Atmospheric and Oceanic Oscillations

The effects of oceans on developing weather patterns have long been known and considered in weather prediction. Atmospheric and oceanic patterns fluctuate over a one—to twenty-year course. These fluctuations, or oscillations, create major climate change, such as the well-known El NiñoSouthern Oscillation (ENSO) in the tropical Pacific, which affects global weather.

Air and Ocean Interaction

Currents in the water and the air push heated water toward cooler climates, where the heat is released, helping to regulate temperature on northern continents. As heat is released, currents in the ocean and atmosphere return the cooled water to warmer climates, and the process continues in an endless cycle. These exchanges are an important way of recycling energy through the ocean-atmosphere system, which tends to balance land temperatures and climate response. Ocean currents, which keep water in constant motion, are affected by Earth’s rotation, the sun’s energy, wind, and salinity (salt content) as well as the temperature of the ocean. Just as there are a series of air streams, pressures, and currents in the various levels of the atmosphere, the ocean also contains a similar network of circulation patterns and pressure zones.

Some deep-sea currents, such as the Gulf Stream, push through the waters much as rivers flow through land. By coupling the sciences of meteorology and oceanography, several major ocean currents have been detected using peak evaporation levels caused by warm, dry air in the subtropics. Strong evaporation throughout the year off the coast of South Africa characterizes the presence of the Agulhas Current, strong evaporation off the eastern United States in January characterizes the Gulf Stream, and strong evaporation throughout the year in the northeast Atlantic characterizes the North Atlantic Drift. These currents all have generally higher surface temperatures in relation to overlying air, especially in winter, and are frequently accompanied by strong winds. Approximately 40 percent of the total heat transported from the Southern to the Northern Hemisphere is through the action of surface ocean currents.

Another type of ocean current, called an upwelling, brings deeper, colder waters to the surface. This colder water replaces warmer surface waters that are pushed away by the strong trade winds. Upwellings carry with them large amounts of nutrients that nourish the plankton, which make up the base of the food chain. Such currents are an excellent example of how atmospheric currents and ocean currents interact: Without the trade winds to push surface waters away, colder water could not surface.

Ocean currents called Ekman spirals also demonstrate how the atmosphere affects ocean waters. Winds drive surface water in the same direction that the wind blows. Water just below the surface moves more slowly due to friction, and the moving water is slightly deflected to the right by the Coriolis effect. Therefore, water at lower depths moves even more slowly and in the opposite direction of the flow of surface water. This causes the water to flow in a downward motion called the Ekman spiral. Because of the Ekman spiral, surface water actually flows at a 90-degree angle to the wind flowing out to sea along coastlines, thus creating the opportunity for upwellings.

Climate Shifts and Global Impact

Changes in ocean temperature and air currents influence weather, and weather changes over time determine climate. As warm-water masses move toward the coasts, they bring with them atmospheric moisture that causes rainfall. When air currents shift, as when strong trade winds die down or reverse course, areas that normally get little rain may be flooded, and areas that normally get a lot of rain may have droughts.

The best-known climate shift has occurred for centuries. It was named El Niño (Spanish for “Christ child” or “the little boy”) by Peruvian fishermen because it occurs during the Christmas season. Peru is known for its anchovy fisheries, and El Niño hampers this important harvest. Normally, strong trade winds push warmer surface waters away from Peru’s coast so upwelling can occur. This upwelling brings rich nutrients from the lower ocean waters, and Peruvian fishermen reap strong harvests. During El Niño years, which cycle approximately every seven years and may last up to two years, trade winds weaken, and warmer surface waters remain along Peru’s coast. Upwellings do not take place, surface waters heat up, and the anchovy harvest suffers.

The effects of El Niño do not end with the Peruvian anchovy fisheries, however. The phenomenon triggers a climatic ripple effect that disrupts weather patterns around the globe. Unusual numbers of storms may rage across North America. Drought conditions have been experienced in northeastern Brazil, southeastern Africa, and western Pacific islands. El Niño effects have also caused unusually wet springs in the eastern United States and elsewhere.

The 1982-1983 El Niño is estimated to have caused more than $8 billion in damage. The severe 1997-1998 El Niño, in turn, is thought to have caused more than $15 billion in damage. It was not until the 1982 El Niño, however, that meteorologists and oceanographers began to seriously study the phenomenon to learn ways to predict both its approach and the severe weather it often causes. Researchers learned that a fluctuating wind pattern known as the Southern Oscillation, which is the same fluctuation that causes the trade winds to weaken, triggers El Niño. Researchers also learned that the ocean-current pattern is dependent upon the wind pattern. Together, this climatic event is known as the El Niño Southern Oscillation (ENSO).

ENSO

Meteorologists and oceanographers continue to learn all they can about ENSO and other atmosphere-ocean oscillations around the globe. The ENSO pattern was the first to be studied, and many weather forecasting models have been created to aid the study of atmosphere-ocean oscillations. These computer models have also helped researchers study ENSO historical trends for the past five hundred years. Many have come to believe that ENSO events may have contributed to plagues and other similar disasters throughout history.

ENSO patterns fluctuate between the extremes of heated tropical waters associated with El Niño and a colder weather-front pattern known as La Niña (“the little girl”). This oscillation pattern takes about seven years to cycle, and each extreme (hot or cold) in the pattern may last up to two years. ENSO researchers have also linked the onset of El Niño with other climate events in surrounding ocean waters. Just before El Niño begins warming the Pacific Ocean, the tropical Indian Ocean warms. Warming then appears in the tropical Pacific, triggering ENSO, which causes warm winds to blow over South America. About nine months after this occurs, the circulation of the tropical Atlantic changes, and the waters there begin to warm.

The study of the ENSO pattern has led researchers to discover a major ocean circulation pattern called the conveyor belt. This conveyor belt of water is thought to have existed for several million years since the continents occupied their current positions in the oceans. It connects the major bodies of water making up the global ocean. Its significance was overlooked until oscillation patterns were studied. The conveyor belt current circulates heated ocean water from the tropical Pacific Ocean to the North Pacific Ocean, turning clockwise around the Pacific to pass westward between Australia and Malaysia across the Indian Ocean, around the southern tip of Africa, and up into the North Atlantic Ocean. As the surface waters cool in the North Atlantic Ocean, they sink to a lower ocean level and return eastward across the Atlantic Ocean, around southern Africa, across the Indian Ocean, passing south of Australia, and back to the tropical waters of the Pacific Ocean.

Some researchers are concerned that if this conveyor belt current were to stop or slow down, heat would build up in the Southern Hemisphere, while the Northern Hemisphere would experience a severe drop in temperature. Such an occurrence may have caused the last ice age, and research findings continue to stress the importance of this conveyor belt current to maintaining global climate. Temperatures of surface water change in direct relation to changes in this conveyor belt current, the mass flow of which is estimated to equal that of the Amazon River a hundred times over, delivering a heat load to the upper North Atlantic equivalent to one-fourth of the solar energy that reaches the surface in that region.

NAO and PADO

The study of the ENSO pattern and the use of computer modeling to forecast weather revealed other atmospheric-oceanic oscillations, such as the North Atlantic Oscillation (NAO), which fluctuates on a twenty-year time scale. The atmospheric behavior of NAO has long been known to meteorologists. It is typically a low-pressure, counterclockwise wind circulation that centers over Iceland. This weather pattern contrasts with a high-pressure, clockwise circulation near the Azores Islands off the coast of Portugal. Strong winds blow west to east between these two weather centers. NAO seesaws between these two centers and strong winds drive heat from the Gulf Stream across Eurasia during high-index years, producing unusually mild winters there. At the other extreme, air pressure builds up over Iceland, weakening the warming winds and delivering bitter winters to Europe and Greenland.

Oceanographers have concluded the ocean must be the cause of the unique atmospheric patterns created by NAO. Because the ocean has a huge capacity for storing heat and reacts at a slower rate than the atmosphere, it must provide the input for atmospheric patterns operating in the same mode year after year. The source of the oceanic oscillator is a pipeline of warm water fed by the Gulf Stream. It takes twenty years to complete one cycle, thus setting the timing for the long-term swings of NAO. Researchers are unsure, however, how the temperature of the waters in the pipeline actually triggers NAO.

The discovery of the Pan-Atlantic Decadal Oscillation (PADO) resulted from a theory that tropical oscillations in the Atlantic actually extended beyond the tropical region. PADO covers an area of more than 11,000 kilometers, extending from the southern Atlantic to Iceland. Meteorologists proposed the existence of PADO, claiming it fluctuates on a ten- to fifteen-year time scale. PADO consists of east-to-west bands of water spanning the Atlantic Ocean. The bands alternate between warmer and cooler water and are accompanied by changes in atmospheric circulation. An oscillation to the other extreme reverses the temperature variance. It is also believed that PADO is triggered by NAO.

The study of ENSO, NAO, and PADO has led many researchers to theorize that such oscillations are linked in a chain that circles the global ocean. The Arctic Oscillation, which reverberates from the far northern Atlantic Ocean to the far northern Pacific Ocean, is associated with the NAO, especially in winter. The two phases that characterize the Arctic oscillation correlate precisely to those of the NAO. Varying between days and decades, the oscillation is a natural response of the atmosphere to the complex interactions of the ocean-atmosphere system.

The Pacific Decadal Oscillation (PDO) is believed to be another link in the chain of atmosphere-ocean oscillations. This high-latitude oscillation was first noted in 1977 when the northern Pacific Ocean cooled dramatically. An atmospheric low-pressure center off the Aleutian Islands intensified and shifted eastward. This brought more frequent storms to the West Coast of North America and warmed Alaska, while Florida experienced periodic winter freezes. A multitude of other environmental changes also resulted. Researchers have pinpointed other shifts occurring in 1947 and 1925 and identified close ties between PDO and ENSO events.

Significance

The greatest impact of studying atmosphere-ocean oscillations has been joining two separate fields of science. As meteorologists and oceanographers research how atmospheric and oceanic patterns rely on each other and ultimately influence climate, they have become more dependent on coupled research.

Computer simulations, sometimes called “oceans in a box,” have become more sophisticated as researchers from both fields collaborate and combine data. Climate scientists began using computer simulations in the mid-1980s to study winds in the atmosphere to determine how they stir ocean currents and alter pressure patterns in the atmosphere that feedback on the ocean again. Joint research between oceanographers and climatologists extends knowledge about the complex interactions of atmosphere-ocean oscillations and their influence on each other and the climate.

Principal Terms

Conveyor belt current: a large cycle of water movement that carries warm waters from the North Pacific westward across the Indian Ocean, around southern Africa, and into the Atlantic, where it warms the atmosphere, then returns to a deeper ocean level to rise and begin the process again

Ekman spiral: water movement in lower depths that occurs at a slower rate and in a different direction from surface water movement

Solar radiation: transfer of energy from the sun to Earth’s surface, where it is absorbed and stored

Trade winds: winds that blow steadily toward the equator; north of the equator, trade winds blow from the northeast, whereas south of the equator, they blow from the southeast

Upwelling: the process by which colder, deeper ocean water rises to the surface and displaces surface water

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