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? Teacher’s Note: Shallow waves and deepwater waves. The combined pull of the sun and the moon on the earth is one cause. Spring tides occur when the sun, moon and earth form a straight line and neap tides occur when the sun, earth and moon form a right angle.

Interactions between the air and the sea are important The El Niño Southern 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 Teacher’s Note: You may want to indicate that La Niña is characterized by unusually cold water temperatures in the equatorial Pacific. Photo: NOAA, http://www.esrl.noaa.gov/gsd/outreach/education/samii/SAMII_Activity5.html, Accessed April 2011

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 Teacher’s Note: *Seawater has a high heat capacity meaning it takes a lot of heat to change ocean temperatures *This also means seawater temperatures don’t change much over time in response to large gains or losses of heat unlike land temperatures that can vary over 60 ºF in a single day *As a result, the ocean may thereby help buffer global temperatures from daily and seasonal atmospheric changes

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 Photo: http://www.pmel.noaa.gov/tao/elnino/nino_normal.html

How does an El Niño occur? Under El Niño conditions….. Ocean heat alters the jet stream, causing rains to occur beyond their normal location As a result of weakening trade winds, warm water (red) moves eastward along the Equator, toward the Americas (white arrows) Trade winds weaken Teacher’s Note: As a simple demonstration of how water temperature affects air temperature (and thus weather), have your students place a hand over a cup of hot water. They should note that their hand gets moist and warm from the heat transfer between the water and the air above it. Likewise, heat rising off the ocean transfers to the air, causing convection patterns and rainfall. Photo: http://www.pmel.noaa.gov/tao/elnino/nino_normal.html; Accessed: November 2010 Teacher’s Note: The following analogy may be useful for your students. Imagine a bathtub, with a fan near the faucet end. In a normal year the fan is blowing full strength (symbolizing the trade winds), pushing the water back toward the end of the tub, with cold water at the bottom filling in the water at the surface blown toward the back. In an El Niño year, the fan slows down greatly (as the trade winds do). This means less water is blown toward the back of the tub and less cold, nutrient rich water on the bottom comes to the surface. In La Niña the exact opposite happens. The fan in this example blows stronger than normal, causing more cold, nutrient rich water to come to the surface. Upwelling along the coast of the Americas also decreases slowing the flow of cool, nutrient rich water to the surface ( Photo: NOAA / OAR / PMEL

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) Teacher’s Note: Students should be familiar with La Niña for the Bowl. For brevity, the PowerPoint does not go into the definition of La Niña except to make mention of it here, but the student handout provides some additional information. As an example for the last bullet: Under normal conditions, the nutrient rich bottom water, brought to the surface through upwelling fuels the base of the food chain creating food for fish like anchovies, which in turn become for food for larger species. During El Niño when upwelling decreases, so do the nutrients, and ultimately anchovy and other marine species that depend upon the nutrients.

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 The red color means SST is warmer, blue is cooler South America Australia During an El Nino year, SST is warmer than normal South America Australia Photo: http://www.pmel.noaa.gov/tao/elnino/nino_profiles.html; Accessed: November 2010 Teacher’s Note: This slide will come together as you advance the slide show presentation. Photo:PMEL/NOAA

Which graph shows a normal year and which shows an El Niño year? The color code corresponds with temperature: orange and red are relatively Warm (27-31oC) and the blues are relatively cool (20-23 oC) A. Teacher’s note: On the graphs above, the left-hand vertical axes show latitude and the horizontal axes show longitude. The right-hand vertical axes show temperature anomaly where cool colors correspond with cooler sea surface temperature (SST). Photo: NOAA/PMEL/TAO, http://www.pmel.noaa.gov/tao/elnino/images/lat_lon_9712_sst_lf.gif ; Accessed: November 2010 Australia South America B.

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

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 Teacher’s Note: You may want to contrast El Niño to bullets 3 and 4. El Niño is characterized by unusually warm ocean temperatures and decreases in the trade winds unlike La Niña that is characterized by unusually cold ocean temperatures and increases in the trade winds.

Decide whether each graph represents normal, El Niño or La Niña conditions A. December 1998 La Niña: relatively cool SST B. December 1993 Teacher’s Note: Each graph shows real SST data in the equatorial Pacific Ocean between Australia and South America. Ask your students to use what they know about typical SSTs during normal, El Niño and La Niña conditions to decide which each graph represents. Answers are given when you advance the slide in presentation mode. Photo: NOAA, http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html; Accessed: April 2011 Normal C. December 1997 El Niño: relatively warm SST

Class activity Test your skills at predicting El Niño!