Quarterdeck, Volume 6, Number 2, Summer 1998
The Pacific Ocean
El Niño, La Niña, and 'decadal variability'
cause long-term temperature changes in Pacific
Dr. Benjamin Giese
If you're interested in taking a swim in the Pacific Ocean, one of the first things you might do--after checking for sharks--is determine the water's temperature.
The surface temperature of the Pacific Ocean can vary from hour to hour, season to season, and decade to decade. Much of the short-term temperature variation comes from the direct influence of the sun. The water is warmer during the day than at night and warmer during the summer than during the winter for the same basic reason: The sun's rays are more intense during the day and in the summertime, and less intense during the evening and the wintertime.
But some of the variation in the ocean's temperature is not directly related to the sun's influence. Two examples of this long-term climate change are El Niño and decadal variability. (The term "decadal" is derived from "decade," a period of 10 years, but we use the term decadal variability to indicate changes in climate that occur over a period that is approximately 10 years long.)
Our research group at Texas A&M University is investigating the relationship between El Niño warming and decadal climate variability. We use computer models of the ocean to study the most recent El Niño, as well as El Niños of the past, in an attempt to better understand the types of climate change that we may expect in the future.
Some of the variation in the ocean's temperature is not directly related to the sun's influence.
Could anyone who opened a newspaper last winter avoid reading about El Niño? The 1997-98 El Niño was one of the most widely discussed climate events in recent history.
Long ago, the fishermen of Peru named the seasonal warming of the waters off the South American coast "El Niño"-Spanish for "the Christ child." During most of a normal year, the waters adjacent to the coast of South America are cold and highly productive, and fishing is bountiful. In December, a summer month in the Southern Hemisphere, normal warming of the water decreases the water's productivity-and the fishermen's catch. Peruvian fishermen associated the arrival of warm water with the arrival of Christmas ... and hence the name El Niño. Usually the cold waters return in June, and the fishing season begins again.
However, in some years the cold waters do not return, and the water stays warm throughout the year. We now use the term El Niño to mean these unusual years, and the 1997-98 El Niño was one of the biggest of the El Niño events in the last 400 years.
Ordinarily, easterly tradewinds blow from the coast of South America toward the western Pacific. The persistent winds blow warm surface water toward the western Pacific, making room for cold water from the deep ocean to rise to the surface in a process called upwelling.
During an El Niño year, the tradewinds weaken so that not as much cold water gets to the surface and the temperature there starts to rise. The rising water temperature causes the winds to weaken even more, which in turn causes the temperature to rise even more, resulting in an El Niño.
This means that the ocean is intimately connected to the atmosphere, and is one of the reasons why we feel the effect of El Niño here in Texas, even though the El Niño warming is thousands of miles away.
The El Niño warming changes the patterns of circulation in the atmosphere, which can in turn change the path of storms coming from the Pacific Ocean onto the United States.
During January and February of 1998, the peak of the 1997-98 El Niño, the storm track brought several storm systems further south than normal so that it rained in southern California, a region that is normally dry throughout the year.
In Texas, we often get a rainy winter during El Niño years, and the first part of the winter of 1997-98 was colder and wetter than normal.
But more important to Texans is not cold and wet periods,
but dry periods. Rainfall is crucial to many aspects of the state's economy,
especially agriculture. Dry periods in Texas are associated not with El
Niño, but with its opposite phase, La Niña.
The 1997-98 El Niño was one of the biggest in the last 400 years. This diagram shows an average of the unusually high sea-surface temperatures during El Niños.
Could anyone who opened a newspaper last winter avoid reading about El Niño?
Whereas an El Niño brings warmer than normal temperatures to the tropical Pacific, La Niña has colder than normal temperatures in the tropical Pacific.
La Niña conditions often follow an El Niño, with stronger upwelling of cold water during the months of June, July and August. Like El Niño, the unusual conditions can last for nine months to a year. La Niña can also distort the normal storm tracks across the United States, and can bring unusual weather to the region.
La Niña is the opposite of El Niño, and the impact on Texas climate is the opposite as well.
We know that El Niño can cause a wetter than normal winter in Texas, and La Niña can cause a drier (and warmer) summer in Texas.
It appears that this year is no exception; we have experienced a dry and warm summer, and yes, conditions in the tropical Pacific are colder than normal.
Decadal variability in climate is similar to El Niño variability, except that it is weaker and lasts for a much longer period of time. And like an El Niño, we experience the effects of decadal warming as changes in climate. Where an El Niño can bring intense storms and flooding for a period of several months and La Niña can cause several months of drought, decadal variability might be responsible for the decade-long periods of drought that we experience in the southwestern part of the United States.
As in the case of El Niño, decadal variability is prominent in both the ocean and in the atmosphere. Whereas the largest El Niño temperature anomalies are found in the tropics, large decadal temperature anomalies are found in the North Pacific, as shown above. We are not sure what causes decadal variations in surface temperature in the Pacific, but it is likely that positive air-sea feedbacks play an important role as they do in the Atlantic. (See page 12 for a diagram of a positive air-sea feedback.) This means that we can feel the effects of decadal warmings across the United States, even though the center of warming in the ocean is thousands of miles away.
These two forms of climate variability, El Niño and decadal warming, may be related to each other. As an example, the decade of the 1930s was extremely dry throughout the southwestern United States, suggesting a connection to decadal variability. Interestingly, it also was a period of few and very weak El Niños.
Finding a relationship
Our research seeks to answer questions about the connection between El Niño and decadal climate variability. The ultimate goal of this work is to better predict the occurrence of these climate phenomena, so that people can prepare for the adverse weather conditions that often arise during periods of climate change. It would be helpful for farmers to be able to predict how much rainfall will occur during the growing season, or for city planners to predict how much water they must reserve for the next year.
We can study El Niño conditions for any period since 1950, and so we can use the computer results to explore the possibility of a change in El Niño over the last 48 years.
For instance, the El Niños of 1982-83 and 1997-98 are two of the strongest on record. Should we expect more unusually strong El Niños? Or is it just a chance occurrence that these strong El Niños happened in close succession?
Our research group is continuing to explore how to best use satellite observations of the oceans to get the most accurate representation of the ocean possible.
We plan to expand our Pacific Ocean studies to the global oceans, in an effort to explore how climate varies globally. Eventually, we hope to be able to predict these climate changes well in advance.
Read about a flip-flopping climate in the tropical Atlantic Ocean.
La Niña conditions
often follow an El Niño
and can also bring
This large area of unusually warm water in the northern Pacific may exist for a decade and have a long-term influence on climate, including long periods of drought in the southwestern United States.
Like tree rings, coral layers (such as in the coral core, above) can reveal climate change. Find out more ...
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Last updated August 1, 1998