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Lesson 11: El Niño Southern Oscillation (ENSO) Physical Oceanography

Lesson 11: El Niño Southern Oscillation (ENSO) Physical Oceanography. We’ve learned a lot about physical forces in the oceans. What are two primary types of waves? What is one major cause of tides? What is the difference between a spring tide and a neap tide ?.

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Lesson 11: El Niño Southern Oscillation (ENSO) Physical Oceanography

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  1. Lesson 11: El NiñoSouthern Oscillation (ENSO) Physical Oceanography

  2. We’ve learned a lot about physical forces in the oceans • What are two primary types of waves? • What is one major cause of tides? • What is the difference between a spring tide and a neap tide?

  3. Interactions between the air and the sea are important • The El NiñoSouthern Oscillation (ENSO) is a disruption of the ocean-atmosphere system in the Tropical Pacific, the ocean basin located between New Guinea, Australia and the Americas • A key characteristic of ENSO is increased surface ocean temperature in the equatorial Pacific • The ENSO greatly affects global weather • Scientists use oceanic and atmospheric data to predict ENSO events

  4. Oceans can affect the weather • Evaporation from the ocean can transfer heat and moisture to the atmosphere and thereby affect weather patterns • Currents can also transport heat from warm areas to cool areas and vice versa • Example: The Gulf Stream transfers heat from the tropics to Europe. Without it Europe would be much colder. • Upwelling, the vertical transport of cold, nutrient-rich water from the deep water to the surface can also cool nearby areas • Example: Temperatures in the Galapagos are slightly cooler than areas at similar latitudes due to upwelling in the region. • ENSO is an example of a periodic interaction between the ocean and atmosphere that can influence global weather and climate

  5. How does an El Niño occur? Under normal conditions….. Strong trade winds (white arrows) blow from the Americas toward New Guinea and Australia Heavy rains are concentrated in the Western Pacific Ocean These winds “push” water that has been warmed by the sun (red) toward the coast of New Guinea and Australia Cool, nutrient rich water (blue and green) upwells along the west coast of North and South America to replace the water being pushed toward New Guinea Photo: NOAA / OAR / PMEL

  6. How does an El Niño occur? Under El Niño conditions….. Ocean heat alters the jet stream, causing rains to occur beyond their normal location Trade winds weaken As a result of weakening trade winds, warm water (red) moves eastward along the Equator, toward the Americas (white arrows) Upwelling along the coast of the Americas also decreases slowing the flow of cool, nutrient rich water to the surface Photo: NOAA / OAR / PMEL

  7. Why do we care about El Niño? • During El Niño (and La Niña) years, more destructive weather events tend to occur: • Droughts and brush fires • Intense hurricanes • Intense tropical storms • Severe coastal flooding • Decline of some marine species (to figure out why, think about what happens during upwelling)

  8. During an El Nino year, SST is warmer than normal The red color means SST is warmer, blue is cooler South America South America Australia Australia How do you think scientists can predict El Niño events? • By looking for abnormal sea surface temperature (SST) – known as SST anomalies • By looking at the Southern Oscillation Index (SOI) • The SOI refers to an oscillation or “seesaw” in air pressure systems between the western and eastern Pacific that is linked to El Niño Photo:PMEL/NOAA

  9. Which graph shows a normal year and which shows an El Niñoyear? The color code corresponds with temperature: orange and red are relatively Warm (27-31oC) and the blues are relatively cool (20-23 oC) A. Australia South America B.

  10. Were you right? The color code corresponds with temperature: orange and red are relatively Warm (27-31oC) and the blues are relatively cool (20-23 oC) A. Normal Australia South America B. El Niño

  11. La Niña • La Niña causes mostly the opposite effects of El Niño • For example: In the Southeastern U.S., El Niño would typically cause cooler winter temperatures while La Niña would typically cause warmer winter temperatures • La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific • La Niña is thought to occur due to increases in the strength of the normal patterns of trade wind circulation • These trade winds increase upwelling off the coast of South America, bringing cool water to the surface • Impacts include increased rainfall in the Western tropical Pacific

  12. Decide whether each graph represents normal, El Niño or La Niña conditions A. December 1998 La Niña: relatively cool SST B. December 1993 Normal C. December 1997 El Niño: relatively warm SST

  13. Class activity • Test your skills at predicting El Niño!

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