Chapter 12
The Southern Ocean
T
he Southern Ocean is unique in having no continental barrier to flow eastward
around the globe; only the bottleneck of the Drake Passage between South America
and the Antarctic Peninsula is a constraint, pinching together the components of
the circumglobal current structure but not eliminating them. The lack of land barriers
also permits the westerly winds of the Southern Ocean to build up a wave field having a
mean height greater than anywhere else; this is the windiest and roughest of all oceans.
It goes without saying that poleward of the Polar Front it is subject to seasonal ice cover,
that the Antarctic coastline bears large permanent ice shelves, and also that here the sun
is never very high in the sky. Winter darkness occurs south of the Antarctic Circle, the
dark period increasing poleward at a rate of 40 additional days for each 5
of latitude.
Thus, the region having some winter darkness includes the entire marginal ice zone and
all three marginal seas: Ross, Amundsen, and Weddell.
The Southern Ocean is also unusual in the richness of the ecological studies that have
been performed there. Although it is distant from all oceanographic centers, the lure of
the rich whaling grounds early attracted the attention of biologists. The list of research
ships that have worked the Southern Ocean is a roll call of early oceanography, strikingly
evoked in a chapter by Deacon (1984). I personally relish the fact that Captain Thaddeus
von Bellingshausen, German leader of Tsar Alexander I’s expedition of 1819–1821 aboard
Vostok (East) and Myrni (Peaceful), caused net tows to be made that first demonstrated
the diel vertical migration of zooplankton: some people are still debating the “real”
significance of that behavior today. From then on, there was a cascade of biological
studies, including the Discovery II expeditions organized by the British Colonial Office a
century after Bellingshausen; Alister Hardy’s 1967 account gives the full flavor of these.
The modern voyages of Polarstern show that the tradition of oceanographic research is
alive and well in the Southern Ocean. JGOFS studies, carried by that ship and others in
the 1990s, gave us new insight into these fascinating seas.
Circulation in the Southern Ocean is carried by the Antarctic Circumpolar Current
(ACC), which is forced by exceptionally strong and stable winds; these build up an equally
exceptionally steep wave field that has infinite fetch. The consequent mean flow of the
ACC is sufficiently rapid that Rossby waves appear to propagate eastward (Killworth et al.,
2004); in all other zonal currents, as in the equatorial current system, they propagate
westward against the baroclinic mean flow. The eastward flow of the ACC is tightly
constrained by both deep and shallow topography, though the frontal systems maintain
their integrity even when squeezed through Drake Passage. Sharp poleward turns in
the ACC flow are induced through a gap in the deep Southwest Indian Ocean Ridge
and through fracture zones in the Pacific-Antarctic Ridge, while the Scotia Arc induces
a northward turn. The ACC is strongly sheared and contains three relatively narrow,
meandering jets between which the flow is significantly slower. Each of these current
cores is associated with one or more oceanic fronts, which form prominent, permanent,
443
The Southern Ocean
T
he Southern Ocean is unique in having no continental barrier to flow eastward
around the globe; only the bottleneck of the Drake Passage between South America
and the Antarctic Peninsula is a constraint, pinching together the components of
the circumglobal current structure but not eliminating them. The lack of land barriers
also permits the westerly winds of the Southern Ocean to build up a wave field having a
mean height greater than anywhere else; this is the windiest and roughest of all oceans.
It goes without saying that poleward of the Polar Front it is subject to seasonal ice cover,
that the Antarctic coastline bears large permanent ice shelves, and also that here the sun
is never very high in the sky. Winter darkness occurs south of the Antarctic Circle, the
dark period increasing poleward at a rate of 40 additional days for each 5
of latitude.
Thus, the region having some winter darkness includes the entire marginal ice zone and
all three marginal seas: Ross, Amundsen, and Weddell.
The Southern Ocean is also unusual in the richness of the ecological studies that have
been performed there. Although it is distant from all oceanographic centers, the lure of
the rich whaling grounds early attracted the attention of biologists. The list of research
ships that have worked the Southern Ocean is a roll call of early oceanography, strikingly
evoked in a chapter by Deacon (1984). I personally relish the fact that Captain Thaddeus
von Bellingshausen, German leader of Tsar Alexander I’s expedition of 1819–1821 aboard
Vostok (East) and Myrni (Peaceful), caused net tows to be made that first demonstrated
the diel vertical migration of zooplankton: some people are still debating the “real”
significance of that behavior today. From then on, there was a cascade of biological
studies, including the Discovery II expeditions organized by the British Colonial Office a
century after Bellingshausen; Alister Hardy’s 1967 account gives the full flavor of these.
The modern voyages of Polarstern show that the tradition of oceanographic research is
alive and well in the Southern Ocean. JGOFS studies, carried by that ship and others in
the 1990s, gave us new insight into these fascinating seas.
Circulation in the Southern Ocean is carried by the Antarctic Circumpolar Current
(ACC), which is forced by exceptionally strong and stable winds; these build up an equally
exceptionally steep wave field that has infinite fetch. The consequent mean flow of the
ACC is sufficiently rapid that Rossby waves appear to propagate eastward (Killworth et al.,
2004); in all other zonal currents, as in the equatorial current system, they propagate
westward against the baroclinic mean flow. The eastward flow of the ACC is tightly
constrained by both deep and shallow topography, though the frontal systems maintain
their integrity even when squeezed through Drake Passage. Sharp poleward turns in
the ACC flow are induced through a gap in the deep Southwest Indian Ocean Ridge
and through fracture zones in the Pacific-Antarctic Ridge, while the Scotia Arc induces
a northward turn. The ACC is strongly sheared and contains three relatively narrow,
meandering jets between which the flow is significantly slower. Each of these current
cores is associated with one or more oceanic fronts, which form prominent, permanent,
443