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
