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| Oceanography
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| The Southern Ocean
Antarctica lies at the bottom of the globe, physically isolated from the rest of the world by plate tectonic forces that moved the continent into its southern position about 30 million years ago, opening up the vast ocean that encircles it in the process. This circumpolar sea, the windy and often very rough Southern Ocean, dominates the climate of the continent and governs the life found there. An intricate system of water chemistry, currents, sea ice, wind and waves shapes the richness of life in the Southern Ocean and the barrenness of life in Antarctica. |
Woven together to create this complex arrangement are currents and upwellings, driven mainly by the wind, superimposed over a large system of water masses. The waters of the Southern Ocean are deep and layered, one mass atop another like a layer cake. Layers of this ocean are made up of water masses with different temperatures and salinity, which typically remain separate from one another.
Deep beneath the surface, flowing across the ocean floor creeps the coldest, densest mass of seawater in the world. This layer is the Antarctic Bottom Water. It forms mainly in shallow places along the Antarctic continent, like the Weddell Sea to the east of the Antarctic Peninsula. As sea ice forms during the autumn and winter months in the Weddell Sea, an enormous quantity of surface water becomes frozen but salt, squeezed out as the ice forms, remains in the water below the sea ice. This salty, icy water produced under the immense plain of sea ice sinks rapidly down, sliding over the edge of the continental shelf and into the abyss. The Antarctic Bottom Water can be detected in all of the world’s oceans, for it travels very slowly northward on a journey that may last thousands of years. Above this deep, cold water mass are other layers that play a critical role in the biology and meteorology of the Antarctic. |
Heading south across the sea towards Antarctica from South America, a ship begins its journey plowing through the warm and highly saline Subantarctic Surface Water, a mass flowing south from the Indian, Pacific, and Atlantic Oceans. Somewhere in the neighborhood of 50º S latitude, a plunge of roughly 5º C in the water temperature, with an accompanying drop in air temperature, signals that the indistinct northern boundary of the Southern Ocean has been crossed. Now the ship travels across the Antarctic Surface Water, a cold, less saline water mass flowing north, away from the icy continent. This boundary marks the Antarctic Convergence, a dynamic and biologically important area.
Cold Antarctic seawater is denser than warm Subantarctic seawater. When these two masses of water thus meet, the dense Antarctic water sinks beneath the Subantarctic layer to continue its slow, spiraling journey north. Some mixing occurs when the two water masses come together, and a mass more dense than the two parent masses forms. This cold, salty water sinks at the convergence to form the Antarctic Intermediate water layer.
The meeting and mixing of the water masses at the Antarctic Convergence form a critical biological boundary. It is enough to kill or incapacitate the tiny plants (phytoplankton) and animals (zooplankton) that cannot survive rapid changes in temperature or salinity. The convergence, where millions of plankton die every day, is a popular place for seabirds to congregate, for here, they find a feast.
It is the uppermost layer, the Antarctic Surface Water that directly affects plants and animals in Antarctica. The Surface Water is a fairly thin layer, only 150 to 250 meters thick. It is chilled by contact with the cold air and ice from the continent. Melting water from winter sea ice and freshwater icebergs, as well as precipitation keep this layer cold and relatively low in salt content. This layer originates at the Antarctic Divergence, a narrow "gap" created by opposing forces of the circumpolar prevailing winds. |
The winds of the Southern Ocean exist in two concentric bands separated by a low-pressure trough that cuts through the Peninsula at approximately 65º S latitude. To the north of this trough, the winds are from the west; to the south, mainly from the east. The Southern Ocean is the windiest major body of water on the planet and intense low-pressure systems often create gale and hurricane force winds associated with heavy seas and high waves.
South of latitude 65º S the easterly winds push the Antarctic Surface Water to the west and south towards the continent, creating a weaker but more complex "East Wind Drift" that narrowly circles the continent.
North of the Divergence is a zone of powerful and persistent westerly winds. Circulating around the Antarctic continent, this "West Wind Drift" is responsible for driving the main body of Antarctic Surface Water slightly north of east. It also pulls the water up from below at the Divergence.
Pulled upward into the Divergence is yet another layer of water: the "Circumpolar Deep Water." This 600-meter thick mass of water, which lies between 1,500 to 3,000 meters below the surface, is formed from highly saline water that sinks in the far North Atlantic. It mixes with warmer, highly saline waters of the Mediterranean Sea as it slowly moves south. It may take many hundreds of years for this water mass to reach Antarctica!
As the Deep Water wells up at the Divergence, it carries dissolved nutrients into the sunlit surface zone. These nutrients, brought to the surface by upwellings generated by winds and currents, provide food for a vast array of plankton which form the basis of the food chain in Antarctica.
In this complex relationship between water masses and winds that encircle Antarctica, biological productivity of the ocean is generated. |
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