Quarterdeck, Volume 6, Number 2, Summer 1998
The Atlantic Ocean
Unusual tropical Atlantic Ocean temperatures
cause climate swings in Brazil and West Africa
Dr. Ping Chang
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.
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.
How and why does a positive air-sea feedback take place in the tropical Atlantic? Find out!
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.
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Last updated August 1, 1998