The Ice. Stephen J. Pyne

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СКАЧАТЬ 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 is almost completely uncoupled from that of the intermediate stratum of Antarctic air. In the interior, to know the depth of the inversion and the topography of site is usually adequate for predicting the surface wind-regime. On the coast, the process becomes more complicated. Topography steepens and becomes irregular; surface winds must mingle with air masses from the pack and beyond; the properties of the air masses in three dimensions and their complex exchanges of heat and mass with sea and broken ice fields become significant. Antarctica has the simplest meteorology of any continent. That simplicity increases with the increase of ice toward the interior. Weather patterns may be fierce and huge, but they show nothing of the complexity of weather elsewhere. Weather seems to be reduced to surface winds and the numbing constancy of the inversion.

      Snow accumulation and ice flow regimes. Accumulation and rates of flow increase toward the perimeter. In general, katabatic wind flow conforms to ice flow patterns. Redrawn, original courtesy Encyclopedia Britannica.

      A special wind mediates between the interior and the coast. Between inversion winds and cyclonal winds, there exists a transitional regime of powerful, gravity-driven winds known as katabatics. Irregular in outflow, yet often dominant locally, katabatics have a much stronger inertial energy than do simple inversion winds. A large drainage area, intense surface cooling, and a convergent flow pattern are all among preconditions for katabatic flow. These furnish an adequate air mass. The dynamics of katabatic winds seem to depend, in part, upon the synoptic weather around the coast, especially the movement of cyclones. Ordinary katabatic winds erupt for periods of hours, perhaps days, then give way to periods of simple inversion winds or even calm. Extraordinary katabatic winds, however, can persist for days or even weeks, completely overriding the otherwise prevalent synoptic weather. For much of Antarctica, outbursts of katabatic winds—blizzards—constitute the local “storms.” Katabatics are the winds of The Ice.

      Typically, katabatic flow begins rapidly, reaches a plateau of gustiness, then abruptly subsides. As the air avalanche rushes downslope, it warms adiabatically, and turbulence with the overlying, warmer air stratum results in further warming. In some cases, the prevailing lapse rate means that this temperature increase still leaves the katabatic wind colder than the coastal air it displaces, but often the katabatic air is actually warmer, although denser. An explanation is that in the process of descending, the wind scours the surface snowfield, entraining a considerable volume of snow, and this increases its overall density such that the warming it experiences is not adequate to slow its gravitationally driven momentum. Where the drop between plateau and coast is steepest, the winds can reach staggering velocities. Where the source region is also vast and air convergence is the norm—for example, near Adelie Land—extraordinary katabatics may be commonplace for months.

      The strength of the katabatics can vary according to their interaction with migrating cyclones. As a storm approaches, relatively warm, moist air is advected inland. As this air mass rides over and against the ice or the cold air of the inversion, it leads to cloudiness and snow drizzles. More importantly, the advected air and unfavorable pressure gradient may dam up the normal outflow of inversion winds or katabatics. As the storm passes, however, a new pressure gradient encourages outflow from the continent. The katabatics rush down the ice, first violently, then steadily, until another cyclone approaches. The blizzards for which Antarctica is so celebrated generally develop when gravity winds (katabatics) and gradient winds (cyclones) act in concert. They are most intense where conditions favor vigorous, extraordinary katabatics—steep slopes, sharp temperature contrasts, developed storm tracks. The winds tumble down the ice dome, sublimating some snow and entraining more, creating a white dust storm from the polar desert. Curiously, the winds in Antarctica know little moderation: they tend to blow either fiercely or mutedly.

      Katabatics are best developed over East Antarctica. Here the polar plateau is so massive and elevated that storms from the thin Antarctic atmosphere can barely penetrate anywhere into the interior. By contrast, the smaller, lower West Antarctic ice sheet is crossed so frequently by storms that katabatic flow may be considered secondary. The topography of the Antarctic Peninsula is nonetheless important for local weather. When shallow winds crossing the Weddell Sea reach the Antarcandes, most are dammed and deflected to the right (north) as СКАЧАТЬ