9. 1997 El Niño

(a.) Prior to 1997, the 1982-83 El Niño was the strongest and most devastating of the century, perhaps the worst in recorded history. During that period, trade winds not only collapsed, they reversed direction. Weather-related disasters occurred on almost every continent. Australia, Africa, and Indonesia suffered droughts, dust storms, and brush fires. Peru was hit with the heaviest rainfall in recorded history: over 3.0 m in areas where 10-20 cm (0.1-0.2 m) was the norm. California had very high rainfall and the year was characterized by extensive flooding and landsliding. The event was blamed for nearly 2,000 deaths and more than $13 billion in damage to property and livelihoods. During this period, the thermocline off the South American coast dropped dramatically. On September 24, in just 24 hours, sea-surface temperatures along a coastal village in Peru shot up about 4 degrees Celsius.

The diagram below shows sea-surface temperature anomalies (that is, differences compared to normal years) during the 1982-1983 El Niño.


How much warmer than normal were surface waters of the eastern equatorial Pacific during the 1982-83 El Niño event?

Which parts of the Pacific Ocean were colder than normal?


(b.) During Spring 1997, my oceanography class at San Francisco State University was keeping track of real-time TAO buoy data via the NOAA/PMEL web site. An advantage of this web site is that anyone can observe conditions as they develop in the tropical Pacific Ocean. We observed the beginnings of El Niño-type conditions long before announcements began to appear in the various news media.

The diagram below summarizes data from the equator during the two-year period from October 1995 to September 1997. The left-side diagram shows the sea-surface temperatures (SSTs) from west to east along the equator during this time period. The right-side diagram shows the temperature difference compared to normal years. The heavy dark line = 0.0 (no temperature difference).

During which month(s) do you think we first suspected that El Niño conditions were beginning to develop in the tropical Pacific?

(Note that SSTs on the eastern side of the Pacific were colder than normal prior to the onslaught of El Niño.)


(c.) The following diagrams show SSTs and winds along the equator in January 1997 and in September 1997. Remember that the top part shows actual temperatures and winds and that the bottom part shows differences compared to normal years.

Describe changes in the temperatures and winds that occurred within this nine-month period.


(d.) The following diagram shows a depth profile of ocean temperatures along the equator during September 1997 (a monthly average).

At which depths did the greatest temperature anomalies occur in the eastern Pacific?

How much warmer were water temperatures in the eastern Pacific compared to normal years?

Compare this diagram with a similar diagram (part 8 of this exercise) from November 1996.

Briefly describe the differences.


(e.) Other data from the tropical Pacific show equally dramatic changes. For example, instruments aboard the U.S./French satellite TOPEX-POSEIDON measure heights of the sea surface. Although the ocean surface appears flat to our eyes, there are subtle variations in height that are due to influences such as shape of the ocean floor, ocean temperatures, and wind stress. For example, when wind blows strongly to the west along the equator, it pushes warm surface water westward, raising the elevation of the surface water in that direction. When the wind weakens, and the warm surface water moves eastward, the height of the ocean decreases in the west and increases in the east.

The diagram below shows changes in the height of the Pacific Ocean surface between March and June of 1997. The colors show sea-surface height relative to normal ocean conditions during this time period. The white and red areas indicate unusual patterns of heat storage; in the white areas, the sea-surface is between 14 and 32 centimeters (6 to 13 inches) above normal; in the red areas, it's about 10 cm (4 in) above normal. The greed areas indicate normal conditions, while purple indicates heights at least 18 cm (7 in) below normal sea level.

Describe the changes in sea-surface height that occurred during this time period.

What are some of the expected manifestations of this mass of elevated, warm water in the eastern Pacific?


(f.) The diagrams below show observed temperature and precipitation anomalies in August of 1997.

Briefly describe the temperature and precipitation anomalies.

Do these anomalies make sense in light of what you have learned about the El Niño phenomenon?


(g.) Compare the temperature anomaly diagram from 1997 in part (f.) above with the temperature anomaly diagram from 1982-83 in part (a.). Does the magnitude of the anomaly seem similar or different?

The diagram below compares the 1997 El Niño (as of July 1997) with the 6 strongest historic events, including the 1982-83 event (purple color). The ENSO (El Niño Southern Oscillation) Index combines measurements such as sea-surface temperature and sea-surface pressure; the higher the number, the stronger the ENSO event.

This diagram shows comparable conditions between the 1997 event and the 1982-83 event. However, there is one large difference between 1997 and previous large ENSO events.

What is this difference?

NOTE: One aspect of the El Niño phenomenon to keep in mind is that timing during the year is very important. For example, in California precipitation is typically restricted to the fall through spring months, primarily in winter. If El Niño peaks in summer it will have much less impact on precipitation in California than if it peaks in winter.


(h.) Final Note. Many phenomena during 1997 were attributed to El Niño. However, El Niño refers specifically to warming of the sea surface in the eastern tropical Pacific. Other manifestations are often associated with El Niño, such as increased or decreased rainfall, although these manifestations can only be forecast as probabilities. For example, along the west coast of the United States, the southern part is very likely to experience higher than average rainfall during El Niño years, while the northern part is likely to experience lower than average rainfall. Places in between, such as central to northern California, have rainfall probabilities closer to 50/50. See meteorologist John Monteverdi's comments on the SFSU Department of Geosciences' home page for a more complete discussion of these issues.


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