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