Purpose: Comparing the Effects of El Nino & La Nina on the Midwest (E4.2c) To compare the effects of El Nino and La Nina on the Midwest United States. Background Knowledge: The El Nino-Southern Oscillation (ENSO) is a global coupled ocean-atmosphere phenomenon. El Nino and La Nina are important temperature fluctuations in surface waters of the tropical Pacific Ocean. ENSO is associated with floods, droughts and other disturbances in a range of locations around the world. ENSO is the most prominent known source of interannual variability in weather and climate around the world. ENSO conditions seem to have occurred at two to seven year intervals for the past three hundred years. The mechanisms which might cause an ENSO are still being investigated. As the phenomenon is located near the Equator, it may affect both hemispheres. The first signs of ENSO are: 1. A rise in air pressure over the Indian Ocean, Indonesia and Australia. 2. A fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean. 3. Trade Winds in the South Pacific weaken or head east. 4. Warm air rises near Peru, causing rain in the Peruvian deserts. 5. Warm water spreads from the West Pacific and the Indian Ocean to the East Pacific and it takes the rain with it, causing extensive drought in the Western Pacific and rainfall in the normally dry Eastern Pacific. El Nino results in a decrease in the earth's rotation rate, an increase in the length of day, and therefore a decrease in the strength of the Coriolis force. La Nina tends to have the opposite effect. El Nino is associated with a weakening of the tropical Pacific trade winds, and also with a strengthening of the midlatitude westerlies both at the surface and aloft. To balance these changes in atmospheric winds, the earth's rotation rate decreases in order to conserve total angular momentum of the earth/atmosphere system. Conservation of angular momentum is a basic physical principal which operates, for example, when a ballerina brings her arms closer to her body to spin faster. La Nina is the name of the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. La Nina causes the opposite effects of El Nino. For example, El Nino would cause a wet period in the midwestern United States, while La Nina would cause a dry period. In the winter of 2010/2011 Michigan experienced our 6 th snowiest winter ever, over 68 inches, due to a strong La Nina effect. The strong La Nina leads to a cooling of the central and eastern Pacific Ocean resulting in a colder pattern in the northern United States and warmer in the South. Materials: Monthly Precipitation & Temperature Impact Line Graphs 1 (1 page) Precipitation & Temperature United States Maps (2 pages) Procedure: 1) Analyze the graphs of average temperature and precipitation changes nationwide during El Nino & La Nina 2) Answer the conclusion questions that follow
Analysis Questions: 1) El Nino is generally cooler/warmer in temperature with higher/lower precipitation than average. 2) La Nina is generally cooler/warmer in temperature with higher/lower precipitation than average. 3) Are all El Nino and La Nina phases the same? Provide evidence to support your answer. 4) The El Nino data shown is only for strong El Nino years. What differences may we see between a strong El Nino year and a weaker El Nino year? 5) How do scientists detect El Nino patterns? 6) The 1997-98 El Nino was the strongest on record, and it developed more rapidly than any El Niño of the past 40 years. As a result, we started to see it impacts on weather, marine ecosystems and fisheries very quickly, and these impacts were spectacular. Early effects in August-October 1997 included record flooding in Chile, Marlin caught off the coast of Washington, the extensive smog cloud over Indonesia, and a quiet Atlantic hurricane season. Given what you ve learned about El Nino what do you think the temperature and precipitation data would look like during the 1997-98 El Nino? Explain your answer. 7) How does ENSO affect continental climate? Explain your answer.