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The Pack

[The ship] came to the limits of the world, to the deep flowing

Oceanus … shrouded in mist and cloud.…

—Homer, The Odyssey, Book XI

I will not say it was impossible anywhere to get in among this Ice, but I will assert that the bare attempting of it would be a very dangerous enterprise and what I believe no man in my situation would have thought of. I whose ambition leads me not only farther than any other man has been before me, but as far as I think it possible for man to go, was not sorry at meeting with this interruption, as it in some measure relieved us from the dangers and hardships, inseparable with the Navigation of the Southern Polar regions.

—Capt. James Cook, Journals, Voyage of the Resolution and Adventure (1774)

PROGRADATION

They raft across a wine-dark sea, sometimes isolated, sometimes strung together, circling the continent like gears within a vast orrery of floating ice.

A veil of low cloud and grey fog, a stygian current of black sea, a mobile breakwater of white ice: these define the terrane of the pack and the oscillating perimeter of The Ice. The boundary is multiple. The cold core of the global atmosphere and the tangled vortex of the world ocean roughly coincide with the ice field of Greater Antarctica, creating a complex zone of mixing. The Ice is surrounded by a circumference of swirling storms, where Antarctic and subantarctic air masses mingle; by braided currents and fronts that mix Antarctic and subantarctic waters; by a shadow line that separates the varied calendar of temperate time from the two seasons of the polar day. That all of these zones approximately overlap accounts for the intensity of Antarctica’s isolation. Of these processes, sea ice is both a product and a producer. As the zones wax and wane with the seasons, the ice field grows and decays on a grand scale, and the pack becomes the effective boundary of the continent.

While the iceberg is the most interesting ice mass in Antarctica, the pack ice is the most interesting ice terrane. Its rapid life cycle, its explosive winter growth and catastrophic summer collapse, the infinite movements of its numberless floes, with their constant rupturing and resuturing, the volatility of its position as a solid-phase boundary between two fluid regimes, air and sea—all give the pack a collective dynamism and variety unparalleled among Antarctic ices. This is the most complex and active of Antarctic systems. In part, the pack reflects this vigor and reduces it. The floes form an ice membrane between air and sea, atmosphere and hydrosphere; between land and the air-sea matrix, Antarctica and the fluids that bind it to the Earth; between biosphere and cryosphere, life and an inorganic lithosphere. In this twilight zone between Earth and Ice are mixed sea and sky, sea and ice, sky and ice, sea ice and land ice, life and lithosphere. Even in their colors and geometry the black sea and white ice recapitulate the two polar seasons: the summer day and the winter night.

This intermingling, so characteristic of the pack and so different from what goes on in the rest of the ice field, leads not to more complexity but to less. As the pack matures, it reduces dramatically the interactions among its component systems, and the reductionism and solipsism of The Ice are boldly extended outward. The mingling of sea, sky, and ice makes this region among the cloudiest on Earth: a perennial fog hangs cloyingly over the pack; the scene is shrouded in a stormy grey twilight. The ragged front of the pack creates a geographic warp, an icescape where space expands and dissolves, where time slows and distorts. Around the ice field the floes orbit, waxing and waning with the seasonal tides: at times loose and free-floating in the circumpolar current, like ice fragments trapped within the rings of Saturn, and at times frozen more or less solid, slowly creaking around, their gyres like Aristotelian spheres of quintessential crystal. Only the power of the outside world reverses the trend to greater and greater simplicity and uniformity. For millennia, the pack was the commanding barrier not only to human travel but to human mind. One could reach The Ice only by passing through the pack.

Glaciology of the Pack

The progradation of the pack begins with the coming of the austral twilight. The sun circles low on the horizon, a cold white globe. As ambient temperatures fall below the freezing point of seawater, sea ice begins to form. Ice forms first in protected embayments along the coast, most rapidly in the protected seas that ring the continent. Some takes the form of congelation ice, which organizes surface crystals into a scaffolding; some takes the form of frazil ice, slushy clumps of ice crystals suspended within supercooled seawater. From their structured mixing the floe evolves, by a series of stages. More ice and snow are added. Metamorphism restructures the ice breccias that comprise the floe. What happens on the microscopic level is then repeated on a macroscopic level. Individual floes interact, form a matrix, accept more ices, move around and away from the coast, and acquire a collective identity as the Antarctic pack. The expansion of the pack measures the outward march of subfreezing temperatures that accompanies the encroaching polar night.

The Southern Ocean is never far from freezing. Much of the surface water is perennially supercooled. As less sunlight heats open waters and as cold air streams off the continent, water turns to ice. The fundamental crystal is a hexagon, but it can aggregate in two habits: sometimes hexagon stacks on hexagon to make long filaments and needles of ice, and sometimes hexagons are annexed side-by-side to make plates and disks. Initially, both habits are apparent. But the crystals exist in two states, and two kinds of ice result. Along the sea surface, needles give way preferentially to plates. The heat of fusion released during crystallization locally warms a site, and platy crystals appear between needles. As the platy structure expands, it coats the surface with a filmy grey sheen known as grease ice. Away from the surface, however, no such preference occurs. Ice crystals proliferate into an unstructured slush called frazil ice. The two kinds of ice evolve in different ways and contribute differently to the mass of the pack.

Which ice predominates apparently depends on the role of snow, the formation of polynyas, and the local hydrometeorology. Congelation ice requires a stable environment, which it progressively sheets over. Frazil ice requires open, turbulent waters. It is assisted by winds that break apart the embryonic ice sheeting, by the convective mixing that results from the liberation of brine during surface-ice freezing, and by the persistence of open water like leads and polynyas. Snowfall assists both ice masses, contributing directly to the surface of congelation ice sheets, and it may be vital to the formation of frazil ice by providing suitable nuclei. Depending on the turbulence of the local seas, ice crystals may coat the surface in the form of congelation ice or they may ride like turbid sediment, frazil ice, within subsurface waters. Congelation and frazil ice frequently combine. Where frazil ice forms a dense sludge, congelation ice may grow along the exposed frozen surface; and where congelation ice evolves a well-developed structure, frazil ice may collect in clumps. Yet the two ices are also competitive. The sealing of the surface by congelation ice, for example, prevents snowfall from furnishing new nuclei for frazil ice.

While some pack expansion is attributable to a simple process of freezing along the perimeter, most seems to occur by a process of interstitial freezing between floes. Storms, offshore winds, and ocean currents break up the ice veneer, the protofloes rift outward, and interstitial leads between them freeze. Where open water persists, sea ice—frazil ice in particular—can form in abundance. Polynyas thus coincide, not accidentally, with major centers of ice production. Some polynyas are semipermanent features of the Southern Ocean—in part the product of warm water upwelling and persistent winds. The pack expands not by a process of simple accretion along its margin but by a more complex interaction of winds, water, and ice.

In many places, the pack will consist of about equal portions of congelation ice and frazil ice. In the Ross Sea, however, congelation ice predominates; and particularly in the protected, polynya-free region of McMurdo Sound, where fast ice is abundant, congelation ice is the norm. But in the Weddell Sea, where polynyas persist, between 50 and 90 percent of sea ice consists of frazil ice. Since nearly one-third of the entire Antarctic pack belongs within the Weddell gyre, frazil ice is a significant constituent of the ice field. Frazil ice tends to be more common near the coast (within 30–40 kilometers), where windblown snowfall is more abundant. Whatever the mixture, however, frazil ice will be supplemented by snow and other ices that become incorporated within the general sea ice matrix.

Congelation ice brings structure to pack ice. Initially, the mingling of ice needles and ice plates creates a porous crystalline scaffolding called skeletal ice. Filamentlike crystals branch outward toward patches of water that are characterized by reduced salinity and higher freezing points. This framework thickens and spreads laterally across the sea surface into a sheen of grease ice. The evolution of congelation ice, if unbroken, interferes with the production of subsurface frazil ice and fundamentally redefines the boundary between air and sea. When this evolution is complete, the exchange of mass and energy between atmosphere and ocean ends. In its place, the pack initiates fluxes of salt, water, and heat between the ice and the ocean. Some of these processes involve positive feedback mechanisms, such that the presence of sea ice encourages the further production of sea ice. Snow insulates the ice floes, the ice floes insulate the sea, the chilled boundary of air and sea promotes fog, which further reduces insolation. Storm tracks and ice edge become interdependent. A dense pack increases albedo and reduces mixing. Ice leads to ice.

The salt flux is especially important. When seawater freezes, it liberates salt and releases the latent heat of fusion. The venting of this heat by and large replaces the direct exchange of heat between ocean and atmosphere. The ice crystals themselves hold little salt. Instead salt is extruded into interstitial pores around which further ice crystals form. Its increased salt content lowers the freezing point of the brine, so that the brine pocket does not immediately freeze but becomes mechanically encased by the rapidly emerging ice lattice. As the lattice freezes inward, the brine pocket eventually shrinks. The more rapid the freezing, the more brine is entrapped within the structure and the less homogeneous is the resulting ice lattice. Where frazil ice is abundant, it brings a high proportion of brine to the overall matrix. The more brine, the weaker the ice structure. Meanwhile, the extrusion of brine salts through capillaries and inter-granular discharge channels upsets the density profile of the subsurface waters, and a convective cell develops in the waters beneath the pack.

As a lattice evolves, the random orientation of the initial ice crystals is replaced by a stronger, more columnar framework. Grease ice and skeletal ice thicken and spread into ice paddies that resemble grey lily pads. As the overall structure grows by the erection of more congelation ice, other ices are captured. Frazil ice attaches in globs to the sides and bottom. Snow falls on the surface. When melted and refrozen, it forms lenses of infiltration ice. Other infiltration ice results from the capture of seawater on the surface by spray and wave. Underwater ice may develop in the form of ice stalactites, growing along brine extrusion channels. Anchor ice—frazil ice that collects on the shallow sea floor—may break loose and rise upward into the ice matrix on the surface. Ice flowers may form on the surface as feathery growths of crystals nucleate on salts excreted along freezing ice columns. Other ices—bergy bits, brash ice, the mechanical debris left by colliding floes—become incorporated into the matrix. The structure may even contain an ice biota, a product of the sudden entrapment of brine impregnated with algae and plankton. From all these sources, out of all these processes, emerges a complex ice fabric, a partially stratified ice breccia that is frequently spongy and malleable.

This concatenation of ices eventually elaborates the ice paddy into a tabular slab called pancake ice. As pancake ice butts and jostles, its edges curl upward. Snow and seawater collect inside to produce infiltration ice. Further thickening, freezing, and deformation convert spongy slabs of pancake ice into a hardened floe. The floes multiply into an ice terrane, the pack.

In its shape a floe crudely resembles the platy ice crystals that constitute its microstructure. Floes are roughly equidimensional, 10–100 meters across. Because of constant collisions, their top edges curl up. The thickness of a floe varies with the relative effectiveness of ice production and ice ablation. The floe can ablate from the bottom by melting and from the top by evaporation, sublimation, melting, and wind scour. The resulting equilibrium thickness varies from 2.75 to 3.35 meters. The actual composition and structure of the floe will depend on its unique history.

This internal history will reflect thermal and mechanical metamorphisms. The collision of floe with floe, driven by wind and wave, can mechanically deform a floe. Ice masses can shear, crumple, and override one another to form pressure ridges and ice massifs. While in the Arctic pack such features are common, in the Antarctic they are restricted to local sites where high stress can accumulate. Some mechanical metamorphism occurs in fast ice, where at least one side of the ice mass is rigidly frozen to the shore, and in the oceanic gyres, like the Weddell, where floes are drawn into a crushing spiral. But more commonly the effect of wind and wave is to shove the pack outward rather than in on itself. This tendency accelerates the overall process of pack formation by pushing floes from sites where ice is readily made into more marginal sites and by exposing, under favorable conditions, new seawater for freezing. The ice terrane becomes larger rather than more complex.

Thermal metamorphism takes several forms. If the insulating snow cover is removed, the exposed surface may contract, leading to thermal cracks. It may also melt, allowing meltwater to percolate into the ice lattice, refreeze as infiltration ice, and release more heat that can induce further local melting. Such melting can occur over and over. The permeation of fresh meltwater through the floe flushes salts out of the interior and strengthens the lattice. In the Antarctic, the presence of brine, algae, and plankton within floes sometimes causes insolation to warm the interior of the floe, not merely the surface. This encourages brine migration and expulsion which, in turn, may lead to the formation of undersea ice—ice stalactites—that protrudes downward from the floe. In ideal systems ice accretes on the bottom of the floe and ablates from the top, and a floe will experience several cycles of metamorphism. Multi-year ice preserves not only more mechanical deformation than single-year ice, but more thermal cycling. There is some multi-year fast ice in protected embayments, and sea ice in the Weddell gyre survives perhaps two seasons. But unlike the Arctic, where sea ice is constantly reworked and converted into fresh ice, the Antarctic experiences an annual renewal of virtually the entire pack. Nearly all sea ice dates from the onset of the austral autumn and expires during the austral summer. The awesomeness of the pack derives from its enormous geographic extent, not its history or internal intricacy.

Once embedded within the pack, a floe enjoys a collective identity. Floe interacts with floe, and the pack with the sea, the air, and other ice terranes. The pack has a collective geography and a collective history. Geographically, it is organized by two boundaries, one rigid and one dynamic. Near the continent, where it originates, the pack is bounded by land, ice shelves, and fast ice. As the season matures, free-floating floes move north as an ensemble of diverging shards. Later in the season the ice near the continent freezes solid or moves by means of the slow shearing of floe past floe. On its outer margin, the ice is not so rigidly confined. The dynamics of air and sea demarcate this outer fringe, and these processes vary by season and year. The outermost boundary of the pack lies somewhat inside the Antarctic convergence, which defines the perimeter of the Southern Ocean. The actual shape of the pack depends on storms that roughly follow, but do not precisely mimic, the contours of the ocean. Near-shore currents and winds drive coastal floes eastward, most spectacularly around the gigantic gyre that forms in the Weddell Sea. More distant floes spin within the larger, westward drift of the Antarctic circumpolar current.

The constant pulverization of floes yields ice fragments. Some of these shards are swept under floes, causing them to thicken; some are carried out to sea in clusters and streamers; some are reabsorbed, in the proper season, to make new floes. To this ensemble land ices also contribute. Grounded bergs keep their surrounding near-shore environs cold, divert winds, damp out approaching waves, and ward off warmer water from entering shelves and bays. The presence of land ice thus encourages the formation of sea ice. Some bergs will be frozen in among the pack during the winter. Others, when wind and current urge them, plow through the pack like icebreakers. Their debris—bergy bits and brash ice—may be incorporated into the churning breccia that constitutes the pack.

The pack experiences a life cycle. The rate of progradation varies considerably by year, but on the average the advancing edge of the sea ice moves 4.2 kilometers per day, and the total ice terrane increases by about 100,000 square kilometers per day. In September the pack reaches a maximum extent of 20 million square kilometers. Retrogradation ends when the last of the shore-fast ice breaks out in late January or early February. Sea ice then has a minimum extent of 4 million square kilometers, virtually all in the Weddell Sea. In an average annual cycle nearly 16 million square kilometers of sea ice freeze and melt. But the process fluctuates enormously by year and place. The annual variation is as much as 75 percent of maximum, and individual seas show more variability than does the whole. Each sea features regionally distinctive meteorological and oceanographic processes and, hence, sea ice production. At the same time, there apparently exist some compensatory mechanisms by which the different sectors of the Southern Ocean adjust to one another. In any calculation, however, the Weddell Sea enjoys a commanding role.

Convergence: The Southern Ocean

It is the great vortex and heat sink of the world ocean, and it girdles The Ice like the River Styx. The Southern Ocean works like a slow centrifugal pump that mixes the major oceans of the globe and supplies out of its peculiar ices the bottom waters which layer the abyssal plains of the Pacific, Atlantic, and Indian oceans. Its geography is defined on one side by the ice coastline of Antarctica, with its multiple seas, and on the other by the Antarctic convergence, the mobile interface the Southern Ocean shares with other oceans. Its dynamics are driven by stark contrasts of heat and cold, both oceanographic and atmospheric. Warm bodies move toward the continent and cold bodies away from it. Within these gradients, mixing occurs by means of anastomosing currents that circle the continent, one to the east and one to the west.

Its interior seas are all arrayed along the crenulated coastline of West Antarctica, a mountainous archipelago welded by land ice into a unified subcontinent. The largest, the Weddell and Ross seas, mark the boundary between West Antarctica and East, a true continent. Smaller seas—the Amundsen and the Bellingshausen—trace the rocky outline of the Antarctic Peninsula. The Scotia Sea, a cold Caribbean, extends the peninsula outward to the South Atlantic along an island arc system. Only where it joins West Antarctica does East Antarctica exhibit anything but a uniform coastline of ice, the flange of a great ice dome, varied only by the proportions of land, sea, and fast ice that compose it. The seas show some local currents, but apart from the gigantic gyre of the Weddell Sea, the dynamics of the Southern Ocean are dominated by its circumpolar currents.

Antarctica in relation to the world ocean, Hammer transverse elliptical equal-area projection. Note position of the Antarctic convergence. Redrawn, original courtesy American Geographical Society.

Between them the two circumpolar currents integrate the protected seas that indent West Antarctica, mix and give new identities to the water masses brought across the convergence, and shape the ice terranes of the berg and the pack. The two currents are countervailing: a nearshore current flows east, while an offshore current flows west. Overall, the dynamics of the Southern Ocean are dominated by the clockwise Antarctic circumpolar current (ACC), driven by the prevailing west wind drift. Nearshore flow, however, is controlled by the Antarctic coastal current. Under the impress of easterly winds (east wind drift) the coastal current is rapid and thorough, extending throughout the water column. When this coastal flow encounters deep embayments, like those containing the lesser seas, gyres of varying sizes and intensities are formed. Where the coastal current is shielded from the outer Antarctic circumpolar current the effect is powerful: the Weddell Sea becomes an extraordinary gyre of ices, chilled air, and unprecedentedly cold water.

The two currents inscribe two important oceanographic boundaries. The Antarctic divergence defines the interface between the coastal current and the circumpolar current. The Antarctic convergence segregates the Antarctic circumpolar current from other oceans. Between the counterclockwise Antarctic coastal current and the clockwise Antarctic circumpolar current is a dynamic boundary that is simultaneously oceanographic and meteorological. It demarks not only two opposing oceanic flows but a zone of atmospheric mixing—of semipermanent cyclones—where the prevailing winds shift from easterlies to westerlies. Its subsurface influence extends downward as the Antarctic front.

The Antarctic convergence is a major feature of the world ocean. Here the waters of Antarctica shear against the waters of the neighboring oceans. The effect is both deep and broad. The surface zone, marked by the convergence, corresponds to a subsurface zone, the polar front. The transition is immediately apparent, marked by discontinuities in the properties of adjacent water masses, especially their temperature and density, their flow regimes, and their biology. In fact, the convergence denotes a biotic no less than a hydrographic and atmospheric front. Species rarely cross from one side to the other; even for a given genus, like krill, species occupy one side or the other. The actual boundary is always apparent but never exact. To cross it perpendicularly gives the sense that the convergence is rigidly drawn, but to cross it obliquely reveals a quantum lumpiness to the boundary, a patchiness full of small eddies and clumps of isolated water masses. At least in the Scotia Sea it appears that the boundary encourages the formation of cyclones that break free to be sent north as enclosed rings of cold Antarctic waters. In general, the actual convergence occupies a 100-kilometer belt around a mean position. The polar front, too, is a broken, fluid plane of exchange, full of interweaving cold and warm waters.

The flow of waters into and out of the Southern Ocean occurs on several levels. Some inflow occurs as fresh water discharged from the continent in the form of icebergs. The greatest inflow—known as circumpolar deep water (or warm deep water)—proceeds at intermediate levels of the water column. It is this water mass that the Southern Ocean mixes, transforms, and ejects. As this water mass approaches the continent, it becomes more homogeneous, weakening in the final 100 kilometers and allowing for fuller, deeper convection. The Antarctic front marks its horizontal limit, and in the process the water loses its original identity. Some of the circumpolar deep water combines with fresher surface waters, from melting icebergs and an excess of precipitation over evaporation, to create the Antarctic surface waters, whose boundary coincides with the Antarctic divergence. Some contributes, primarily by mixing with surface waters, to Antarctic intermediate water, which moves north across the polar front. And some contributes, in complex ways, to the formation of Antarctic bottom water, destined for the abyssal plains of the world ocean. Mixing is deep and continuous because the water masses never achieve equilibriums of density or temperature. The salt flux from surface-ice formation, the temperature differences between intermediate and surface waters, turbulence along the boundaries of the strata, and circumpolar flow all result in constant stirring. A stable surface layer never forms.

But not only does Antarctica transform the circumpolar deep waters: they also transform Antarctica. The release of heat brought by the circumpolar deep waters to the region alters circumpolar air masses and helps direct storm tracks. The mixing of Antarctic surface water with circumpolar deep water results in a net loss of heat to the Antarctic atmosphere and a net loss of salt through dilution with fresh water. The upwelled waters bring to the surface high concentrations of nutrients that are in good measure responsible for the phenomenal biotic richness of the Southern Ocean.

This inflow is balanced by an outflow. Most, by volume, takes the form of Antarctic intermediate waters, the product of mixing deep and surface waters. But two water masses above and below these intermediate strata are distinctive to the Southern Ocean, and both are profoundly influenced by the ice terranes with which they interact. Antarctic surface water—lighter, fresher than Antarctic intermediate waters—reflects the presence of icebergs, relatively poor evaporation, and the seasonally important plating of sea ice. Eventually, most of the surface water is reconstituted with deep water to make the Antarctic intermediate waters that are drafted north across the polar front. The mechanism of Antarctic bottom water formation is less well understood but appears to be intimately connected to the presence of persistent ice, both ice shelves and pack ice.

Specifically, the vast proportion of Antarctic bottom water seems to come from the Weddell Sea. 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 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 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 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 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 biology of Antarctica is almost wholly a marine biology. There are several hundred species of mosses and lichens at selected land sites, a few insects and a spider, a handful of higher plants, but these are confined to subantarctic islands, the milder west coast of the peninsula, and far-flung deglaciated oases. Other than a few cryoalgae, who colonize melting snowfields, no organisms live on land ice exclusively; there is no terrestrial cryo-ecosphere. The biota of Antarctica—proverbially productive and exotic—is confined to the Southern Ocean. A much impoverished terrestrial biota thrives best in areas subject to maritime influences. For geologic eons, the continent has been isolated from any land connection, and its ice sheets have been unable to support an indigenous biota. For a while in the Tertiary Period, a land bridge joined the peninsula to South America, much as the Panamanian isthmus now joins the Americas; but this was severed. Unlike Australia, which was also isolated, Antarctica could not sustain terrestrial life on its own. Unlike the Arctic, where seasonally exposed land supports a terrestrial population that can inhabit sea ice or amphibious mammals who can occupy sea and land, the Antarctic lacks organisms who can live off the icescape or who can occupy the interior from the sea. And unlike the Arctic pack, the Antarctic pack is not continuous enough in space and time to weld land mass to land mass. Its biological connections are wholly maritime, sustained by upwelling from the nutrient-rich circumpolar deep waters. Biologically, the continent is a vast, cold, desert island, surrounded by a formidable moat of frigid surface waters.

Milieu of terrestrial and Martian biomes. Note the intermediary status of Antarctic biomes, especially away from the peninsula. Redrawn, original courtesy Smithsonian Institution.

With the exception of far-ranging migratory species, such as some birds and whales, the great proportion of Antarctic species are unique and endemic, confined to the continent or within the convergence. The geography of krill, for example, conforms to the frontal systems of the Southern Ocean. Most krill live within the Antarctic divergence, and the Antarctic convergence segregates the Antarctic from subantarctic species. Similarly, 86 percent of Antarctic coastal fishes are found nowhere else. The isolation and cold of Antarctica—both long-standing environmental parameters—have greatly simplified the Antarctic ecosystem. Marine life in The Ice shows the same traits as the Southern Ocean and the pack. It is mobile and migratory, strongly seasonal, with powerful circumpolar mixing superimposed over regional diversity. Terrestrial life shows an even stronger tendency toward reductionism. There are few species; they experience simple life histories, form discrete populations, and occupy circumscribed sites; their interactions are few and direct. It has been said that Antarctica has the most diminutive continental flora and fauna on the planet. For all this The Ice is responsible. Once ice has claimed an area, there is little opportunity for organisms to recolonize. The species that exist are survivors from the last glaciations.

The pack, however, does provide a matrix for life. It exerts an indirect influence through its effects on the Antarctic atmosphere and the Southern Ocean. But its biotic services are direct, too, and seasonal productivity closely parallels the annual cycle of sea ice. The pack furnishes a necessary platform for many marine mammals and birds. The heaviest concentrations of krill, squid, and fish are somewhat away from the ice-abraded shoreline; the pack furnishes rafts to carry seals and penguins to primary feeding grounds, small islands upon which they may rest, sleep, and even mate. Paradoxically, unlike the Arctic, where the continuous pack provides a platform for humans, the Antarctic pack has been a primary barrier for human movement into and around the Antarctic. In this respect, the sea ice again functions as an insulator, a filter, not between air and sea but between civilization and terra nunquam cognita.

Perhaps most spectacularly, the pack features a special biota of microorganisms. Unlike land ice, sea ice is not a desert. Two communities of organisms actually exist within it. A snow community forms from sea wash collected on the snowy surface of floes. More productive by several orders of magnitude is an epontic community of microorganisms that thrives on the bottom and interior of floes. The sudden freezing of congelation ice traps algae, diatoms, ciliates, and flagellates within a crystalline scaffolding. Other organisms are caught up in brash-ice slush and frazil-ice clumps that are incorporated into the larger floe. Thus there is a surface snow-biota and a subsurface ice-biota. There are ample nutrients for each. Nitrates and phosphates gravitate to the lower strata of floes, where biological productivity is greatest, and microalgae migrate along capillaries and through brine channels within the floe—weakening the structural strength of the floe and tinging the floe with a brownish stain. Incredibly, the density of microalgae populations and the productivity of the ice biota are perhaps greater than in seawater. An ice fauna, in turn, grazes on this ice flora. Ice biotas contribute as much as 20 percent of the total primary production of Southern Ocean biomass. Whether or not the nutrients and biota released by the recession of the pack actually “seed” the phytoplankton bloom that occurs at the same time is undetermined, but this bloom contributes significantly to the seasonal cycle of life.

The trophic hierarchy of Antarctica is comparatively simple. The food chain is characterized by large numbers of a few species, an enormous biomass within a less diverse ecosystem than those typical of temperate or tropic lands. Virtually all of its bird biomass (99 percent) consists of penguins, and they are dominated by one species, the Adelie. Nearly three-fourths of all Antarctic fishes belong to one group, the nototheniformes. The crabeater seal is the most abundant seal in the world; this single species accounts for 85 percent of Antarctic pinnipeds. The baleen whale dominates the Antarctic whale population, the greatest herd in the world ocean. The impressive yields are best explained by an abbreviated food chain. Even compared to the Arctic—with its biotic connections to the land masses of Asia, Europe, and North America—the Antarctic is almost artlessly uncomplicated. The marine ecosystem thus mimics The Ice: great bulk in an equally awesome simplicity. Among the advantages enjoyed by this marine ecosystem, it should be noted that sea temperatures remain relatively constant and that the main seasonal change is restricted to the oscillation of the pack. The tremendous seasonal fluctuations in sunlight and ice cover, however, are vital for controlling the variable productivity of the system. Continental influences act indirectly on the biota through their control over pack ice production.

Phytoplankton consist principally of diatoms and dinoflagellates. Their abundance is legendary. The figures may be exaggerated, but the primary productivity of the system is often estimated to be the richest in the world ocean, perhaps four times greater than anywhere else. Primary production is greatest along the coastal areas, excluding the nearshore environments that are scoured or coated by ice, and amid the coastal seas, the deep embayments that surround West Antarctica. The nutrient-rich cold broth that upwells from the circumpolar deep water, along with possible contributions of phosphate or other minerals from discharged icebergs, accounts for much of this abundance. Accordingly, productivity is most prominent just below the cold surface waters that veneer the nearshore drift, and it shows pronounced geographic and seasonal variations.

But it is the next trophic layer that provides a universal link in the food chain. The euphausiid shrimp known as krill is the only significant connection between the primary producers and all the higher trophic feeders. Like the phytoplankton on which it grazes, krill converges around the Antarctic Peninsula; the Scotia Sea, nourished by the cold waters of the Weddell Sea, is especially favored. But the importance of krill to the circumpolar ecosystem depends also on its mobility. It is found everywhere around the continent, though the heaviest concentrations of Euphasia superba are embedded within the east wind drift of the Antarctic circumpolar current. Krill feeds by migrating in a daily rhythm vertically through the water column, exploiting the upward leaching of nutrients, and it migrates around the continent in vast surface or near-surface swarms, with tons of krill to a swarm. Because of its universal importance, directly or indirectly, to all subsequent trophic levels, krill establishes the basic geography and dynamics of biology in the Antarctic. Life is pelagic, migratory, seasonal, abundant in mass and scanty in variety.

On the krill swarms feed squid and fish; on them feed other fish, birds, and mammals. Although relatively constant in its temperature, the Southern Ocean is bitterly cold. Cold-blooded species, such as fish, adapt by several means to temperatures that would otherwise freeze internal fluids, including the production of several chemical antifreezes that swirl through their body fluids. Warm-blooded species acquire insulating layers, such as blubber or down, by which to retain heat. Virtually all Antarctic birds are pelagic. Some, like the albatross, live and breed outside the pack; others, like penguins, live on the pack; a few, like skuas and penguins, reside at least seasonally along the coastline. But, apart from its penguins—eleven of the eighteen species are present—Antarctica is best known for its marine mammals. There is a fur seal that inhabits subpolar islands and there are true seals—the leopard, crabeater, Weddell, Ross, and elephant—that thrive on the pack. There is a porpoise, the killer whale. And, of course, there are the famed true whales. Whales migrate to the Antarctic during the austral summer, but their abundance is (or was) astonishing. The presence of fur seals and whales first drew humans to the south polar regions.

Humans, of course, are the great anomaly in the Antarctic ecosystem. In some respects—notably their migratory and seasonal habits—they resemble typical Antarctic organisms. But in other ways they are ill-adapted aliens who find the Antarctic as disruptive as the Antarctic biota finds them. They arrived on the continent only in the twentieth century, and they have never become an integral part of the marine ecosystem. They extract from the system, removing organisms to take back to civilization, but they never contribute to the ecosystem’s productivity. Their best adaptations are simply to limit the amount they take from the system, or to substitute for the higher trophic feeders, such as whales, whose numbers they have reduced. It is not that humans cannot cope with polar environments; they have adapted famously to the Arctic. Rather, the peculiar isolation and reductionism of The Ice render its occupation problematical for humans. In coping with The Ice, humans must overcome not only the energy gradient, with its abstraction of accessible food and water, but the information gradient, which strips the region of meaning. By its nature the polar vortex repulses rather than beckons. It drains rather than contributes. By its awesome simplicity The Ice becomes exclusive.

Yet humans have one singular achievement: they have bound the marine ecosystem to those terrestrial ecosystems which humans inhabit elsewhere and, through them, have begun the biotic occupation of the Antarctic continent. No other organism systematically lives on the interior ice sheets. It is precisely because humans need not live off the ice, however, that they can live on it, that they alone have crossed the biotic boundary shielding the lifeless Ice from the living Earth. For the most part, inland from the coastline biological complexity in the Antarctic ceases. It is the peculiar burden and desire of humans—those from certain civilizations—to extend that complexity inward. It is a complexity of information, not merely of ecology. It is not what they find in Antarctica that sustains these humans but what they bring to it and surrender to The Ice.

A Kinetic Art: The Esthetics of the Pack

The pack marks an esthetic no less than a geographic border. In its light, colors, shapes, and motions, the pack defines the ragged transition from landscape to icescape, from Earth art to Ice art. Because it is a vast zone of mixing, the pack is the most active and variously populated ice terrane; the site of the most striking contrasts of sky, sea, light, shape, motion, and ice; the region richest in sensory data, information, and perspectives. Of all the terranes within the Antarctic ice field, the pack is the most complex. It is no accident that so many artists who come to Antarctica confine themselves to the pack, where access is easiest, where life is abundant and exotic, where light effects are varied and subtle, where (if near the coast) mountainous backdrops offer traditional perspective and a reassuring allusion to alpine esthetics.

Much of the variety results from seasonal changes. During midsummer, after the pack has rapidly disintegrated and the fog that normally shadows it has vanished, something resembling a modified seascape is possible. Sunlight plays with storm cloud to illuminate a silver-grey sea, dappled with ripples and swells, ice flakes and floes. The sky is colored with subdued pastels of yellow, grey, and blue, mixed with grey and white cloud. In the distance, bounded by the pack, the horizon is obliterated by a grey fogbank, flaked by white scuds of cloud. Sea and sky—roughly in equal proportion, with a minimum of ice—mirror one another.

With the progradation of the pack, with the deepening of winter, with proximity to the continent, the proportion of ice effects increases. On the seaward fringe of the pack there is not enough ice, while along the shore there is too much. Between these extremes, however, the pack is at its most attractive. There are variations in the objects that populate the scene, in the mixtures of sea, light, cloud, air, and ice, in the choice of perspective. In particular, the appearance of the pack changes dramatically with changes in the character and the distribution of incident light. For much of the year, a veil of snow drizzle, sea smoke, and ice fog envelops the terrane and blocks, distorts, and reflects incoming sunlight. But breaks in the clouds, new orientations of the object to sunlight, and movement of the ice can swiftly recompose and color the same collection of objects.

During the austral summer, sunlight is rarely blocked completely. The cloud deck is low, often consisting of sea smoke or ice fog. Yet there are breaks from time to time, and the dense haze is partially translucent. With or without openings light diffuses through the veil, reduced in intensity and sometimes scattered to bluish hues. Often light is so altered that apparent shadows replace true shadows. It is possible to look to one side of a scene, immersed in diffused sunlight, to discover a virtual whiteout, with heavy pack ice, ghostly icebergs, and finely sprayed fog merged into a single pale luminescence. To the other side, the shadow zone, objects are distinct. Ice floes and bergs gleam iridescently, as if from some inner radiance; the sea glowers in dull black; and the clouds thicken gloomily. In the same way, an iceberg may present a greyish or dull ivory color at one locale, then after passing out of the shadow zone glow a brilliant white.

The surface texture and composition of the ice masses also vary the character of the reflected light. Fresh snow on a floe or berg radiates a dazzling white; meltwater, dull yellow; exposed sea ice, a yellow grey, often lightly stained brown; and exposed glacial ice, blue and white. Just beneath the sea surface, washed free of snow, ice appears green and turquoise. At times, in vigorous sunlight, the ice gleams like whalebone; and in light more strongly filtered and scattered, it is a pale blue. Apertures of blue and turquoise sky may momentarily open in the cloud deck to immerse the surface in brilliant, if localized, lighting effects. Sunlight on ice clouds and ice surfaces induces optical spectacles—haloes, parhelia, arcs—and the surface inversion above the pack promotes mirages. Where clouds reflect open leads, a dark water-sky results. Where the pack is reflected, the yellow-white flush of iceblink brightens the cloying haze.

Seawater, too, exhibits a range of colors as a function of how much incident light is scattered and absorbed. The waters of the Southern Ocean are famously pure, adding little coloration through light scatter from contaminants. When the sun shines directly on open water, the sea appears a dull green-grey. When viewed in thin shadow, the sea seems darkly blue-grey. In deep shadow, with maximum absorption and minimum scatter, the sea is as black as tar. Thus, a composite scene of bergs, floes, and sea may be swallowed up into whitish obscurity when viewed in strong light and low fog; or reveal muted contrasts of shape and design when observed in a shadow zone; or starkly counterpoint black sea and white ice, bluish berg and turquoise streaks of open sky, when seen under a deep or fractured overcast.

The pack may be profusely populated with scenic objects. Small chunks of ice litter the sea like frozen foam. Sea ice floes endlessly change shape, color, and motions, both individually and collectively. Bergs, like prisms, may capture and refract light differently as they raft across the scene. No other ice terrane possesses so many kinds of ices or such a welter of ice masses of different dimensions. Even more, there is life. Penguins and seals adorn floes with color, shape, and movement. Sea ice biotas stain floes. The combinations are endless.

Seen from beneath, the translucent ice suffuses the interacting blue, white, and green light with a subfloe topography punctuated by spires and hummocks. The view is arresting, but too strongly filtered by the ice to hold interest for long. Viewed from above, the pack presents a wonderfully abstract geometry. Its pattern of ruptures and sutures makes an ice embroidery of floes. Their different ages, as they congeal or dissolve, make floes differentially translucent and colored. A melange of dark sea and white ice, of serpentine polynyas and rigid ice polygons, combines the genres of action painting, collage, and abstract expressionism. Where the pack is dense and snow-covered, the white of the ice overpowers the white of the clouds; clouds glide over the icescape like disembodied grey shadows. All these perspectives of the pack, moreover, have in common that they are pictures of surfaces, patterns of colors on a flat canvas of sea and ice. They lack depth, borders. Yet this variety of perspectives is nowhere else approached by the ice terranes of Antarctica.

The pack is most interesting when it exhibits, within a single scene, a good mixture of all its potential effects. Such an ensemble is rare. More typical is the modulation of a given scene by variations in the intensity and the distribution of light. The result is a subtle permutation of color or of apparent composition. There is a darkening or lightening of hues or a reconstitution of ice and sea amid a somber twilight. The scene simplifies into a vast duotone of grey, dull ivory, or pale blue. In dense, overcast pack, sky and sea take on the same color, sheen, and texture. In the absence of motion and color the horizon vanishes into a common smudge of grey. The sparseness of strong light blurs shape as well as color into an immense spectral monotone. Gneisslike bands of slightly lighter or darker greys differentiate the elements of the scene—if anything can. This oppressive uniformity is broken only by the irregular arrangement of ice. White on white, grey on grey, opaque cloud on opaque floe, flat ice on a flat horizon of floe and fog, a collage of white particles on a white surface. Only icebergs, mountainous white shadows, manage to interrupt the scene. Occasionally, a berg of blue ice, still enveloped in mist, captures a shaft of sunlight and gleams like sapphire, an effect both stunning and enchanting—the setting of a blue sun amid a grey twilight.

As autumn deepens and the pack stiffens near the coast, the terrane sheds the variability of light, motion, and objects that makes it attractive. Instead, by a process of subtraction, the scene simplifies and intensifies. More and more is removed. The scene is compressed, like the Antarctic sea and atmosphere, into a shallow surface—a linearity of banded low clouds, floes, and tabular bergs. Perspective lapses into indeterminacy. Composition blurs as borders proceed to infinity and objects lose their mass and structure. Color is erased into whiteout and greyout. Even motion slows. The pack, welding into a continuous sheet, dampens ocean swells. The only movement is the slow freezing of open leads, the grappling of frazil ice to floe, the muted impact of floe upon floe, the stately, imperceptible tread of the bergs—luminous shadows, ice sphinxes, full of grey inscrutability. The pack as a whole may move under the impress of tides and deep ocean swells, but it does so with ponderous undulations, like an earthquake in slow motion. Near the shore, as winter approaches, motion ceases. Ice fragments weld into a rigid mosaic of inertness, a still life painted in white and grey. Then, amidst the polar night, all discriminations are lost in a frozen entropic darkness.

These are the common esthetics of all The Ice. What makes the pack special among the icescapes of Antarctica is its relative abundance and variability, its dramatic mixing of Earth and Ice. Compared to normal landscapes, the pack is esthetically impoverished. What dramatic spectacles it contains appear episodically and mechanically. Compared, however, to the interior icescape—monolithic and seasonally invariant—the pack offers a bewildering ensemble of effects and scenery. Its matrix of fluids—the air and the sea—brings far more mobility and uncertainty to the scenery of the pack than is possible where an ice mass is wholly embedded in solids. There is an element of randomness. Surprise is possible. Accordingly, the pack is most spectacular where it mixes floes, sea, bergs, and broken sky; when the ice grows and moves and storms reshuffle; when the ice is not complete, the fog not total, the sky neither wholly obscured nor utterly open; when there is a certain proportion of light and dark in vivid contrast, not homogenized into a uniform twilight; when some ice surfaces reflect incident light and some ice prisms refract them. White ice abuts black sea, with no gradation between them other than the muted light that diffuses through mixed clouds.

It is an ensemble that mimics the ebb and flow of the polar day, that instantly encapsulates the seasonal progradation and retrogradation of the pack. The contrast—the proportions—accounts for the effectiveness of the scene. No single process or esthetic dominates completely. In the scene’s finest expressions, all are mixed and in motion. Each, by its contrast, heightens the other.

RETROGRADATION

The floes hesitate, suspended in an instantaneous equilibrium of seasons, like the globular clusters of an expanding universe caught at their maximum extent before gravity induces a slow collapse.

Then the retrogradation of the pack begins. Maximum progradation is reached sometime in October, just after the vernal equinox. Now comes the recession. It is not an implosion so much as an erosion of the ragged perimeter, a steady exposure of sea at the expense of ice. The processes that directed the progradation of the pack now guide its retrogradation. The same principles of positive feedback accelerate the trend. What has changed is the general climate under which these processes operate. The polar night favors ice production; the polar day, ice erosion. What makes the recession possible is not simply the removal of old ice but the failure to regenerate new ice. The net balance between ice growth and ice decay tilts toward loss. The areal extent of the Antarctic ice field is halved as the pack retreats inward, drawing back the veil, a disintegrating vortex of ice floes and bergs.

The individual floes ablate in several ways, but surface meltwater, so integral to the destruction of sea ice in the Arctic, has little role in the Antarctic. Winds from the interior polar desert sublimate the surface snow off floes and evaporate any standing meltwater. The katabatics of Antarctica are much drier and 60–100 percent stronger than Arctic winds. Only thin films of water develop over the surface, although melting does occur internally, concentrated around the black bodies of inclusions like microorganisms trapped within the rapidly assembled skeleton of the floe. Instead, open leads between floes take the place of surface meltponds. Winds drive apart those floes along the outer edge, but instead of refreezing, the interstitial leads remain open. The growth of open water reduces the albedo of the pack, encouraging solar heating of open water; warmer water upwells into the pools, promoting further melting of floes; waves penetrate the pack, leading to more thermal decay and mechanical disintegration; icebergs, increasingly mobile in the more fluid matrix of the collapsing pack, plow through floes, widening leads and crushing smaller ice masses. Since storms tend to hover over the circumpolar ring of maximum thermal upwelling, which generally coincides with the perimeter of the pack, the seasonal temperature change brought on by the waxing polar day can initiate the process of retrogradation. The storm belt, in turn, moves inward with the receding pack. Curiously, the pace of the recession outstrips that of the progression.

By February sea ice is at a minimum. Over half the total decay occurs from mid-November to mid-January. For the most part, the pack contracts steadily southward toward the coast. The Weddell Sea is again an exception. Here the pack persists, although it retreats along an axis from east to west. But the collapse is never complete. Some floes linger, mingling with bergs and other ice breccia. Some are trapped in the Weddell Sea gyre, adding another year to their life cycle. Some are caught as fast ice in protected areas of the shore, part of a coastal ice terrane that, unlike the pack, rings the continent with a nearly immobile, quasi-permanent shield. A hybrid of sea ice, snow, and firn, fast ice connects the ice terrane of the pack to the ice terrane of the continent. It is itself a partial metamorphosis of sea ice into land ice, floe to glacial tongue, pack ice to ice shelf.

The processes that shape sea ice and land ice are also responsible for fast ice, with the addition that both sets of processes here interact and their ices intercalate into a unique stratigraphy. The ice begins under the same conditions that generate sea ice. A protected embayment, perhaps sprinkled with grounded bergs; light winds, currents, and tides; and open water (a polynya is ideal) to generate fresh crystals—all lead to an ice matrix and the creation of a sheet of sea ice frozen to coastal ice or land. Anchor ice and frazil ice add to the expansion of the lower stratum. Crystal growth by congelation ice and the incorporation of other ice fragments expands the floe along its margins. Snow, recrystallizing meltwater, and (when the snow weights the floe so that the ice layer falls beneath sea level) percolating seawater all add ice strata to the surface. The saturation of surface snow (and sometimes firn) by seawater, which then freezes, can account for 25–90 percent of the total ice mass. Some seawater represents simple wash, trapped on the surface; but some enters the floe interior from beneath, along fissures that then freeze to form lenses and veins of infiltration ice. Grounded bergs promote fast ice by creating a breakwater that dampens sea and wind, traps snow, cools the adjacent sea-waters. Fast ice, in turn, helps shield the bergs from erosion.

But fast ice also erodes, and the resulting terrane represents a new outcome between the addition of new snows and ices and their subtraction. Open water seasonally erodes the sides and bottoms of the ice, winds and tides contribute to its mechanical disintegration, and winds ablate some surface snows. Solar heating melts other snows, particularly when they are contaminated with dark organic and abiotic debris. The uneven topography of the resulting ice surface leads to differential filling and emptying, an alternating relief of scourings and deposits. This asymmetric erosion is even more intense in the presence of dirty ice. By absorbing more radiation than pure ice does, dirty ice surfaces can erode into fantastic sculpturings, the most exotic shapes in Antarctica. Shore ice that is only seasonally fast breaks out in sudden surges.

In general more ice is formed than removed, and the special circumstances that favor the formation of fast ice also favor its preservation. Protected from ocean swells and chilled by a matrix of land ice—glaciers and bergs both—fast ice persists. It experiences some disintegration, some ablation and breakage. But new ice replaces the old; ices of different composition, origins, and structure combine into a unique amalgamation. Snow cover and frozen sea-slush are underlain by strata of congelation ice and veins of infiltration ice, which are in turn underlain by loosely consolidated frazil ice, congelation ice, anchor ice, and ice stalactites. In place of the annual ontogeny of growth and decay that characterizes individual floes in the open sea or the seasonal waxing and waning that accompany the life cycle of the pack, fast ice acquires an internal structure and a history.

This fragment of sea ice, now firmly fastened to the coast, is all that remains of the mighty pack. Part of an inner, less mobile perimeter, the spotty terrane of fast ice traces a new zone of cold mixing, one that mingles solids rather than fluids. The particle of fast ice thus has symbolic as well as geographic importance. It marks a ragged cryospheric boundary, the first one that is internal to The Ice. Elsewhere land ice is sloughed off into sea ice, to join the phylogeny of the pack. But here land ice replaces or metamorphoses sea ice, and sea ice is transformed into an approximation of land ice. Land replaces sea, land ice displaces sea ice, Ice supersedes Earth.