El Niņo, La Niņa, and the Southern Oscillation
ERTH 303, Weather and Climate, Fall 2009: Assignment 2

*DUE 11 am, Tuesday November 17th - hand in only the answer sheet attached to the end of this document*

1. Objectives

  1. To learn the inter-relationships in the tropical Pacific between the ocean and the atmosphere.
  2. To learn the features associated with El Niņo, La Niņa, and more normal phases of ENSO.
  3. To learn what the Southern Oscillation (SO) is, and how it’s measured, and its association with El Niņo and La Niņa.
  4. To learn how swings in the SO impact weather in North America.
  5. To study the recent strong El Niņo and La Niņa events in the late 1990’s.

satellite image of sea surface temperatures with red being warmest SST (http://earthobservatory.nasa.gov/)

2. What is El Niņo?


Fig. 1: El Niņo is a disruption of the “normal” ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.

In normal, non-El Niņo conditions (Fig. 1, top panel), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that 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.

During El Niņo (Fig. 1, bottom panel), the trade winds relax in the central and western Pacific allowing warm water to move eastward. Rainfall follows the warm water as heat and moisture are added to the atmosphere, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

Use Figure 2 to answer the questions below

 2: Ocean surface temperatures and atmospheric surface winds in the tropics as measured primarily by an anchored-buoy array along the Equator. The mapped area extends ten degrees of latitude north and south of the Equator and from near the South American coast on the east to near Indonesia on the west. The top view, labeled “November 1997 Means” is the average sea surface temperatures (SST; shaded with isotherms at one-half degree intervals) and surface winds (as shown by arrows) for the month of November 1997, near the peak of the 1997-98 El Niņo episode. The bottom view is a depiction of SST and wind anomalies, that is, departures of the observed values shown in the top view from the long-term average. (Note: Positive anomaly isotherms are solid lines in intervals of one-half degree Celsius. Negative anomaly isotherms are dashed lines. A heavy line labeled 0 shows where no temperature anomaly exists, i.e. conditions are average.)

1. The sea surface temperatures (SSTs) across the region ranged from about 26 °C as the "coolest" in the southeast corner to above __________°C just south of the Equator, west of center in the upper panel.

a. 20

b. 25

c. 30

2. The warmest SSTs were located in the neighborhood of about __________ Longitude in the Pacific.

a. 170° W

b. 120° W

3. The wind directions in the eastern Pacific were generally from the southeast. However, in the west-central Pacific along the Equator (from about 160° E to 150° W) winds were from the west and generally __________. (Use November 1997 Means).

a. light

b. strong

4. SSTs in the eastern Pacific during an El Niņo event are __________ the "Long-Term Average" values. This is shown in the November 1997 Anomalies diagram above.

a. above

b. below

5. The SST anomalies in the eastern Pacific were __________.

a. positive

b. negative

6. The highest anomalies are more than __________ °C.

a. 1.5

b. 4.5

c. 7.5

3. What is La Niņa?

La Niņa is characterized by unusually cold ocean temperatures in the Equatorial Pacific (signified by blue and green shading in Fig. 3), compared to El Niņo, which is characterized by unusually warm ocean temperatures (signified by red and orange shading in Fig. 2) in the Equatorial Pacific.

Use Figure 3 to answer the questions below

Fig. 3: A similar depiction of the tropical Pacific sea surface temperatures (SST) and wind conditions for November 1998, one year later than Fig. 2, showing the La Niņa conditions that had replaced those of El Niņo.

7. In November 1998, the sea-surface temperatures along the Equator in the eastern Pacific were near 22 °C, several degrees __________ than those in the same area in November 1997 during the El Niņo.

a. warmer

b. cooler

8. The winds across the entire Pacific area of the depiction were now generally from the __________.

a. east

b. west

9. The lower panel of Figure 3 shows the Pacific SST anomalies along the Equator being almost all __________.

a. positive

b. negative

10. These anomalies are denoted by the dashed lines, with values going below __________°C. This relatively cool (compared to the Long-Term Average) water marks the identification of La Niņa.

a. -1

b. -2

Use Figure 4 to answer the questions below

 4: The upper panel of is the depiction of the five-day mean tropical Pacific SST and wind conditions ending on 6 June, 2005. The lower panel displays the anomalies of SST and wind conditions.

11. The shading and isotherms indicate that the coolest waters across the tropical Pacific are located in the __________ Pacific near and just south of the Equator.

a. western

b. central

c. eastern

12. In the lower view (anomalies), the SST anomalies are generally positive in (the) __________ tropical Pacific.

a. eastern

b. central

c. western

d. none of the

13. In the lower view (anomalies), the SST anomalies are __________ over most of the tropical Pacific.

a. negative

b. positive

c. neutral

The latest forecast for the tropical Pacific can be found at http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.html.

4. How Strong Where the Recent El Niņo and La Niņa Events of the Late 1990’s?

Navigate to the website http://instructional1.calstatela.edu/sladoch/web_page2/HowStrong_good.html to view an animation from the Topex-Poseidon satellite used to show changes in sea surface temperatures (SST) and in sea surface heights. [This link will also include the text and questions of this section – only use the link to look at the animation]. You will compare the sequence of events from 1996 through 1999 documenting one of the largest El Niņo events of the century followed by a strong La Niņa event.

14. What does the globe on the left show?

a. SST

b. SST anomalies

c. sea surface height

d. sea surface height anomalies

15. What does the globe on the right show?

a. SST

b. SST anomalies

c. sea surface height

d. sea surface height anomalies

16. About when did the El Niņo reach its maximum strength?

a. Mar. 1997

b. Oct. 1997

c. Feb. 1998

d. June 1998

17. At the time of the maximum El Niņo strength, what was the largest sea level height anomaly in the eastern Pacific?

a. 2 cm.

b. 5 cm.

c. 8 cm.

d. over 10 cm.

18. For the same time as in #’s 3 and 4, what was the greatest SST anomaly?

a. 0.5°C

b. 1.0°C

c. 1.0-2.0°C

d. over 3.0°C

19. In February, 1998, southern California experienced a very wet month. How strong was the El Niņo in terms of SST anomalies?

a. weak, but growing stronger

b. continues showing strength

c. strong, but growing stronger

d. weak, and growing weaker

20. What indicates the weakening of the El Niņo and a rapid change to La Niņa conditions in May and June of 1998?

a. sea level heights rise in the central equatorial Pacific

b. sea level heights fall in the central equatorial Pacific

c. SST’s are much cooler in the Alaskan Gulf

d. SST’s are warming in the eastern tropical Pacific

21. What is the sea surface height anomaly in the eastern Pacific in January, 1999?

a. 10 cm higher

b. 5 cm higher

c. 5 cm lower

d. 10 cm lower

22. What is the SST anomaly in the eastern Pacific in January, 1999?

a. 3° C cooler

b. 1° C cooler

c. 3° C warmer

d. 1° C warmer

23. The Scripps, UCSD, website, http://meteora.ucsd.edu/~pierce/elnino/en97/en97.html offers an interesting 3-dimensional look at the development of the 1997-98 El Niņo. From the monthly 3-D figures, about when did the actual El Niņo begin (when warm water at depth begins to move eastward)? If you have a problem opening this site go here and click on "a 3-D look at the El Nino of 1997/1998"

a. Jan. 97

b. Mar. 97

c. May 97

d. July 97

e. cannot tell

5. What is the Southern Oscillation?

El Niņo is normally accompanied by a change in atmospheric circulation called the Southern Oscillation. Together, the ENSO (El Niņo-Southern Oscillation) phenomenon is one of the main sources of interannual variability in weather and climate around the world. Since recognizing some 25 years ago that the oceanic and atmospheric parts of ENSO are strongly linked, scientists have moved steadily toward a deeper understanding of ENSO. Climate forecasters have taken the first steps toward predicting the onset of El Niņo and La Niņo events months in advance. Still, much remains to be learned.

The basics of ENSO

It was the atmospheric part of ENSO–the Southern Oscillation, or SO–that first attracted the attention of scientists. Sir Albert Walker documented and named the SO in the 1930s. Other persistent patterns of high and low pressure had been previously noted in the North Pacific and North Atlantic; thus, the "southern" in SO.

The clearest sign of the SO is the inverse relationship between surface air pressure at two sites: Darwin, Australia, and the South Pacific island of Tahiti. High pressure at one site is almost always concurrent with low pressure at the other, and vice versa. The pattern reverses every few years. It represents a standing wave or "see-saw"-a mass of air oscillating back and forth across the International Date Line in the tropics and subtropics. When Tahiti pressure is high, that indicates winds blowing towards the west (normal trade winds), and when it is low winds blow to the east (El Niņo).

The persistent easterly trade winds are a key ingredient in the ENSO process. They have two major effects:

The most widely used scale for ENSO is known as the Southern Oscillation Index (SOI), which is based on the surface (atmospheric) pressure difference between Tahiti and Darwin, Australia. We use the Southern Oscillation Index for many reasons, although it is an indirect measure of El Niņo, and these locations are not ideally sited for this purpose. The main reason being that the time series at Darwin and Tahiti are more than 100 years long, and there is no other record that would allow us to categorize the El Niņo cycle that far back.

The SOI is given in normalized units of standard deviation. It can be used as an intensity scale. For example, SOI values for the 1982-83 El Niņo were about 3.5 standard deviations, so by this measure that event was roughly twice as strong as the 1991-92 El Niņo, which measured only about 1.75 in SOI units. However, the sea surface temperature anomaly in November 1997 was about as large as in 1982-83, and some might say that is a more important measure. This shows that there is no single number that summarizes the intensity of events.

Use Figure 5 to answer the questions below.

Fig. 5: The Southern Oscillation Index (SOI) from 1950 through 1999 (source:Wm. Kessler, PMEL, NOAA).

24. When the pressure in Tahiti is higher and the pressure in Darwin is lower, the SOI would be__________.

a. positive

b. negative

c. neutral

25. When was the greatest negative (El Niņo) SOI?

a. 1976-1977

b. 1982-1983

c. 1991-1993

d. 1997-1998

26. In the 1990s, the most frequent SOI values were __________.

a. positive (La Niņa)

b. negative (El Niņo)

c. neither one dominated

6. Impacts of ENSO

The El Niņo and La Niņa, especially strong ones, impact the weather and climate around the world. Although their greatest impacts are in the tropics, we can see definite patterns of change over North America as well (Fig. 6).


Fig. 6: The seasonal impacts of El Niņo and La Niņa events on regions of N. America (Source: FSU, COAPS, http://www.coaps.fsu.edu/lib/booklet)

Use Figure 6 to answer the questions below

27. During the warm event (El Niņo), in which season does southern California have wetter than normal weather?

a. fall

b. winter

c. spring

d. summer

28. During winter, which phase of ENSO brings dry weather (shown in yellow and orange) to southern California?

a. warm event

b. cold event

c. both events

29. Which phase of ENSO brings a cold spring to the Great Lakes?

a. warm event

b. cold event

c. both events

30. Which phase of ENSO brings a wetter year to the Northwest (Oregon, Washington)?

a. warm event

b. cold event

c. both events

31. In general, the Gulf States experience _____ weather during El Niņo’s.

a. wetter than normal

b. drier than normal

c. normal

California’s weather is thought to be quite mild and sunny. However, during the warm and cold phases of ENSO, there can be large changes (Fig.7).



Fig. 7: Precipitation anomalies in the continental U.S. during strong El Niņo and La Niņa events (Source: Jan Null, Golden Gate Weather, http://ggweather.com/enso/calenso.htmg)


32. Figure 7 represents the precipitation year (July 1-June 30) during strong El Niņo and strong La Niņa events. How does southern California’s precipitation compare with northern California’s during strong El Niņos?

a. both are wetter than normal

b. both are drier than normal

c. south is wetter, north is drier

d. south is drier, north is wetter

33. How does southern California’s precipitation compare with northern California’s during strong La Niņas?

a. both are wetter than normal

b. both are drier than normal

c. south is wetter, north is drier

d. south is drier, north is wetter

7. Review Questions

34. During the El Niņo phase of ENSO the __________ equatorial Pacific is warmer than normal.

a. western

b. central and eastern

c. central and western

d. northern

35. During the 1997-1998 El Niņo event, ENSO Index was __________.

a. positive

b. negative

c. neutral

d. warm

36. During the La Niņa phase of ENSO, Southern California weather is __________.

a. unusually dry

b. unusually wet

c. near normal

d. extremely stormy

37. After doing this exercise I feel I know more about air-sea interactions.

a. Strongly agree

b. Somewhat agree

c. Somewhat disagree

d. Strongly disagree

38. What did you like most about the activity?

39. What did you like least about the activity?

8. Sources for Additional Information

The following websites are good sources for information on the impacts of ENSO on:

Global Weather



North American Weather



California Rainfall










Fisheries, Biology










Measuring ENSO From Space


JPL Topex-Poseidon


JPL Jason-1


ENSO Newsletter


Media (El Nonsense)


9. Acknowledgements

This assignment is was developed by Steve LaDochy and Pedro Ramirez, Californa State University, Los Angeles, and was provided by the Science Education Resource Center at Carleton College (http://serc.carleton.edu/quantskills/workshop05activities/ladochy.html). It was very slighly modified by Philip Higuera, Montana State University (10/2008) and Dave McWethy (10/2009).

The El Niņo-Southern Oscillation activity receivedsupport from California State University, Los Angeles Innovative Instruction Awards. While animations, illustrations and some text are from Internet websites, the authors wish to express thanks to JPL, NASA, and Project Atmosphere, American Meteorological Society, for providing guidance on questions, and Richard Medina, geography student assistant for web design. Any errors are the responsibility of the authors.

10. Answer Sheet

Place your answers in the spaces provided below and hand in this sheet only.

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