consecutive months. Here, negative

3 means temperatures are 1.5 degrees less than normal. Neutral is characterized by what are considered to be normal conditions. Normal conditions are defined as when the sea surface change in temperature in the Niño 3.4 region is between -0.5 and 0.5 degrees Celsius averaged over three consecutive months. In these definitions, sea surface temperatures in region Niño 3.4 are used to classify the season as an El Niño, La Niña, or neutral phase. As shown in the definition, it is not the absolute temperature in the region that is important, but rather changes in the temperature from normal or a base temperature. The current base period to define normal temperatures is the years Changes from normal are known as anomalies. In an El Niño or the warm phase, the anomalies are warmer than the base. Colder anomalies indicate a cold or La Niña phase. The location of the Niño 3.4 region is shown in the following figure. This region is in the tropical Pacific Ocean between Australia and South America. A world map with the approximate location of these regions is provided at the end of this Learning Module. Historical Perspective - Optional Sir Gilbert Walker, a mathematician, preformed the first rigorous scientific study of ENSO. In 1924, he observed and reported that the sea surface air pressure oscillates across the Pacific Ocean. When the sea surface air pressure rises in the eastern Pacific, it tends to fall in the western Pacific. Likewise, when the sea surface air pressure falls in the eastern Pacific it tends to rise in the western Pacific. Walker named this oscillation the Southern Oscillation. He identified relationships between the Southern Oscillation and precipitation patterns in Australia, Africa, the Indian subcontinent, and South America. Walker thought some oceanic mechanism was responsible for the oscillation. However, he could not establish any link with the ocean. Walker s work resulted in the Southern Oscillation being the first identifiable meteorological process that can be associated with global climate variability. Interestingly, initial reaction to Walker s work was highly skeptical, because his work had no theoretical justification.

4 Importantly, scientists never completely dismissed Walker s study because of the rigor of his analysis. Consequently, a few scientists continued to explore the possibility of a relationship between the sea surface air pressure in the equatorial Pacific and climatic variability. Jacob Bjerknes provided a key breakthrough by associating changes in sea surface temperatures to differences in the trade-wind circulation patterns in both the Northern and Southern Hemispheres of the Pacific. Bjerknes theoretical developments lead to the conclusion that the ocean s influence cannot be ignored in describing the Southern Oscillation. Acknowledging Walker s earlier work, Bjerknes named this airflow the Walker Circulation. This early work provides the basis for climate science today. In fact, recently one scientist, Michael H. Glantz of the National Center for Atmospheric Research, has gone as far as stating knowledge of ENSO is science s gift to the twenty-first century. Other Phenomena Affecting Climate Variability - Optional The ENSO phenomenon is the most highly recognizable physical phenomena affecting global seasonal climate. Other physical phenomenon and interactions between the phenomenons are, however, important. Of all the many phenomena affecting climate, four are briefly mentioned. One is similar to ENSO, but is found in the Atlantic Ocean, one has a major impact on a large population base, and the last two are longer cycles. Similarities have been established between the North Atlantic Oscillation and ENSO; however, the North Atlantic Oscillation is not as well understood. The North Atlantic Oscillation is a large seesaw pattern in atmospheric mass between a subtropical high and a polar low. Associations between the North Atlantic Oscillation and climate variability have been established for parts of Europe, northern Africa, southeastern United States, and northeastern Brazil. The Indian monsoon cycle refers to changes in seasonal winds in the Indian Ocean. During the summer winds blow from the Indian Ocean towards the Indian sub-continent; bringing with them moisture. This causes a wet season during the summer. During the winter, the winds reverse, blowing from land toward sea. Thus, the winds are dry and a dry season occurs. Gaining a fuller understanding of this process is important because such a large population (over one billion people) is dependent on the Indian monsoon cycle for food production. The Pacific Decadal Oscillation is associated with sea surface temperatures in the northeastern and tropical Pacific Ocean basin. This oscillation has been described as a long-lived ENSO-like pattern of Pacific climate variability. The Pacific Decadal Oscillation is distinct from ENSO in that Pacific Decadal Oscillation phases are persistent with periods lasting between 20 and 30 years instead of approximately six to 18 months that ENSO lasts. Causes of the Pacific Decadal Oscillation are not known. The North American climate anomalies associated with Pacific Decadal Oscillation are similar to those associated with ENSO, but are usually not as extreme. The affect of ENSO events on climate in North America may be dependent on the Pacific Decadal Oscillation phase. A cycle of approximately 80 years in length, the Gleissberg Cycle, appears to influence climate by causing the atmosphere to cool for 40 years and then warm for 40

5 years. Such cycles will affect predictions of global warming. Currently, it appears we maybe entering a cold cycle whose minimum is expected around Further studies are necessary to fully understand the relationships the different phenomena and climate variability. This creates interesting career paths for students. For additional information North Atlantic Oscillation Indian Monsoon ian_monsoon.html Pacific Decadal Oscillation Gleissberg Cycle idt-1.html

6 1. What does the acronym ENSO stand for? ENSO Worksheet -What is ENSO? 2. What is the definition of ENSO? 3. Do scientists completely understand the way ENSO works? Yes No 4. What is the difference between climate and weather? 5. Year-to-year variations in climate patterns are associated with climate changes between years. List 2 examples of year-to-year climate variability. 6. What phenomena is the basis for most current forecasts of climate conditions for more than one month into the future? 7. The acronym ENSO is not always used by everyone. What other term / acronym is often used in the popular media to describe changes in Pacific sea temperature and air pressure differences? 8. Why might knowledge of ENSO be important to Joe Soccer?

7 ENSO Worksheet - ENSO Phases Defined 1. Indicate on the following map the location of the three Niño regions, Niño 3, Niño 3.4 and Niño 4 in the Pacific Ocean. 2. Label the equator on the above map. 3. What Niño region is used to classify the different ENSO phases? 4. List the three common phases of ENSO. What type of sea temperature anomalies in the Niño Region 3.4 are observed with each phase? 5. What are anomalies?

8 ENSO Worksheet - Historical Perspective 1. Why are not all scientific discoveries accepted by scientists and the general public when they are discovered? 2. What mechanism was the first identified as affecting global climate variability? Who discovered this mechanism? 3. Why were Sir Gilbert Walker s studies not completely dismissed at the time they were published? ENSO Worksheet - Other Phenomena Affecting Climate Variability 1. Why is improved understanding of the Indian Monsoon so important to society? 2. Which cycle may be important in help explaining global warming? 3. Name a phenomenon that is similar to ENSO that is found in the Atlantic Ocean. 4. Name two ways the Pacific Decadal Oscillation varies from ENSO. 5. Why are there important and exciting career opportunities in climate science?

9 Approximate Locations of the Niño Regions in the Pacific Ocean Niño 3.4 Equator Niño 4 Niño 3

Name Date Class. growth rings of trees, fossilized pollen, and ocean. in the northern hemisphere.

Name Date Class. growth rings of trees, fossilized pollen, and ocean. in the northern hemisphere. Lesson Outline LESSON 2 A. Long-Term Cycles 1. A(n) climate cycle takes much longer than a lifetime to complete. a. To learn about long-term climate cycles, scientists study natural records, such as growth