30
K. WYRTKI:
Arafura Seas through the Timor Sea. With the onset of the NE monsoon, when the intertropical convergence lies south of Java, the South Equatorial Current deviates southward
from Java, and a coastal current to the east develops along the south coast of Java. It
is an extension of the Equatorial Countercurrent and carries low salinity and warm water
into the area.
The Agulhas Current is the western boundary current of the Indian Ocean and the
strongest part of the Agulhas Current system, which is rather complex and somewhat
different from other western boundary currents. This current system draws its water
essentially from the South Equatorial Current through the Madagascar Channel and to
the south of Madagascar from a rather narrow and swift current flowing south along its
east coast. In the Madagascar Channel a strong, apparently permanent anticyclonic
geostrophic eddy is developed, intensifying flow to the south along the coast of Africa and
causing northward flow along Madagascar.
The Agulhas Current itself is a narrow high-speed flow along the SE coast of South
Africa. Between Durban and Port Elizabeth it flows close to the shore, has average velocities of 1 m sec- 1 and often exceeds 2 m sec- 1 at its core. The current is strongly barocline,
but since parts of it are over the continental shelf and the high-speed core is often over the
continental slope, geostrophic computations of its transports are difficult to make.
Computations and estimates by DUNCAN (1970) give transports as high as 80 megatons
sec- 1 , making the Agulhas Current the strongest western boundary current of the southern
hemisphere, although average transports are more like 50 megatons sec- 1 • The current
reaches great depths and its baroclinic structure is apparent in hydrographic sections
deeper than 2000 m, Fig. 6. When reaching the Agulhas Bank and the longitude of Cape
Agulhas the current turns south and east in a sharp anticyclonic eddy and forms the
Agulhas Return Current. The Agulhas Current and Agulhas Return Current form the
large elongated Agulhas eddy, which is a permanent feature of the circulation and stretches
along the coast approximately 300 km offshore. The center of this eddy is filled with
warm, high-salinity subtropical surface water, which with salinities of 35.4%0 extends
down to 500 m depth (Fig. 6). From this analysis it appears that the Agulhas Current and
the Agulhas Return Current are the geostrophic flow around a thick tongue of light subtropical water, which presses southwestward from the subtropical gyre into the wedge
between the African continent and the Circumpolar Current. This situation may also
explain the strong variability of the Agulhas Current and the frequent shedding of eddies
from the main anticyclonic flow. These eddies drift west into the Atlantic Ocean. There
is no direct and continuous flow of subtropical water from the Indian to the Atlantic
Ocean.
At the confluence of the Agulhas Return Current and the west wind drift the subtropical
convergence forms a rather pronounced front near 41
0
S. However, the Agulhas Return
Current soon broadens, one branch turns north and recycles water into the Agulhas
Current, the other branch continues east, but widens into a wind drift. Over the central
part of the ocean, where surface flow usually has a northward component, the subtropical
convergence is not well marked, especially not during summer, when a shallow summer
thermocline develops. Convergent movements near 40 0 S may temporarily occur, when
strong west winds drive cooler water from the more temperate regions to the north and
force it to sink below the warmer subtropical water. During the winter, when strong
cooling leads to vertical convection, a deep uniform mixed layer develops, which can
exceed 400 m in depth. Under favorable wind conditions, considerable sinking must occur
K. WYRTKI:
Arafura Seas through the Timor Sea. With the onset of the NE monsoon, when the intertropical convergence lies south of Java, the South Equatorial Current deviates southward
from Java, and a coastal current to the east develops along the south coast of Java. It
is an extension of the Equatorial Countercurrent and carries low salinity and warm water
into the area.
The Agulhas Current is the western boundary current of the Indian Ocean and the
strongest part of the Agulhas Current system, which is rather complex and somewhat
different from other western boundary currents. This current system draws its water
essentially from the South Equatorial Current through the Madagascar Channel and to
the south of Madagascar from a rather narrow and swift current flowing south along its
east coast. In the Madagascar Channel a strong, apparently permanent anticyclonic
geostrophic eddy is developed, intensifying flow to the south along the coast of Africa and
causing northward flow along Madagascar.
The Agulhas Current itself is a narrow high-speed flow along the SE coast of South
Africa. Between Durban and Port Elizabeth it flows close to the shore, has average velocities of 1 m sec- 1 and often exceeds 2 m sec- 1 at its core. The current is strongly barocline,
but since parts of it are over the continental shelf and the high-speed core is often over the
continental slope, geostrophic computations of its transports are difficult to make.
Computations and estimates by DUNCAN (1970) give transports as high as 80 megatons
sec- 1 , making the Agulhas Current the strongest western boundary current of the southern
hemisphere, although average transports are more like 50 megatons sec- 1 • The current
reaches great depths and its baroclinic structure is apparent in hydrographic sections
deeper than 2000 m, Fig. 6. When reaching the Agulhas Bank and the longitude of Cape
Agulhas the current turns south and east in a sharp anticyclonic eddy and forms the
Agulhas Return Current. The Agulhas Current and Agulhas Return Current form the
large elongated Agulhas eddy, which is a permanent feature of the circulation and stretches
along the coast approximately 300 km offshore. The center of this eddy is filled with
warm, high-salinity subtropical surface water, which with salinities of 35.4%0 extends
down to 500 m depth (Fig. 6). From this analysis it appears that the Agulhas Current and
the Agulhas Return Current are the geostrophic flow around a thick tongue of light subtropical water, which presses southwestward from the subtropical gyre into the wedge
between the African continent and the Circumpolar Current. This situation may also
explain the strong variability of the Agulhas Current and the frequent shedding of eddies
from the main anticyclonic flow. These eddies drift west into the Atlantic Ocean. There
is no direct and continuous flow of subtropical water from the Indian to the Atlantic
Ocean.
At the confluence of the Agulhas Return Current and the west wind drift the subtropical
convergence forms a rather pronounced front near 41
0
S. However, the Agulhas Return
Current soon broadens, one branch turns north and recycles water into the Agulhas
Current, the other branch continues east, but widens into a wind drift. Over the central
part of the ocean, where surface flow usually has a northward component, the subtropical
convergence is not well marked, especially not during summer, when a shallow summer
thermocline develops. Convergent movements near 40 0 S may temporarily occur, when
strong west winds drive cooler water from the more temperate regions to the north and
force it to sink below the warmer subtropical water. During the winter, when strong
cooling leads to vertical convection, a deep uniform mixed layer develops, which can
exceed 400 m in depth. Under favorable wind conditions, considerable sinking must occur