 Although the El Niño phenomenon in the Pacific
has a profound impact on global weather patterns, not all anomalous patterns
are linked to El Niño. It has become increasingly clear that other
modes of climate variation besides El Niño can have significant influences
on regional climate. Among them is a variation of sea-surface temperature
(SST) seemingly unique to the tropical Atlantic Ocean.
During some years, the water to the north of the equator
in the Atlantic is anomalously, or abnormally, warmer than that to the south.
In other years, the opposite condition occurs, and warm water appears in
the southern tropical Atlantic. This pattern of sea-surface temperature
seems to alternate every 10 years or so, and has a dramatic impact on regional
climate in Africa and the Americas. The pattern is often called the Atlantic
sea-surface temperature "dipole" because the temperature anomaly
on one side of the equator tends to be the opposite of the temperature on
the other side (although the temperature change is not always simultaneous).
How can sea-surface temperatures in the tropical Atlantic
Ocean influence regional climate? Intense tropical rainfall is confined
to the Intertropical Convergence Zone (ITCZ), a band of strong air currents
rising through the atmosphere (convection). The ITCZ locates over the area
of the warmest sea-surface temperature just north of the equator. When sea-surface
temperature is warmer than normal in the northern tropical Atlantic (and/or
colder than normal in the southern tropical Atlantic), the sea-surface temperature
difference between the Northern and Southern Hemispheres (the SST gradient)
generates a north-south surface pressure gradient in the atmosphere.
This "pushes" the ITCZ farther north from
its normal location. As a result, the band of heavy precipitation migrates
northward and creates persistent drought conditions in some South American
countries, particularly in northeast Brazil. At the same time, it creates
and wetter-than-normal conditions in the Sahel region of Africa.
During the early 1980s, a persistent drought occurred
in Brazil's Nordeste region, while temperatures in the northern tropics
were unusually higher than temperatures in the southern tropics. The conditions
apparently reversed during the mid-1970s and mid-1980s when rainfall was
unusually heavy in the Nordeste region. Recent research also suggests that
variations in tropical Atlantic sea-surface temperature has an impact on
weather patterns in the southeast United States.
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It has become increasingly
clear that other modes of climate variation
besides El Niño can have significant influences on regional climate.
Compare four decades of rainfall anomaly
(rainfall above and below normal) in northeast Brazil with rainfall anomaly
in West Africa. |
What causes the long-term variations
in sea-surface temperature in the tropical Atlantic? For a long time scientists
have tried to understand the physics behind this phenomenon. Recent studies
suggest that the SST variability involves interactions between the tropical
atmosphere and ocean. Over the tropical Atlantic Ocean, between 30°
north and 30° south, the prevailing winds are usually from the northeast
in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
This wind system, called the tradewinds, is sensitive to changes in the
tropical SSTs.
An unusually warm sea-surface temperature in the northern
tropics, for example, will produce a southerly wind anomaly near the equator
because of the greater difference in surface air pressure between regions
to the north and south.
Owing to the effects of Earth's rotation, counterclockwise
circulation of winds will be set up in each hemisphere, which weakens the
northeast trade winds in the Northern Hemisphere and strengthens the southeasterly
trade winds in the Southern Hemisphere.
As a result, less heat is transported through evaporation
from the ocean surface to the atmosphere in the northern tropics. (Evaporation
and heat removal are largely controlled by the wind speed in the region;
weaker winds can remove less heat from the ocean's surface.) Therefore,
the already warm surface water in the northern tropics will warm even more,
which will allow an even weaker northeasterly trade wind, and so on. This
process is called a positive air-sea feedback because
it enhances the unusual climate conditions.
As the warm surface temperature builds up in the northern
tropics due to the positive feedback, ocean circulation gyres carry relatively
cold waters from the south Atlantic and from the northern midlatitudes into
the northern tropical Atlantic to counteract the warming effect. This process
is called a negative feedback.
The negative feedback eventually cancels out the warm
anomaly and leads to a reversal of the sea-surface temperature in northern
tropical Atlantic, bringing colder-than-normal surface temperatures. The
positive feedback then acts to enhance this reversed SST pattern. The interplay
between the positive and negative feedbacks can produce a decadal oscillation
in the coupled ocean-atmosphere system, which could provide an explanation
for the Atlantic SST "dipole" oscillation.
If the tropical Atlantic variability can indeed be described
as a self-sustaining oscillation, then there is a good possibility that
this variability can be predicted using a computer model that captures the
essential physics of the oscillation. The predictability follows simply
from the fact that a cycle is sequential, and therefore one part of the
cycle must follow another. The possibility of predicting low-frequency SST
variability in the tropical Atlantic region has long been a central concern
because of its significant influence on rainfall over northeast Brazil and
the Sahel region of Africa. Rainfall changes in both areas have an extraordinary
economic and societal impact.
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How and why does a positive air-sea feedback take place in the tropical
Atlantic? Find out!
Tropical economies rise and fall with the rain
Major swings in decadal climate have a significant impact on the economies
and societies of the Sahel region of Africa and northeastern Brazil.
Read more ... |
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A computer model of the
tropical Atlantic coupled system has been developed recently at Texas A&M.
This model simulates both the atmosphere and the ocean, allowing simulated,
or modeled, air-sea feedbacks to take place. We model atmospheric behavior
by statistically relating historical patterns of sea-surface temperature
to corresponding patterns of wind. The ocean model is based on first principles
of physics. It uses mathematical equations based on the laws of conservation
of mass and momentum.
The equations yield numbers to describe the behavior
and properties of water-velocity, temperature, salinity, and other amounts
of chemical tracers-at each point on a set of nested numerical grids. The
results are then further processed by a computer to display graphics.
Numerical simulations indicate that the coupled ocean-atmosphere
model is capable of capturing many important features of the decadal SST
variability in the tropical Atlantic. In particular, the model produces
a self-sustaining "dipole-like" SST decadal oscillation whose
pattern bears a close resemblance to the one we observe in nature.
The computer model is further used to predict changes
in the tropical Atlantic SST. The prediction consists of two steps. First,
we ask the computer model to generate a realistic starting point, called
the "initial conditions" of the modeled ocean and atmosphere.
The initial conditions depend on observed values for wind stress on the
ocean surface and observed changes in heat caused by the sun's energy and
other processes. Then the model simulates the passage of time, season-to-season
and year-to-year, and predicts how SST will change from the initial conditions.
The diagram above shows the predicted SST averaged over
a northern tropical region. The cold period in the early 1970s and the warm
period in the early 1980s, as well as the gradual warming in the late 1970s,
were reasonably well predicted one year in advance.
Further analysis indicates that forecasts with this
model improve on a persistence forecast, in which the SSTs are assumed to
persist, or remain the same, for a long time.
We hope that the initial success of this model will
promote further research in the area of decade-long climate cycles. It will
be interesting to see how much the tropical Atlantic SST predictability
can be translated into useful rainfall predictions seasonally and year-to-year
in northeast Brazil and the Sahel of Africa as well as in other regions
that surround the Atlantic basin.
Read about the ocean-temperature influences on climate in
the Pacific. |
How accurately can scientists predict sea-surface temperatures? Compare the simulated, predicted, and observed sea-surface
temperatures from 1970 to 1990 in this figure. |
