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
