 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.
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Some of the variation in the ocean's
temperature is not directly
related to the sun's influence. |
El Niño
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.
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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?
View a timeline
of El Niño and La Niña years. |
La Niña
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
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
unusual weather.
What's the difference
between climate
and weather? Find out! |
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 ... |
