11
the two wind systems the upper layer divergence is compensated by upwelling (the
Antarctic Divergence). A major flow of this upwelled water extends northward
as far as the Antarctic Convergence or Polar Front (Mann and Lazier 2006). The
Antarctic Convergence, which encircles Antarctica roughly 1,500 km off the coast,
divides the colder and fresher southern water masses and the warmer and saltier
northern waters; creating the largest pelagic boundary of the world ocean (Sournia
1994). Based on water mass properties within the Antarctic Circumpolar Current
(ACC) three major transitions are apparent, referred to as the Subantarctic Front,
the Polar Front, and the Southern ACC Boundary (Orsi et al. 1995). Changes in sea
surface height determined from satellite altimeter indicate each of these fronts is in
fact formed by three coherent fronts (Sokolov and Rintoul 2009). The most conspicuous open ocean fronts are those formed at the transitions between the poleward extensions of warm-salty western boundary currents (e.g. the Gulf Stream;
Kuroshio; Agulhas and Brazil currents) and cold-less saline subpolar waters. As
the ACC deflects northward downstream of Drake Passage the Subantarctic Front
penetrates northward in the South Atlantic and nearly merges with the Subtropical
Front creating even more intense surface gradients. These fronts are characterized
by strong frontal jets and strong surface temperature, salinity and nutrient gradients,
and are frequently associated with intense eddies and meanders that developed by
instabilities of the mean flow (Fig. 1.1).
2.7 Frontal Eddies
Eddies and large-scale meanders are ubiquitous features of the ocean circulation and naturally emerge from instabilities of the mean flow. There are several
classes of rings or eddies in the ocean, originated by different forcing and covering a range of spatial and temporal scales; we consider here just one type: the
frontal eddies. They contain pockets of moving water that break off from the main
body of a front and can travel independently, covering long distances before dissipating. Eddies are commonly found in the vicinity of faster flowing currents
that form intense fronts with the surroundings waters, such as the Gulf Stream,
the Kuroshio Current, the Brazil Current, the Agulhas Current and the Antarctic
Circumpolar Current. Strong currents meander in a wave-like fashion and become
unstable; these flow instabilities lead to pinching off of relatively warm or cold
waters that act as a seed for frontal eddies. The water within such eddies has temperature and salinity characteristics different from the surrounding waters. Frontal
eddies can take the shape of warm-core (masses of warm water turning within
colder ocean waters) or cold-core (masses of cold water within warmer waters)
eddies (Fig. 2.3c). Eddies nearly always contain embedded frontal interfaces, and
like other frontal types, embody mechanisms by which the physical energy of the
ocean system can be converted to trophic energy to support biological processes.
Recent high resolution observations and numerical models also indicate that relatively short lived (~1 day) submesoscale structures (1–10 km) may significantly
2.6 Fronts Associated with the Convergence …
the two wind systems the upper layer divergence is compensated by upwelling (the
Antarctic Divergence). A major flow of this upwelled water extends northward
as far as the Antarctic Convergence or Polar Front (Mann and Lazier 2006). The
Antarctic Convergence, which encircles Antarctica roughly 1,500 km off the coast,
divides the colder and fresher southern water masses and the warmer and saltier
northern waters; creating the largest pelagic boundary of the world ocean (Sournia
1994). Based on water mass properties within the Antarctic Circumpolar Current
(ACC) three major transitions are apparent, referred to as the Subantarctic Front,
the Polar Front, and the Southern ACC Boundary (Orsi et al. 1995). Changes in sea
surface height determined from satellite altimeter indicate each of these fronts is in
fact formed by three coherent fronts (Sokolov and Rintoul 2009). The most conspicuous open ocean fronts are those formed at the transitions between the poleward extensions of warm-salty western boundary currents (e.g. the Gulf Stream;
Kuroshio; Agulhas and Brazil currents) and cold-less saline subpolar waters. As
the ACC deflects northward downstream of Drake Passage the Subantarctic Front
penetrates northward in the South Atlantic and nearly merges with the Subtropical
Front creating even more intense surface gradients. These fronts are characterized
by strong frontal jets and strong surface temperature, salinity and nutrient gradients,
and are frequently associated with intense eddies and meanders that developed by
instabilities of the mean flow (Fig. 1.1).
2.7 Frontal Eddies
Eddies and large-scale meanders are ubiquitous features of the ocean circulation and naturally emerge from instabilities of the mean flow. There are several
classes of rings or eddies in the ocean, originated by different forcing and covering a range of spatial and temporal scales; we consider here just one type: the
frontal eddies. They contain pockets of moving water that break off from the main
body of a front and can travel independently, covering long distances before dissipating. Eddies are commonly found in the vicinity of faster flowing currents
that form intense fronts with the surroundings waters, such as the Gulf Stream,
the Kuroshio Current, the Brazil Current, the Agulhas Current and the Antarctic
Circumpolar Current. Strong currents meander in a wave-like fashion and become
unstable; these flow instabilities lead to pinching off of relatively warm or cold
waters that act as a seed for frontal eddies. The water within such eddies has temperature and salinity characteristics different from the surrounding waters. Frontal
eddies can take the shape of warm-core (masses of warm water turning within
colder ocean waters) or cold-core (masses of cold water within warmer waters)
eddies (Fig. 2.3c). Eddies nearly always contain embedded frontal interfaces, and
like other frontal types, embody mechanisms by which the physical energy of the
ocean system can be converted to trophic energy to support biological processes.
Recent high resolution observations and numerical models also indicate that relatively short lived (~1 day) submesoscale structures (1–10 km) may significantly
2.6 Fronts Associated with the Convergence …