LITMIR.BIZ

      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 barrier winds. A small proportion crosses the summit to form foehn winds. Of the winds greater than 10 meters per second in the region, 79 percent are cold barrier-winds from the south and 20 percent are foehn winds from the west. The climate on the east side of the mountains, which is consequently severe, helps account for the prevalence of the pack in the Weddell Sea and the otherwise anomalous presence of the Larsen Ice Shelf.

      The barrier winds sustain a geophysical “conveyor belt” to transport ice, cold air, and cold water north within the Weddell gyre. The great outward swelling of the convergence, the string of bergs and broken pack that flare north from the peninsula, the persistent pack ice and shelf ice so influential in the formation of Antarctic bottom water—all depend on this peculiar wind regime. In no other sector of the Antarctic has a similar pattern so fully developed, and only the Ross Sea offers even a mild analogue. The interruption of the polar easterlies exposes the upper portions of the west side of the peninsula to marine influences that make it a distinctive climatic region of the Antarctic. It alone is spared a wind regime connected directly to the polar plateau. In this balmier state rain rather than snow is a frequent occurrence, and some influence from South America is manifest across the Drake Passage.

      The katabatic winds have many effects. They scour some snow off the surface, sublimate other snows, and redeposit still more. This erosion helps to maintain the steep topographic gradients that, in turn, encourage katabatic flow. Where the cold katabatic wind slides over warmer seas, snowspouts—whirls of entrained snow—may dance along the coast, the product of violent mixing. On a larger scale, the interaction of katabatic outflow and marine air helps account for the belt of shallow cyclones that encircles the continent. The effects are limited to a few kilometers beyond the coast, but they can be dramatic—a nearly perpetual veil of clouds and blowing snow, torn only by occasional outbreaks of wind associated with frontal passages. Unstable lapse rates, associated with katabatics, promote the transfer of heat and moisture from the ocean to the atmosphere. Offshore winds interact with the pack ice in important ways, too. They sublimate and redeposit snow, remove surface meltwater, and drive floes outward. During pack progradation, floe separation is an important mechanism for promoting interstitial freezing and frazil-ice formation. During storms, when pack coverage is not total, floe separation exposes seawater, whose released heat and moisture may intensify the storm. The katabatics extend the influence of the interior ice outward.

      Everywhere the presence of ice is felt. Sea ice is a filter, intervening between air and sea, land and ocean; a lever, amplifying small changes in environmental conditions into larger effects; and a matrix, for a mixture of ice masses and for life. The explosive growth of pack ice—in effect, an extension of the Antarctic land mass—is one of the fundamental facts of Antarctic and Earth weather. The pack severs the connection between ocean and atmosphere around the continent, expanding and intensifying the polar heat sink. It reflects incident radiation, cools air and water in contact with it, and breaks the exchange of heat from ocean to atmosphere. Compared to the atmosphere, the oceans have higher heat capacity (1,600: 1), greater mass (400:1), and larger momentum (4:1). The atmosphere drives ocean currents, mainly by an exchange of momentum from wind to wave; the ocean, in turn, drives atmospheric processes, primarily by a transfer to heat. Open water transmits nearly one hundred times as much heat to the atmosphere as ice does. Regionally, ice amplifies the conditions that generate the ice; several mechanisms unite ice cover, temperature, and albedo in positive feedback. Globally, The Ice can amplify small changes of atmospheric conditions into larger effects, perhaps even full-blown ice ages.

      Biotic Barrier

      The pack ice is one of the great biotic boundaries on the planet. It divides the biotic from the abiotic environments of Antarctica, and it marks the limits of life on Earth. Except along the pack, Antarctica constitutes an enormous abiotic oasis segregated from the planetary biosphere. The pack is both a matrix for indigenous life and a biological filter against migration—the only ice terrane with indigenous life. Its biota contributes immensely to the complexity and attractiveness of the terrane.

      The

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