Indian Ocean and Australian sectors.
(^) The density-driven meridional flows are associated with ascent of temperature and
salinity isopleths toward the south, their rise occurring in steps. Bands of horizontal
density gradients alternate with zones (fronts) with steep slopes of density isopleths,
which are thus the cores of strongest current. These fronts divide the region into zones
distinguished by temperature, nutrient availability, and biota. The most important are
the polar front (PF) at ∼50°S, and the subantarctic front (SAF) at ∼46°S (for both at
the Greenwich meridian east of the Weddell Sea; Orsi et al. 1995). Both fronts are
associated with sea-surface temperature rises south to north. However, the vertical rise
of density isopleths extends quite generally from the SAF to 60°S, associated with the
very strong, very deep flow of the ACC. Density isopleths are flatter and temperature
is more uniform from the SAF out to the subtropical convergence (STC) at ∼40°S.
The STC stays close to 40°S, except for swinging south around Tasmania and New
Zealand, then north to almost 30°S as it crosses the Pacific toward Chile.
(^) As the concentric rings of flow approach Drake Passage (Mar de Hoces) between
South America and the Palmer Peninsula, part of the more northern, subantarctic flow
peels off to the north, becoming the Humboldt Current. The rest of the flow and the
frontal sequence compress through the gap. At many other points, the flow and thus
the fronts are subject to topographic steering over submarine ridges and plateaus,
causing the concentric rings to oscillate across latitudes. There are topographically
steered excursions of the SAF toward the north to the east of New Zealand, at the
Crozet Islands and Kerguelen Plateau. In winter, sea ice forms and solidifies
progressively northward, reaching ∼60°S (varying ∼5° longitudinally, also
interannually), then it melts back toward the south in summer, with at least open leads
reaching the outer shelf waters around much of the continent: in the Bellingshausen
Sea, along the west side of Palmer Peninsula, and the northern reaches of the
permanently (at least up to now) frozen Ross and Weddell Seas.
Antarctic Waters to the Edge of Seasonal Ice
(^) In the Antarctic, in the seasonal ice zone, the upwelling of nutrient-rich deep water
under the receding ice provides copious major nutrients. There is a phase in which
epontic algae living in frazil (loose needle-shaped crystals) on the undersurface of the
ice are abundant, followed by a strong diatom bloom in the open meltwater as the
melt proceeds southward. Dust accumulated on the ice over winter provides iron.
Meltwater provides stability and critical depths are well below the mixing depth,
enabling blooms of >10 mg Chl m−3). Diatom production is sufficiently prodigious
that formation of opal sediments in this circumglobal band constitutes the principal
removal term in the dissolved silicate budget of the world ocean (Nelson et al. 1995).
However, Nelson et al. also maintain that the input to sediment of diatom silica in this