LITMIR.BIZ

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Here circumpolar deep water is modified first with surface water, cooled and freshened by the winter waters that result from the intense production of sea ice during the polar night. This altered circumpolar deep water then mixes with shelf water from the western Weddell Sea region—a site almost constantly under the influence of shelf ice and pack ice. This new mixture interacts again with circumpolar deep water as it flows out of the Weddell Sea. Deep-water circumpolar currents and the topography of the deep ocean basins carry the Antarctic bottom water clockwise around the continent. The final composition of Antarctic bottom water is one-eighth winter waters, one-fourth western shelf waters, and five-eighths circumpolar deep water. Other bottom waters, notably from the Ross Sea, add to the volume as the mass circulates around the Southern Ocean. But the Weddell Sea is clearly the primary source, and the properties of the mass blur as it distances itself from the Weddell Sea and as portions are siphoned off to fill the abyssal plains of the Atlantic, Indian, and Pacific basins.

      Because of its ice regimes, the continental shelf harbors another zone of distinctive water masses. The continental shelf of Antarctica is not extensive. The weight of the land-based ice sheets so depresses the continent that its shelves are the deepest in the world, and the flooding of the larger embayments with land ice to form enormous ice shelves further reduces their areal dimensions. In effect, ice shelves replace continental shelves. The ice shelves are extensions of terrestrial ice sheets that at some point float. These floating shelves redefine the contour of the continent and influence the flow regimes and characteristics of waters in the Southern Ocean. There is little encroachment, for example, by circumpolar deep waters onto the continental shelves. Other areas, like the Weddell Sea, are subjected to almost perennial sea ice that also greatly influences the character of the subsurface waters.

      Land ice affects subsurface waters in somewhat different ways than does sea ice. Beneath the pack, winter water collects in large quantities, vertical mixing is good, and a deep layer of surface water develops. By contrast, the ice shelves encourage the production of lesser quantities of very cold water. Water beneath the floating shelves is subjected to higher hydrostatic pressures than water at an equivalent depth in the open sea. This increase in pressure lowers the freezing temperature of the water, allowing for subshelf waters to reach much lower temperatures than they otherwise could. The liberation of this very cold water onto continental shelves may be responsible for some of the peculiarities of Antarctic bottom water. Thus, again, land ice affects sea ice, which in turn influences the weather patterns that sustain the continental source regions.

      The Antarctic ice field becomes one vast self-reinforcing system in which air, water, and land are integrated through the medium of ice, a system in which The Ice transforms everything into more ice. The pack contributes directly to such parameters of the Southern Ocean as its salinity and temperature profiles, its vertical turbulence, its density structure and momentum, and its production of shelf, surface, and bottom waters. Other contributions are more indirect, a consequence of the pack’s role as a thermal insulator and reflector. The geography of the pack affects weather patterns, the distribution of warm and cold waters, and the relative proportions of sea to ice, with their differential abilities to absorb and reflect sunlight. Yet despite an annual balance, the processes are at any one time out of synchronization. Salt flux is at a maximum during winter freezing, heat flux during summer, when there is abundant open water; fresh water flux requires the melting of icebergs. The Southern Ocean is constantly imbalanced. The integrating medium, ice, lags. What ultimately unifies these processes is a shared geophysical core: the great ice continent itself.

      This whole cryospheric cycle has to begin somehow, and the establishment of the Antarctic circumpolar current is the most likely source. The Southern Ocean has evolved piecemeal over the course of 120 million years. The Drake Passage—formed by the complex displacement of the mountain chain binding the Andes to the Antarctic Peninsula, an island arc system—appeared only in late Eocene times, 38 million years ago. The establishment of a proto-ice sheet dates from this event. The ancestral Antarctic circumpolar current developed within a few million years afterward; for the last 30 million years or so, although the Southern Ocean basin has continued to expand outward, the current has been stable. There has been a change of size, a migration northward, but not a fundamental reconstitution of the flow regime. The present-day characteristics of the Southern Ocean apparently date from the early Pliocene, 3–4 million years ago. That this date coincides with the onset of the most recent planetary glacial epoch is no accident. Currently, the circumpolar deep waters circulate between Antarctica and the world ocean on a cycle of about one thousand years.

      The Antarctic, then, makes an almost perfect antipode to the Arctic. The Arctic is a true ocean surrounded by continents; the Antarctic, a continent surrounded by oceans. Their climates, ocean currents, ices—all differ. And from these geographic differences derive the distinctive human histories of the two regions, which are antithetical in nearly every aspect.

      In the world ocean the anastomosing Antarctic is a central vortex, a primary zone of mixing. The Arctic Ocean, connected only by narrow straits, is a virtual eddy. Its waters, air masses, and ices circulate in the polar gyre, occasionally discharging in streams southward. Its fundamentally maritime climate is comparatively mild, with a mean temperature at the pole of −18 degrees C. Its sea ice persists for years, acquiring structure; fast ice and shore effects are common along the coastline; and glacial ice is rare, confined to isolated ice caps like those on Baffin Island and to the Greenland ice sheet, along the margin of the Arctic. Its biota is splendidly varied, with a strong terrestrial component. And the circum-Arctic, including the fringes of the Greenland ice sheet, has long been integrated into the biotic and human history of the planet. There are economic resources to exploit. Geopolitical considerations have superimposed anthropogenic boundaries over Arctic geography and directed much of its contemporary history. There has even evolved an indigenous art of Eskimos, Indians, Siberians, and Lapplanders.

      None of this is true of the Antarctic. Its climate is continental. Nearly all of its land mass is submerged beneath crushing ice sheets. Its pack ice ebbs and flows with the seasons. It mixes the world ocean and serves as a depository for the world’s surplus heat and moisture. The mean temperature at the pole is a numbing −50 degrees C. Its ice terranes far exceed those of the Arctic. Its pack ice is larger and thicker than that of the Arctic, and it is reproduced annually. Its land ice comprises 90 percent of the world’s total. It produces nearly all of the world mass of icebergs. The effect of its ice field is correspondingly more pronounced. Ice is only one component of Arctic geography: in the Antarctic it becomes increasingly the only component.

      Although it stands at the central vortex of the world ocean—in fact, partly because of that—Antarctica as a continent exists in almost extraterrestrial isolation. There is a circumpolar uniformity imposed by The Ice, but it is a shared emptiness. Glaciology replaces geology, biology, meteorology. Like the bottom waters of Antarctica, the geophysical sinews that bind Antarctica to the Earth are remote, unobvious, and abstract. An ecosystem exists, directly or indirectly, only on the ocean and the pack. There is no human history in a traditional sense. Explorers did not sight the continent until the mid-nineteenth century, did not make true landfall until the twentieth, and did not establish quasi-permanent colonies until the post-World War II era. The geopolitics of the region belong with that of the deep oceans and the Moon.

      Cold Core: The Antarctic Atmosphere

      Antarctica is the cold core of the atmosphere, a region so intensely frigid that it deflects the meteorological equator of the globe northward nearly 10 degrees latitude. The solar radiation balance of Antarctica is negative all year round. In the winter night, no radiation enters; in the summer, the snow and ice reflect virtually all of the incoming radiation back into space, with little interference from the dry clear atmosphere over the continent. Even the Arctic enjoys a positive radiation balance for at least a portion of the year. Not so the Antarctic. It is the great refrigerator of the planetary atmosphere. Its unremitting cold is the supreme reality of Antarctic meteorology. The ice terranes of the Antarctic are both an outcome and a contributor to that fact.

      Storm rings around Antarctica. The actual pattern is a spiral, with cyclones veering inward to the Ross and Weddell seas. West Antarctica is frequently

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