The Ice. Stephen J. Pyne
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Название: The Ice
Автор: Stephen J. Pyne
Издательство: Ingram
Жанр: Журналы
Серия: Weyerhaueser Cycle of Fire
isbn: 9780295805238
isbn:
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 crossed, East Antarctica almost never. Courtesy NOAA.
The Antarctic climate consists of three terranes, each with its own subclimate: the continent, the ice-free sea, and the pack ice. The continent is a heat sink; the ocean, a heat source; and the pack, a great filter that regulates the exchange of heat and moisture between ocean and atmosphere, sea and land. Each of the three regions has its own zone of mixing, and the pattern of atmospheric circulation closely conforms to the cycle of atmospheric heat loss. As the polar night deepens, the temperature gradient between perimeter and core increases, storms acquire more vigor, and the polar winds rush more ferociously. Compared to the Northern Hemisphere, the Southern has a high proportion of ocean to land; and a good chunk of its terra firma, Antarctica, is a high-albedo ice field, not a heat-exchanging land mass. Continental warming is meager. The coupling of ocean and atmosphere is only feebly interrupted by lands, and the kinetic energy of air movement (as east-west flow) is nearly double that of the Northern Hemisphere. The perimeter of the pack is among the stormiest sites on the planet.
The south polar atmosphere mirrors, by inversion, the dynamics and structure of the Southern Ocean. There is a similar stratification (in this case of air masses), a similar gradient flow into and out of the region, and a similar continental circulation, dominated by a circumpolar vortex. A vertical profile shows three prominent strata: a layer of surface air, powerfully influenced by ice; an intermediate stratum of warm air, flowing from the temperate regions to the polar interior where it is chilled, transformed, and returned outward; and a remote upper layer, the high-latitude stratosphere, only tenuously bound to the others. The upper and lower strata transport cold air away from the continent, while the intermediate layer brings heat and moisture from more temperate regions inward to the pole by means of a circumpolar vortex. The heat of this intermediate stratum is exchanged by simple advection to the interior, by adiabatic sinking, and by turbulent mixing along the boundary it shares with the surface inversion. Its ambient humidity and clouds trap heat reradiated from the surface. Return flow outward from the continent develops from both the bottom and the top of the Antarctic air mass, with a variety of surface winds off the ice dome and, during the austral summer, a circumpolar anticyclone in the stratosphere. The linkages between these strata are uncertain. But the intensity and magnitude of the surface outflow demand a major inflow, and much of this converging air is transferred to the surface stratum.
The spatial distribution of Antarctic air masses mimics that of the ocean masses to which they are intimately coupled. Subpolar, polar, and Antarctic fronts segregate polar from temperate air masses and define the general zones of mixing. Two patterns of storms are typical. Around the coastline, within the Antarctic front, storms occupy a narrow belt and involve relatively shallow air masses. Here the surface winds that prevail over the continent intermingle with air ultimately derived from marine systems. This type of storm rings the continent with a veil of cloud and snow drizzle. Sea fog forms as warm air is advected over the ice; sea smoke collects as cold offshore winds interact with exposed leads; ice fog and snow haze drape across the horizon from fine crystal precipitates in the air; whiteouts result from various combinations of clouds and snow which so scatter incoming light that all shadow is lost; and blizzards add violence to the opaque curtains of cloud that commonly envelop the continental fringe.
Further outward, along or beyond the perimeter of the pack, the polar front generates deeper storms. It is here that the major mixing of polar and temperate air occurs, that storms are most vigorous. These storms, too, tend to revolve around the continent, but being better developed, they also spiral inward, like eddies caught in a slow, larger vortex. This storm belt oscillates in rough synchroneity with the pack. Sea ice retards that exchange of energy between ocean and atmosphere which helps sustain major cyclones. At the same time, winter storms are capable of penetrating more deeply into the interior than summer storms because winter cooling encourages a much more intense temperature gradient between the Antarctic and temperate regions. But for the most part, Antarctic cyclones require heat released from the ocean, and they tend to follow areas of open water. Frequently, however, storms cross the West Antarctic ice sheet, and occasionally they make inroads into the colossal East Antarctic ice dome. Precipitation—always as snow—is important for local glaciation, for ice shelves, and for floes.
The polar and Antarctic fronts, then, are not fixed by continental boundaries. They fluctuate and are fragmented, much like the pack with which they are associated. Occluded fronts can regenerate over exposed waters along the coast, especially in the Bellingshausen and Ross seas. Once rejuvenated, they may СКАЧАТЬ