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Myneni Lecture 20: ENSO Mar-07-07 (1 of 14). Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University. Further Reading: Chapter 07 of the text book. Outline. - terminology. - characteristics of ENSO. - impacts. Myneni Lecture 20: ENSO Mar-07-07 (2 of 14).

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  1. Myneni Lecture 20: ENSO Mar-07-07 (1 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Further Reading: Chapter 07 of the text book Outline - terminology - characteristics of ENSO - impacts

  2. Myneni Lecture 20: ENSO Mar-07-07 (2 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Today: We want to look at another source of variability in the atmosphere and ocean circulations, namely ENSO (El Nino Southern Oscillation) ENSO represents one of the best examples of ocean/atmosphere feedbacks in the climate system Also has impacts outside of the tropical Pacific due to teleconnections (linkage between changes in atmospheric circulation occurring in widely separated parts of the globe) within the general circulation of the atmosphere Introduction • Key Properties: • Pseudo cyclic: occurs every 2-7 years • Varies in intensity: 1997-98 Very strong; 1992 Weak • Largest effects occur during hemispheric winter

  3. Myneni Lecture 20: ENSO Mar-07-07 (3 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University El Nino Warming of the sea surface temperatures in the eastern tropical Pacific La Nina Cooling of sea surface temperatures in the eastern tropical Pacific Southern Oscillation Changes in surface winds/atmospheric circulations over the equator associated with El Nino ENSO (El Nino/Southern Oscillation) The coupled response between ocean and atmosphere circulations over the tropical Pacific Terminology

  4. Myneni Lecture 20: ENSO Mar-07-07 (4 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Normal Conditions Exceptionally intense trade winds come together in the Intertropical Convergence Zone (ITCZ), and the huge convective zone is to the west of the date line, where sea surface temperatures exceed 28C. The eastward surface current between 3N and 10N, which is known as the North Equatorial Countercurrent because it flows counter to the prevailing winds, is relatively weak. The westward South equatorial current is extremely strong, especially near the equator, where divergent motions cause intense upwelling and hence low sea surface temperatures. The thermocline, the layer of large vertical temperature gradients that separates the warm surface waters from the cold waters at depth, slopes steeply to the west, where its depth is approximately 150 meters

  5. Myneni Lecture 20: ENSO Mar-07-07 (5 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University El Nino Conditions The trade winds have collapsed, to be replaced in the west by westerly winds. The eastward movement of the convective zone is associated with – - an eastward expansion of warm surface waters, - athermocline that is depressed in the east, - an intensified eastward North Equatorial countercurrent, - a weakenedwestward South Equatorial Current that is replaced by an eastward equatorial jet in the west, - very weak equatorial upwelling

  6. Myneni Lecture 20: ENSO Mar-07-07 (6 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University • We find that the waters over the eastern Pacific cool down • This increases the pressure gradient from east to west • Hence the trade winds intensify • This pushes more warm water to the west • The western Pacific warms up • Convection intensifies • Hence the trades intensify even more, resulting in more upwelling in the eastern Pacific and a further cooling of the temperatures. La Nina Conditions

  7. Myneni Lecture 20: ENSO Mar-07-07 (7 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Southern Oscillation Index (SOI) The Southern Oscillation Index(SOI) is defined as the difference in surface level pressure Between Tahiti (in central Pacific) and Darwin (Australia). During an El Nino there is high pressure at Darwin and low pressure area near Tahiti, in the mid-equatorial Pacific. The SOI is therefore negative. Likewise, during the La Nina events, the SOI is positive. H L L H

  8. Myneni Lecture 20: ENSO Mar-07-07 (8 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Recent ENSO Events • More warm events since the mid 70s • 97-98: strongest event on record • 82-83: second strong event • 90-95: series of successive warm events • 88-89: strong cold event – severe drought in the US • Warm event strength, persistence, periodicity changed – is • this due to global change?

  9. Myneni Lecture 20: ENSO Mar-07-07 (9 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Movement of Tropical Convection • As the water moves east, the region of convection moves east • In addition, the rainfall associated with the convection moves east -> find droughts over the western Pacific • At the same time, rainfall increases over the eastern Pacific -> floods in Peru

  10. Myneni Lecture 20: ENSO Mar-07-07 (10 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Impacts-El Nino

  11. Myneni Lecture 20: ENSO Mar-07-07 (11 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Impacts-La Nina

  12. Myneni Lecture 20: ENSO Mar-07-07 (12 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University El Nino Impacts - USA The impacts of El Niño upon climate in temperate latitudes show up most clearly duringwintertime. For example, most El Niño winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida. El Niño affects temperate climates in other seasons as well. But even during wintertime, El Niño is only one of a number of factors that influence temperate climates. El Niño years, therefore, are not always marked by "typical" El Niño conditions the way they are in parts of the tropics.

  13. Myneni Lecture 20: ENSO Mar-07-07 (13 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University ENSO Impacts - Highlights • ENSO impacts are the effects (positive and negative) of El Niño- and La Niña-related • climate conditions on society and the environment. • Perhaps the most famous impacts of El Niño events are: • The damages from floods and landslides caused by very high rainfall in Peru and southern • California • The forest fires in Indonesia that have caused serious air pollution problems • The crop failures and sometimes famine from droughts in southern Africa and Australia • The collapse of the Peruvian anchovetta fisheries because of warmer coastal waters. • But ENSO events (in particular to the strong El Niño events of 1982–83 and 1997–98) have • been connected to impacts on almost every aspect of human life: • disease outbreaks, low and high agricultural yields, natural disasters, availability of water • resources, energy demand, disruption to hydropower generation, price fluctuations, fishery • catch fluctuations, animal movements, forest fires, the economic well-being of nations …

  14. Myneni Lecture 20: ENSO Mar-07-07 (14 of 14) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University ENSO On The Web Current state of ENSO 50 years of ENSO NOAA ENSO theme Page More about ENSO

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