32
K. WYRTKI:
The Antarctic surface water is cold and of low salinity because of excessive precipitation and melting ice. Salinites are generally below 34.0%0 and temperatures near freezing
in winter. During the summer months, a shallow, slightly warmer layer forms at the
surface, leaving a temperature minimum near 100 m depth, a remnant of the last winter
water. This cold low-salinity water is pressed north by the prevailing west winds, where
it meets the warmer waters of higher salinity in the Antarctic polar front. Along this front,
which stretches all the way around Antarctica, and where strong currents flow to the east,
the 2 water masses mix intensively. The mixing products sink down under the lighter
water to the north, and this process is aided by the winds. This sinking water forms the
layer of the Antarctic intermediate water, which spreads north as the uppermost water
mass in the cold water sphere. Simultaneously the continuous removal of the mixing
products by sinking keeps the front strong and well marked. In the western Indian Ocean
the polar front is near 48 a S, but shifts progressively farther south, and to the south of
Australia it lies near 53 a S. This position appears to be rather stable, and seasonal and
irregular fluctuations do not seem to move it more than 2 degrees of latitude in either
direction from its mean position, demonstrating the great stability of the large-scale
baroclinic structure of the oceans.
The water forming the main oceanic thermocline under the subtropical gyre is generated between the polar front and the subtropical convergence in the transition area by sinking, deep convection and downward sliding along surfaces of constant density. These
processes may at various seasons and wind conditions affect water of different properties
and lead to the renewal of different strata. Since all this water derives from the sea surface
and is of high oxygen content, an oxygen maximum forms within the main oceanic
thermocline at temperatures between 10 a and 12
0
C.
The Antarctic Circumpolar Current ist the strong geostrophic current associated with
the baroclinic structure resulting from the surfacing of the main oceanic thermocline.
Although this structure is most intense in the upper 1000 m, the slope of lines of equal
density continues down to the bottom. Geostrophic transports calculated relative to 3000
decibars are about 120 megatons sec-I, making the Circumpolar Current by far the most
powerful current in the oceans. The fact is that the path of this current is influenced by the
bottom topography, and recent measurements of flow near the bottom make it likely that
transports may be even higher (REID and NOWLIN, 1971). The strongest surface flow lies
to the north of the polar front where the meridional slope of the sea surface is strongest.
Beside the polar front, 2 other frontal systems are associated with the Circumpolar
Current, the subtropical convergence and the Antarctic divergence. The subtropical convergence, discussed earlier, is the northern boundary of the transition water. It coincides
approximately with the northern limit of strong westerly winds, and can be taken as the
northern boundary of the Circumpolar Current. The Antarctic divergence, found in the
Indian Ocean near 65
0
S, is near the southern boundary of the prevailing west winds or
along the ice edge. There, west winds cause a divergence and ascending movements by
which nutrient-rich subsurface water (Fig. 2) is brought into the surface layer. The upwelling water is also of higher salinity than the Antarctic surface water. The relations of
these frontal systems to the Circumpolar Current and to the wind conditions in summer
and winter have been explained by WYRTKI (1960), who showed a strong dependence of
the development of convergences and divergences on the changing wind system.
The circumpolar water is situated below the Antarctic surface water of low salinity.
K. WYRTKI:
The Antarctic surface water is cold and of low salinity because of excessive precipitation and melting ice. Salinites are generally below 34.0%0 and temperatures near freezing
in winter. During the summer months, a shallow, slightly warmer layer forms at the
surface, leaving a temperature minimum near 100 m depth, a remnant of the last winter
water. This cold low-salinity water is pressed north by the prevailing west winds, where
it meets the warmer waters of higher salinity in the Antarctic polar front. Along this front,
which stretches all the way around Antarctica, and where strong currents flow to the east,
the 2 water masses mix intensively. The mixing products sink down under the lighter
water to the north, and this process is aided by the winds. This sinking water forms the
layer of the Antarctic intermediate water, which spreads north as the uppermost water
mass in the cold water sphere. Simultaneously the continuous removal of the mixing
products by sinking keeps the front strong and well marked. In the western Indian Ocean
the polar front is near 48 a S, but shifts progressively farther south, and to the south of
Australia it lies near 53 a S. This position appears to be rather stable, and seasonal and
irregular fluctuations do not seem to move it more than 2 degrees of latitude in either
direction from its mean position, demonstrating the great stability of the large-scale
baroclinic structure of the oceans.
The water forming the main oceanic thermocline under the subtropical gyre is generated between the polar front and the subtropical convergence in the transition area by sinking, deep convection and downward sliding along surfaces of constant density. These
processes may at various seasons and wind conditions affect water of different properties
and lead to the renewal of different strata. Since all this water derives from the sea surface
and is of high oxygen content, an oxygen maximum forms within the main oceanic
thermocline at temperatures between 10 a and 12
0
C.
The Antarctic Circumpolar Current ist the strong geostrophic current associated with
the baroclinic structure resulting from the surfacing of the main oceanic thermocline.
Although this structure is most intense in the upper 1000 m, the slope of lines of equal
density continues down to the bottom. Geostrophic transports calculated relative to 3000
decibars are about 120 megatons sec-I, making the Circumpolar Current by far the most
powerful current in the oceans. The fact is that the path of this current is influenced by the
bottom topography, and recent measurements of flow near the bottom make it likely that
transports may be even higher (REID and NOWLIN, 1971). The strongest surface flow lies
to the north of the polar front where the meridional slope of the sea surface is strongest.
Beside the polar front, 2 other frontal systems are associated with the Circumpolar
Current, the subtropical convergence and the Antarctic divergence. The subtropical convergence, discussed earlier, is the northern boundary of the transition water. It coincides
approximately with the northern limit of strong westerly winds, and can be taken as the
northern boundary of the Circumpolar Current. The Antarctic divergence, found in the
Indian Ocean near 65
0
S, is near the southern boundary of the prevailing west winds or
along the ice edge. There, west winds cause a divergence and ascending movements by
which nutrient-rich subsurface water (Fig. 2) is brought into the surface layer. The upwelling water is also of higher salinity than the Antarctic surface water. The relations of
these frontal systems to the Circumpolar Current and to the wind conditions in summer
and winter have been explained by WYRTKI (1960), who showed a strong dependence of
the development of convergences and divergences on the changing wind system.
The circumpolar water is situated below the Antarctic surface water of low salinity.