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

2. Introduction Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (2 of 14) 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 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. Terminology Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (3 of 14) 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

4. Normal Conditions Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (4 of 14) 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. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (5 of 14) 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, - a thermocline that is depressed in the east, - an intensified eastward North Equatorial countercurrent, - a weakened westward South Equatorial Current that is replaced by an eastward equatorial jet in the west, - very weak equatorial upwelling

6. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (6 of 14) La Nina Conditions 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.

7. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (7 of 14) 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. L H L H

8. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (8 of 14) 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. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (9 of 14) 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. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (10 of 14) Impacts-El Nino In the Tropics, El Niño episodes are associated with increased rainfall across the east-central and eastern Pacific and with drier than normal conditions over northern Australia, Indonesia and the Philippines. Elsewhere, wetter than normal conditions tend to be observed 1) during December-February (DJF) along coastal Ecuador, northwestern Peru, southern Brazil, central Argentina, and equatorial eastern Africa, and 2) during June-August (JJA) in the intermountain regions of the United States and over central Chile. Drier than normal conditions generally observed over northern South America, Central America and southern Africa during DJF, and over eastern Australia during JJA. El Niño episodes also contribute to large-scale temperature departures throughout the world, with most of the affected regions experiencing abnormally warm conditions during December-February. Some of the most prominent temperature departures include: 1) warmer than normal conditions during December-February across southeastern Asia, southeastern Africa, Japan, southern Alaska and western/central Canada, southeastern Brazil and southeastern Australia; 2) warmer than normal conditions during June-August along the west coast of South America and across southeastern Brazil; and 3) cooler than normal conditions during December-February along the Gulf coast of the United States.

11. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (11 of 14) Impacts-La Nina During La Niña episodes rainfall is enhanced across the western equatorial Pacific, Indonesia and the Philippines and is nearly absent across the eastern equatorial Pacific. Elsewhere, wetter than normal conditions tend to be observed during December- February (DJF) over northern South America and southern Africa, and during June-August (JJA) over southeastern Australia. Drier than normal conditions are generally observed along coastal Ecuador, northwestern Peru and equatorial eastern Africa during DJF, and over southern Brazil and central Argentina during JJA. La Niña episodes also contribute to large-scale temperature departures throughout the world, with most of the affected regions experiencing abnormally cool conditions. Some of the most prominent temperature departures include: 1) below-normal temperatures during December-February over southeastern Africa, Japan, southern Alaska and western/central Canada, and southeastern Brazil; 2) cooler than normal conditions during June- August across India and southeastern Asia, along the west coast of South America, across the Gulf of Guinea region, and across northern South America and portions of central America; and 3) warmer than normal conditions during December-February along the Gulf coast of the United States.

12. The impacts of El Niño upon climate in temperate latitudes show up most clearly during wintertime. 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. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (12 of 14) El Nino Impacts - USA

13. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (13 of 14) 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. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 20: ENSO Mar-07-07 (14 of 14) ENSO On The Web Current state of ENSOENSO 50 years of ENSOENSO NOAA ENSO theme PagePage More about ENSOENSO