the global ocean influences the Earth's climate by storing and transporting vast amounts of heat, moisture, and carbon dioxide. Huge quantities of carbon are cycled annually among the biosphere (forests, grasslands, and marine plankton), the atmosphere, and the oceans. The oceans are the largest active reservoir of carbon, containing 50 times more carbon than the atmosphere. Of the 6 to 7 billion tons of carbon currently released into the atmosphere by human activities, approximately 3 billion tons remain in the atmosphere, 1 to 3 billion are absorbed by the oceans, and up to 2 billion appear to be absorbed by the terrestrial biosphere.
Oceanographers commonly refer to the oceanic region that surrounds the continent of Antarctica as the Southern Ocean. The northern boundary of the Southern Ocean is not well defined, but it coincides approximately with a broad zone of transition between the warm, saline surface waters of the subtropical regime and colder, fresher subantarctic waters, called the Subtropical Front, which occur between 40 degrees S and 45 degrees S. Using this definition, the surface area encompassed by the Southern Ocean represents approximately 29.7 million sq. mi. (77 million sq. km.), or 22 percent of the global surface ocean. Its unique geography makes it a key player in global climate.
The Southern Ocean is the only ocean that encircles the globe unimpeded by a land mass. It is home to the largest of the world's ocean currents: the Ant arctic Circumpolar Current (ACC). The ACC carries between 135 and 145 million cu. m. of water per second from west to east along a 12,427 mile-(20,000-km.) path around Antarctica, thus transporting 150 times more water around the globe than the total flow of all the world's rivers. By connecting the Atlantic, Pacific, and Indian Oceans, the ACC redistributes heat around the Earth and so exerts a powerful influence on global climate.
Near the Antarctic continent, the Southern Ocean is a source of cold, dense water that is an essential driving force in the large-scale circulation of the world's oceans. The cooling of the ocean and the formation of sea ice during winter increases the density of the water, which sinks from the sea surface into the deep sea. This cold, high-salinity water includes Antarctic Bottom Water and Antarctic Intermediate Water. Antarctic Bottom Water originates on the continental shelf close to Antarctica, spills off the continental shelf, and travels slowly northward, hugging the seafloor beneath other water masses, moving as far as the North Atlantic and North Pacific. Antarctic Intermediate Water is less saline and forms farther north, when cold surface waters sink beneath warmer sub-Antarctic waters at the Antarctic Convergence at about 55 degrees S. Together, these motions form a complex, three-dimensional pattern of ocean currents that extends around the globe, known as the thermohaline circulation, or "great ocean conveyor." The thermohaline circulation has a critical influence on climate by transporting heat efficiently around the globe and by controlling how much dissolved inorganic carbon is stored in the ocean.
At the sea surface, seawater exchanges gases such as oxygen and carbon dioxide with the atmosphere at the same time that it is being cooled. As a result, sinking water efficiently transfers changes in temperature, fresh water, and dissolved gases into the deep ocean 2.5 to 3 mi. (4 to 5 km.) beneath the sea surface; in terms of carbon sequestration, this is called the solubility pump. Biological processes also play a role in the surface layer, where photosynthesis by single-celled marine phytoplankton can sequester carbon dioxide in the surface water and, through the process of sedimentation, transfer this organic carbon to deeper waters—the so-called biological pump.
The Southern Ocean is distinguished as a region of high levels of dissolved nutrients, but with mod est rates of annual net primary production, so that the biological pump appears to be operating well below its maximum capacity. An interesting idea of recent years is that it may be possible to sequester much more atmospheric carbon if iron, an essential micronutrient, is added to the ocean to encourage the growth of marine phytoplankton, and thus stimulate the biological pump. The overall effect would be to lower the concentration of dissolved carbon dioxide in surface waters, allowing more atmospheric carbon dioxide to dissolve into the sea.
Understanding the global circulation and conditions under which surface waters penetrate into the deep ocean is critical for scientists estimating the timing and magnitude of climate change. At this time, the Southern Ocean is considered to be a net sink for atmospheric carbon dioxide; however, the magnitude of this sink has a high uncertainty, with mean annual estimates ranging between 0.5 and 2.5 billion tons. The degree of interannular variability in the Southern Ocean carbon sink, and its possible future response to climate change, is still poorly understood. However, climate model projections indicate that the Southern Ocean overturning circulation will slow down as the Earth warms. A decrease in the rate of overturning circulation will result in a decrease in the rate of carbon dioxide absorbed by the Southern Ocean, which represents a positive feedback and tends to increase the rate of climate change.
The presence of sea ice in the Southern Ocean is another factor that contributes to the Southern Ocean's important role in climate. Sea ice formation during the winter months is the largest single seasonal phenomenon on Earth, with approximately 7.7 million sq. mi. (20 million square km.) of ice formed annually, effectively doubling the size of Antarctica. This has a profound effect on both regional and global climate processes. Because of its high albedo, sea ice reflects the sun's heat back into space, intensifying the cold. However, it can also act as a blanket, insulating against heat loss from the ocean to the atmosphere. Its yearly formation injects salt into the upper ocean, making the water denser and causing it to sink downward as part of the deep circulation. As ocean temperatures increase in response to the global warming, the amount of sea ice is expected to decrease; this has already been observed in the Arctic Ocean. The resulting decrease in the planetary albedo would act as a positive feedback, increasing the amount of energy from the sun absorbed by the Earth and tending to further increase the rate of climate change.
SEE ALSo: Albedo; Antarctic Circumpolar Current; Arctic Ocean; Carbon Cycle; Climate Models; Phytoplankton; Sea Ice; Thermohaline Circulation.
BIBLioGRAPHY. Sayed Z. El-Sayed, Southern Ocean Ecology: The Biomass Perspective (Cambridge University Press, 1994); Kate A. Furlong and Kate A. Conley, Southern Ocean (ABDO Publishing Company, 2003); George A. Knox, Biology of the Southern Ocean, 2nd ed. (CRC, 2006); A.T. Ramsey, J.G. Baldauf, eds., Reassessment of the Southern Ocean Biochronology (Geological Society Publishing House, 1999).
Al Gabric Griffith University
FIRST described extensively by British meteorologist Sir Gilbert T. Walker in the 1920s, the Southern Oscillation refers to the periodic exchange of mass across the equatorial Pacific that is recorded in sea level pressure fluctuations between the eastern and western Pacific. Under normal conditions in the tropical Pacific, surface high (low) pressure prevails in the eastern (western) Pacific, with the easterly trade winds dominating surface wind and ocean flow.
This pressure pattern, also known as the Walker circulation, tends to support rising air motions and convectional precipitation near eastern Australia, as well as sinking air motions and dry conditions near coastal northern Peru. Every two to seven years, this generalized atmospheric surface pressure pattern weakens as equatorial Pacific air pressure rises in the west and lowers in the east. This shift in the pressure field considerably weakens the trade winds and promotes the eastward movement of warm surface water across the tropical Pacific. The associated abnormal warming in the eastern Pacific is known as El Niño. Because the reversals in pressure and associated ocean temperature fluctuations are often simultaneous, this coupled climate variability between the tropical Pacific Ocean and atmosphere is often collectively referred to as the El Niño/Southern Oscillation (ENSO).
measuring southern oscillation
The mode and relative strength of the Southern Oscillation during a given time period is determined using one of several indices that signifies changes in the Walker circulation. A relatively simplistic and common method employed to gauge this change is the Southern Oscillation Index (SOI), which measures the monthly or seasonal sea level pressure differences between two stations, one located in the central Pacific at Tahiti and the other in the western Pacific at Darwin, Australia. Negative SOI values result from abnormally low pressure occurring in Tahiti and high pressure occurring at Darwin, which tends to indicate an El Niño episode; positive SOI values indicate the cold phase of ENSO, or La Niña. The sea level pressures at these two stations thus are negatively correlated and are associated with significant, yet contrasting shifts in regional temperature and precipitation patterns. Some of the most severe Australian summer droughts and heat waves (e.g., in 1983) have been associated with a strongly positive SOI.
ENSO events often affect the temperature and precipitation regimes in tropical regions. The magnitude of these effects differs with the intensity of individual ENSO events. Climatic anomalies associated with ENSO's warm phase in other tropical regions include dry summers and autumns for northern South America, Central America, and southeastern Africa (including Madagascar), as well as less rainfall during the Indian monsoon. Drier-than-normal conditions negatively affect crops—a particularly serious concern in developing regions. Such atmospheric conditions are also conducive to the threat of wildfires. Wetter conditions pervade the Chilean coast, as well as parts of east-central Africa.
Despite being primarily a tropically located phenomenon, ENSO also has extensive effects on extratropical global precipitation and temperature variability. This is achieved, in part, by shifts in storm
tracks. Changes in large-scale atmospheric circulation include deviations from the normal jet stream paths and persistent pressure systems, which in turn steer storms in new directions. During the warm ENSO phase in winter, a deepened Aleutian Low moves southeast of its average position. This is coupled with a strong subtropical jet stream and a weak polar jet stream over eastern Canada, setting up the circulation pattern that redirects storms into the southern United States. The winter cold ENSO phase is characterized by a blocking high forming in the Gulf of Alaska and a split polar jet. The main branch flows from Alaska and northern Canada south toward the western and northern United States; the jet's southern branch moves from the Pacific Ocean toward the Pacific Northwest.
Winter tends to bring the strongest North American precipitation and temperature responses to ENSO, though effects are noted in other seasons. Warm ENSO events typically result in wetter-than-normal conditions for much of the southern United States, with California often experiencing flooding as a result of the position of the stronger-than-normal subtropical jet stream, which directs storms into the region. Conversely, dry conditions tend to occur in the midwestern United States. Warm ENSO events also bring mild, warm winters into western Canada and Alaska, as well to Canada's Maritime Provinces. Cold ENSO events bring considerably drier, warmer winters to the American Southeast and below-aver age temperatures to western and central Canada and the northern tier of the United States.
Global climate models have been used in recent decades to predict how changes in our current climate will affect the frequency, strength, and position of cycles of interannular variability such as the Southern Oscillation. Although the predictability of the Southern Oscillation has been subject to differences between observed and predicted timescales, researchers have shown that the current climate models do tend to place warm ENSO events within the observed two- to seven-year timescale. Ensemble forecasts, in which average forecasts are generated by running models with slight variations in initial conditions, produce good results, with multimodel ensembles generally outperforming single-model ensembles. These models serve to enhance our understanding of atmosphere-ocean interactions, such as ENSO, for improving long-range weather and climate forecasts.
SEE ALSO: El Niño and La Niña; Walker Circulation.
BIBLIOGRAPHY. C. Donald Ahrens, Meteorology Today (Thomson Brooks/Cole, 2007); S. George Philander, ed., El Niño, La Niña, and the Southern Oscillation (Academic Press, 2006); Cynthia Rosenzweig and Daniel Hillel, Climate Variability and the Global Harvest: Impacts of El NiñO and Other Oscillations on Agro-Ecosystems (Oxford University Press, U.S., 2007); John M. Wallace and Peter V. Hobbs, Atmospheric Science: An Introductory Survey
(Academic Press, 2006); Warren M. Washington and Claire L. Parkinson, Introduction to Three-Dimensional Climate Modeling (University Science Books, 2005).
Jill S. M. Coleman Petra A. Zimmermann Ball State University
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