1 The African Marginal and Enclosed Seas: An Overview
13
Mozambique Channel. The exact causal mechanism and location of the eddy formation are still debated. The frequency of eddy occurrence reduces from around seven
per year in the north of the Channel, to around four per year in the south. Once
the eddies exit the Mozambique Channel, they contribute to the variability of the
Agulhas Current, including the generation of Natal Pulses (meanders of the current
itself) and of Agulhas Rings (eddies shed into the South Atlantic by the current,
past its retroflection site). On the eastern side of Madagascar, the East Madagascar
Current flows southward from 20
◦ S to the island southern limit, and subsequently
feeds the Agulhas Current, also by virtue of large cyclonic and anti-cyclonic eddies
propagating westwards, which induce a strong variability in the current transport.
The Agulhas Current is the real western boundary current of the (southern) Indian Ocean – one the largest in the world’s oceans, as comparable western boundary
currents, i.e. the Gulf Stream in the North Atlantic and the Kuroshio in the North
Pacific, or the Brasil Current in the South Atlantic, are believed to transport much
less water, from approximately one half (Gulf Stream and Kuroshio) to one fourth
(Brazil Current), of the Agulhas – flowing southwestward as a narrow jet over a steep
continental slope, off the east coast of Africa and further south, between 27
◦ S and
40
◦ S. Large solitary meanders, called Natal pulses, can occur within this region.
These extend about 170 km offshore, with downstream propagation rates decreasing
as the continental shelf broadens near 34
◦ S. At around 36
◦ S the Agulhas separates
from the coast, leaves the continental shelf and develops oscillations of increasing
amplitude. In the southeastern Atlantic Ocean, at about 16–20
◦ E, due to shear interactions with the Antarctic Circumpolar Current, eventually it turns back on itself in
the Agulhas Retroflection, forming the Agulhas Return Current and rejoining the
main Indian Ocean Gyre. The retroflection loop usually encloses a pool of Indian
Ocean surface water south of Africa, the temperature of which is more than 5
◦ C
warmer than that of South Atlantic surface water at similar latitudes. While most
of the warm and salty water transported from the tropical zone to the southern tip
of Africa loops around to remain in the Indian Ocean, some leaks into the cooler,
fresher Atlantic Ocean via giant eddies (Agulhas Rings) and filaments. Once in the
Atlantic, the warm, salty Agulhas Leakage eventually flows into the South Atlantic
Gyre and ultimately reaches the northern hemisphere, strengthening the Atlantic
overturning circulation by enhancing deep-water formation. The Agulhas Current
region constitutes a moderately productive ecosystem, with localized, episodic upwelling of colder water in the Mozambique Channel and at the southeastern tip of
Madagascar, and in general on the shoreward edge of the current itself (in particular
where the current passes from a narrow to a wider shelf). As a consequence of these
(and other, poorly sampled and not well understood) factors, the area presents an
enhanced primary productivity in comparison to surrounding waters. This is especially notable in the Agulhas Retroflection, where primary productivity tends to be
significantly higher than that in the surrounding waters of the South Indian Ocean
and South Atlantic Ocean. Conversely, when the current turns back on itself, and the
loop of the retroflection pinches off (in general after the passage of a Natal pulse),
the eddy released into the South Atlantic Gyre is a warm core ring, having lower
primary productivity than the surrounding colder waters. These Agulhas Rings have
13
Mozambique Channel. The exact causal mechanism and location of the eddy formation are still debated. The frequency of eddy occurrence reduces from around seven
per year in the north of the Channel, to around four per year in the south. Once
the eddies exit the Mozambique Channel, they contribute to the variability of the
Agulhas Current, including the generation of Natal Pulses (meanders of the current
itself) and of Agulhas Rings (eddies shed into the South Atlantic by the current,
past its retroflection site). On the eastern side of Madagascar, the East Madagascar
Current flows southward from 20
◦ S to the island southern limit, and subsequently
feeds the Agulhas Current, also by virtue of large cyclonic and anti-cyclonic eddies
propagating westwards, which induce a strong variability in the current transport.
The Agulhas Current is the real western boundary current of the (southern) Indian Ocean – one the largest in the world’s oceans, as comparable western boundary
currents, i.e. the Gulf Stream in the North Atlantic and the Kuroshio in the North
Pacific, or the Brasil Current in the South Atlantic, are believed to transport much
less water, from approximately one half (Gulf Stream and Kuroshio) to one fourth
(Brazil Current), of the Agulhas – flowing southwestward as a narrow jet over a steep
continental slope, off the east coast of Africa and further south, between 27
◦ S and
40
◦ S. Large solitary meanders, called Natal pulses, can occur within this region.
These extend about 170 km offshore, with downstream propagation rates decreasing
as the continental shelf broadens near 34
◦ S. At around 36
◦ S the Agulhas separates
from the coast, leaves the continental shelf and develops oscillations of increasing
amplitude. In the southeastern Atlantic Ocean, at about 16–20
◦ E, due to shear interactions with the Antarctic Circumpolar Current, eventually it turns back on itself in
the Agulhas Retroflection, forming the Agulhas Return Current and rejoining the
main Indian Ocean Gyre. The retroflection loop usually encloses a pool of Indian
Ocean surface water south of Africa, the temperature of which is more than 5
◦ C
warmer than that of South Atlantic surface water at similar latitudes. While most
of the warm and salty water transported from the tropical zone to the southern tip
of Africa loops around to remain in the Indian Ocean, some leaks into the cooler,
fresher Atlantic Ocean via giant eddies (Agulhas Rings) and filaments. Once in the
Atlantic, the warm, salty Agulhas Leakage eventually flows into the South Atlantic
Gyre and ultimately reaches the northern hemisphere, strengthening the Atlantic
overturning circulation by enhancing deep-water formation. The Agulhas Current
region constitutes a moderately productive ecosystem, with localized, episodic upwelling of colder water in the Mozambique Channel and at the southeastern tip of
Madagascar, and in general on the shoreward edge of the current itself (in particular
where the current passes from a narrow to a wider shelf). As a consequence of these
(and other, poorly sampled and not well understood) factors, the area presents an
enhanced primary productivity in comparison to surrounding waters. This is especially notable in the Agulhas Retroflection, where primary productivity tends to be
significantly higher than that in the surrounding waters of the South Indian Ocean
and South Atlantic Ocean. Conversely, when the current turns back on itself, and the
loop of the retroflection pinches off (in general after the passage of a Natal pulse),
the eddy released into the South Atlantic Gyre is a warm core ring, having lower
primary productivity than the surrounding colder waters. These Agulhas Rings have