LITMIR.BIZ

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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 settle for days. Air masses that attempt to cross the continent must confront the topography of the ice sheets and the two great chains of mountains—the Antarctic Andes (Antarcandes) of the peninsula and the Transantarctics that extend between East and West Antarctica. The ice dome itself is a formidable barrier. Considering the thinness of the polar atmosphere (at the equator the atmosphere is twice as dense), the elevation of the ice sheets removes them from most storms. The mountains deflect surface winds in characteristic patterns.

      Overall circulation is vortical. A belt of low pressure, populated by a chain of major cyclones, spirals around the continent with the westerlies, roughly between latitudes 60 and 70 degrees South. This is the polar front, the atmospheric equivalent to the convergence. Closer to the coastline, there is a narrower belt of cyclones where the polar easterlies shear against the westerlies. This is equivalent to the Antarctic front. It is from this zone, not from Antarctica proper, that cold outbreaks of Antarctic air seem to emanate. The atmospheric mechanics thus differ from those typical of the Northern Hemisphere. The great ice sheets create a continual sheath of cold air, which they shed by surface-wind flow and occasional cyclonic mixing to the zone of coastal convergence. From here—once mixed—the cold air participates in outbreaks to the north. The bulk of warmer air drafted above the surface—the atmospheric equivalent to the circumpolar deep water—spirals into the interior in what is known as the Antarctic circumpolar vortex. In the winter, when temperature gradients are greatest, the circumpolar vortex intensifies as it reaches upward well into the stratosphere.

      The Antarctic atmosphere most differs from the Southern Ocean in that air extends over the continent itself. The atmosphere must interact with all the ice terranes, not merely with the pack. Contact with these ices creates an intense layer of dense, frigid air. The surface weather of Antarctica is dominated by the permanent presence of this sheath. The inversion forms because the ice sheet is cold and elevated, the extraordinary albedo of snow reflects most of the incident sunlight, and the clear dry skies allow reradiated heat to escape. Air near the surface becomes chilled and dense, and during the polar night the inversion deepens. Normally, a temperature inversion of this sort makes for a stable atmosphere, with little vertical mixing. The cold air collects quietly in topographic basins. Not in Antarctica. The elevation and topography of the ice dome shape an abrupt plateau of enormous dimensions; more than half of the ice surface exceeds elevations of 2,000 meters, and nearly everywhere the 1,000-meter contour line can be found within 200 kilometers of the coast, often less. Instead of pooling tranquilly in local basins, the dense air is shed outward and down the ice dome to form the surface winds—and the perceived weather—of Antarctica. Much as pack ice simplifies the atmosphere and ocean, so terrestrial ice simplifies weather into a meteorology of surface-air dynamics, for which simple rules of synoptic meteorology, which relate winds to pressure gradients, are not adequate to explain the consequences.¹

      Instead the atmosphere is seemingly reduced to the interaction of air and ice. Ice not only creates the sheath of inversion air but directs it. The dense air sloughs off the ice dome like sheet runoff on a desert slope. There is some accommodation to geostrophic effects; the Coriolis force, strong at the poles, deflects the flow to the left, thus forming the polar easterlies. Special flow regimes result from the interaction of inversion winds with topographic features. In some places the surface air diverges, weakening as it splays outward from the dome, while in other places it converges through valleys or mountain passes and intensifies. In still other places major mountains act as barriers that redirect airflow or that, when air occasionally spills over them, establish foehn winds. And there is some association with cyclonic storms along the coast, as they alternately dam up and release outflows of surface air.

      The surface weather is by and large the weather of the inversion. It is most intense where the inversion is deepest, most vigorous where the terrain is steepest. In the short term, the surface weather

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