What is the nature of El Niño and how is it predicted?

1. CLIM 101: Weather, Climate and Global Society What is the nature of El Niño and how is it predicted? Emilia Jin Lecture11: Sep 30, 2008

2. Contents • What is El Niño and La Niña? • Basics of El Niño • Impact of El Niño • ENSO (El Niño and Southern Oscillation) • Teleconnection • Global and regional consequences • Prediction of El Niño • Benefits of El Niño Predicting • Defining • Monitoring • Predicting

3. What is El Niño? (a) 1998 JFM SST [oC] (b) JFM SST Climatology [oC] (a) Minus (b): 1998 JFM SST Anomaly [oC]

4. What is El Niño/La Niña? In late 1800s, Fishermen coin the name El Niño to refer to the periodic warm waters that appear off the coasts of Peru and Ecuador around Christmas. El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas. La Niña means The Little Girl. La Niña is sometimes called El Viejo, anti-El Niño, or simply "a cold event" or "a cold episode". El Niño is often called "a warm event". Normal El Niño Marine ecosystem along the Ecuador and Peru coasts

5. What is El Niño/La Niña? La Nina (‘88.12) Normal (‘90.12) El Nino (‘97.12)

6. Evolution of El Niño (1997-98 case)

7. Evolution of La Niña (1998-99 case)

8. Contents • What is El Niño and La Niña? • Basics of El Niño • Impact of El Niño • ENSO (El Niño and Southern Oscillation) • Teleconnection • Global and regional consequences • Prediction of El Niño • Benefits of El Niño Predicting • Defining • Monitoring • Predicting

9. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition) Wind effect 1: Net transport of trade winds in Pacific is nearly always from east to west. Hadely cell: The Intertropical Convergence Zone (ITCZ) is where the trade winds from the Northern and Southern Hemispheres converge into a narrow belt close to the equator, a result of the general Hadley circulation which dominates the tropics and subtropics. Walker cell: Westward at the surface and eastward in the upper troposphere  They cause a general westward motion of surface waters and warmest waters pile up at the western Pacific.

10. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition)

11. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition) Wind effect 2: Ekman Effect: Ekman upwelling due to wind divergence in the eastern tropical Pacific.

12. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition)

13. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition) Upwelling (Coastal Zone Color Scanner)

14. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition) • The sea surface is about 1/2 meter higher at Indonesia than at Ecuador. • The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. • This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.

15. Mean Tropical Pacific Ocean-Atmosphere Climatology (Normal Condition) How the ocean affects the winds?

16. El Niño and La Niña • Strengthening and weakening of the Hadley and Walker circulations play a crucial role in reinforcing El Niño/La Niña perturbations to the mean tropical Pacific ocean-atmosphere climatology • La Niña Stronger than average trade winds tend to push the warm surface layer of the ocean (upper few 100 meters) towards the western end, creating a thick warm layer. It has higher than average precipitation in Australia, India & Indonesia. • El Niño: Weaker trades relax pressure on surface ocean layer & it starts to move back across Pacific from west to east, raising SST in the eastern tropical water, including Peru, with the zone of heavy rains shifting out over the central Pacific islands.

17. Developing El Niño Images & text from NASA Goddard Space Flight Center

18. El Niño and La Niña Normal conditions :The warmest water is found in the western Pacific, as is the greatest rainfall. Winds near the ocean surface travel from east to west across the Pacific (these winds are called easterlies ). El Niño conditions: The easterlies weaken, warmer than average sea surface temperatures cover the central and eastern tropical Pacific, and the region of heaviest rainfall moves eastward as well. La Niña conditions:Could be thought of as an enhancement of normal conditions. During these events, the easterlies strengthen, colder than average ocean water extends westward to the central Pacific, and the warmer than average sea-surface temperatures in the western Pacific are accompanied by heavier than usual rainfall.  It oscillates with a periodicity of 2-7 years

19. El Niño and La Niña in the tropics La Niña El Niño

20. Contents • What is El Niño and La Niña? • Basics of El Niño • Impact of El Niño • ENSO (El Niño and Southern Oscillation) • Teleconnection • Global and regional consequences • Prediction of El Niño • Benefits of El Niño Predicting • Defining • Monitoring • Predicting

21. Southern Oscillation During an El Niño, sea level pressure tends to be lower in the eastern Pacific and higher in the western Pacific while the opposite tends to occur during a La  Niña. This see-saw in atmospheric pressure between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as simply the SO (Sir Gilbert Walter, 1928).

22. Center of Ocean-Land-Atmosphere studies El Nino/Southern Oscillation 3 month moving average NINO 3 Index = SSTA (5°N-5°S, 150°W-90°) SO Index =Standardized TAHITI SLPA - Standardized DARWIN SLPA NINO 3 Index SO Index  The El Niño-Southern Oscillation, or ENSO for short. Often the term ENSO Warm Phase is used to describe El Niño and ENSO Cold Phase to describe La Niña.

23. Global Impact • Dense tropical rainclouds distort the air flow aloft (5-10 miles above sea level) but on a horizontal scale of thousands of miles. • The waves in the air flow, in turn, determine the positions of the monsoons, and the storm tracks and belts of strong winds aloft (commonly referred to as jet streams) which separate warm and cold regions at the Earth's surface.

24. Rainfall Anomaly (a) 1997/8 DJF (b) DJF Rainfall Climatology • Minus (b): • 1997/8 DJF Rainfall Anomaly

25. Global Impact Teleconnection Teleconnections: physical relationships that result from the dynamics of atmospheric and oceanic waves.

26. Tropical Convection Tropical Convection Global Impact Northward Propagating Rossby-Wave Train (Trenberth, et al. 1998)

27. Global Impact PNA (Pacific/North America) Pattern 97-98 El Nino

28. Global Impact Jet Strem

29. Global Consequences of El Niño This schematic shows areas that have a consistent change in precipitation pattern linked to the first year of an El Niño event. The months in which the effect is seen are grouped by their first initials (e.g., OND is "October-November-December").

30. Global Consequences of El Niño

31. Global Consequences of El Niño/La Niña

32. Global Consequences of El Niño/La Niña Impacts of 1982/83 El Nino episode • One of the two largest amplitude El Nino of this century. • Most recent (1997-98) El Niño was comparably large as well. • Droughts in Australia, India, Southern Africa. • Floods in Peru, Ecuador, USA Gulf of Mexico states, & Colorado River basin. • Collapse of coastal fishery in Peru (largest average annual catch of marine fish in world).

33. Global Consequences of El Niño/La Niña Impacts of 1982/83 El Nino episode The widespread impact of the 1982-83 El Niño is evident in this portrayal of extreme temperature events from 1982 to 1984. Months are grouped by their first initials (for instance, JJA is "June-July-August"). SST denotes sea-surface temperature.

34. Global Consequences of El Niño/La Niña Damage caused by1982/83 El Nino episode

35. Global Consequences of El Niño/La Niña Impacts of 1997/98 El Nino episode

36. China: drought USA: Flood Peru: Flood Africa: drought Global Consequences of El Niño/La Niña Impacts of 1997/98 El Nino episode

37. Regional Consequences of El Niño (United States) • Hurricanes: Below normal number of tropical storms/hurricanes in the Atlantic, although this does not imply any limits on the strength or location of any given tropical system. • Monsoons: A drier-than-normal North American Monsoon, especially for Mexico, Arizona and New Mexico. • Drought: A drier-than-normal fall and winter in the U.S. Pacific Northwest. • Wintertime Storms: A wetter-than-normal winter in the Gulf Coast states from Louisiana to Florida, and in central and southern California if El Niño is strong (Gulf states cooler and wetter. California can be wetter or drier). • Warmer Temperatures: A warmer than normal late fall and winter in the northern Great Plains and upper Midwest (Warmer winters across the northern US ) • Pacific salmon and other fisheries disrupted

38. Regional Consequences of El Niño (United States) • In the El Niño winter, 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.

39. Contents • What is El Niño and La Niña? • Basics of El Niño • Impact of El Niño • ENSO (El Niño and Southern Oscillation) • Teleconnection • Global and regional consequences • Prediction of El Niño • Benefits of El Niño Predicting • Defining • Monitoring • Predicting

40. Benefits of El Niño Prediction Subtle changes in the interplay of wind and water in the tropical Pacific can affect local ecosystems and human lives in far flung regions of the globe. The Influence of ENSO on Climate Once developed, El Niño and La Niña events typically persist for about a year and so the shifted rainfall patterns associated with them typically persist for several seasons as well. This can have a significant impact on people living in areas of the tropical Pacific since the usual precipitation patterns can be greatly disrupted by either excessively wet or dry conditions. Even before we know when or how a particular El Niño or La Niña is going to evolve, we can say something about the regional and global effects it is likely to have due to teleconnections. Various flavors of ENSO on Climate In several parts of the tropics, and some areas outside of the tropics, these seasonal shifts are fairly consistent from one El Niño and La Niña event to the next. It is important to remember, however, that no two El Niño or La Niña events are identical and that the seasonal shifts in temperature and precipitation patterns associated with them can vary from one event to the next. Thus, when an El Niño or La Niña  develops, it does not guarantee that regions which are typically affected by them will be affected, only  that there is enhanced probability that this will be the case.

41. ENSO, Climate, and Society Simple Picture The More Realistic Picture

42. Benefits of El Niño Prediction Seasonal climate forecasts made possible The persistence of tropical sea surface temperature (and rainfall) patterns (such as those associated El Niño and La Niña) plays a fundamental role in making seasonal (3-month) climate forecasts possible. In the absence of El Niño and La Niña, seasonal climate forecasts are still possible because unusually warm or cold sea surface temperatures in other parts of the tropics can still occur. Other influences on seasonal climate While ENSO is the largest known source of year-to-year climate variability, there are other known causes of seasonal climate variability that have nothing to do with ENSO: Tropical Oceans (Atlantic Ocean, Indian Ocean), snow cover and soil wetness, etc.

43. Effects of SST Anomaly

44. Tropical Convection Tropical Convection Global Impact Northward Propagating Rossby-Wave Train (Trenberth, et al. 1998)

45. The “Charney” Diagram Observations Theory Modeling

46. El Niño prediction depends on observed data and numerical models. Reliable data on existing conditions and realistic numerical models that project this picture forward in time are at the crux of researchers continuing efforts, not only to understand El Nino, but also to predict when future events will arise and what their impacts will be.

47. ENSO Predictability and Prediction • ENSO phenomena: Walker (1924), Bjerknes (1969), Wyrtki (1975), Rasmussen and Wallace (1983) • ENSO theory: Philander, Yamagata and Pacanowski (1984); Schopf & Suarez (1988), Battisti & Hirst (1989) • ENSO simulation: MacCreary (1976), Busalacchi and O’Brien (1981), Philander and Seigel (1985) • ENSO prediction: Cane et al. (1986), Zebiak and Cane (1987)

48. ENSO Prediction • Encouraged by the progress of the past decade, scientists and governments in many countries are working together to design and build a global system for • observing the tropical oceans, • predicting El Niño and other irregular climate rhythms, • making routine climate predictions readily available to those who have need of them for planning purposes, much as weather forecasts are made available to the public today. • Scientists are now taking our understanding of El Niños a step further by incorporating the descriptions of these events into numerical prediction models (computer programs designed to represent, in terms of equations, processes that occur in nature). • The results thus far, though by no means perfect, give a better indication of the climatic conditions that will prevail during the next one or two seasons than simply assuming that rainfall and temperature will be "normal."

49. Operational ENSO Observing System Long term operational support for Pacific Ocean observations that are the foundation of skillful ENSO forecasts.

50. ENSO Observing Experiment A vast array of ships, aircraft, and buoys collected oceanographic and atmospheric data throughout the western tropical Pacific as part of the Tropical Ocean and Global Atmosphere Program's Coupled Ocean - Atmosphere Response Experiment (TOGA-COARE) from November 1992 through February 1993. Shown is one of the 70 TAO instrument buoys being deployed as part of TOGA. (NOAA image.)