enced by local winds, but severely controlled by bottom topography. For example, the inner boundary of the current
is usually found approximately above the 200 m depth contour and thus close to the shore in places. As the Agulhas
Current flows south and west of Port Elizabeth, a portion of
it tends to leave the coast and turn back eastwards in a Uturn known as a retroflection. Only small amounts of warm,
high-salinity Agulhas water round the Cape and possibly
proceed up the west coast. After the U-turn, some of the
Agulhas Current flows eastwards just north ofthe Subtropical Convergence zone, and executes a number of north- and
southward meanders, largely due to the changing depth of
the sea in this area. The main inputs to the Agulhas Current
are the Mozambique Current (flowing southwards from the
Mozambique Channel) and the East Madagascar Current,
which join just north of Durban. Because the Agulhas Current originates in warm tropical regions and is low in nutrients and oxygen, it does not support as much fish life as
does the Benguela Current system. There is, however, a
greater diversity of species in these warm east -coast waters.
The large-scale circulation in the Indian Ocean is important to marine biologists wishing to understand the drift of
larvae of various marine species. Drift cards and drifting
buoys indicate that it takes about two years for a complete
circulation of the Indian Ocean surface water from Durban past Port Elizabeth and around towards western Australia. Recently, a lifeboat left to the mercy of the currents
near Port Elizabeth arrived 15 months later on an Australian beach. A similar circuit was executed by one of the
satellite-tracked free-drifting buoys. However, if the water
parcel were to be caught up in the Agulhas Return Current,
it would be recycled to Durban in a few months. Countercurrents can also develop inshore of the Agulhas Current
and so transport animals northwards, as happens during the
Natal sardine run.
The cold, sluggish, nutrient-rich Benguela Current off
the western coast of southern African has normal surface
speeds of only 0.15 m/s or about 15 km per day. This eastern
(Atlantic) boundary current differs from the Agulhas Current in that local winds, blowing equatorwards, have a considerable effect on the upper coastal waters. The net effect
at a coastal boundary of these southerly and southeasterly
winds, coupled with the earth's rotational effect (Coriolis
acceleration), is to drive the surface and upper waters
offshore. Colder water from intermediate depths wells up
vertically to adjust the sea level at the coast, replacing the
warm surface water. These deep waters are rich in nitrate
and phosphate nutrients, and the areas of upwelling often
support massive blooms of phytoplankton. These blooms
start the biological food chain with zooplankton feeding on
the phytoplankton, and pelagic filter-feeding fish preying
on the zooplankton. Blooms of phytoplankton can sometimes be seen as brown murky water in the surf zone, especially off the Cape south coast, and occasionally as red
OCEANOGRAPHY OF THE SOUTHERN AFRICAN REGION
water (when there is a little wind or wave activity).
Off the Cape Peninsula the cold upwelled surface water
forms medium-sized plumes in summer, with extremely
sharp temperature increases at the oceanic boundary some
16-30 km offshore. Often warm-water species of fish like
tuna feed at the open ocean cold/warm-water interface.
Cold surface temperatures (as low as 9° or 10 0c) of the west
Cape coastal waters are found soon after severe southeasterly or southerly winds. As these winds are at their
strongest in summer, the coldest surface waters are found in
this region at that time. In winter, the prevailing north-westerly wind causes downwelling and warmer surface water
(15° -16°C) moves inshore. However, off Namibia the prevailing southeasterly winds are steadier in winter than in
summer, resulting in a regular upwelling of cold, nutrientrich water during winter. This, together with the wide continental shelf, is partly responsible for the extremely rich fishing grounds off the west coast of southern Africa.
The West Wind Drift or Antarctic Circumpolar Current,
a slow-moving, deep current, flowing from west to east
around the Antarctic continent, is caused by the atmospheric circulation at these high latitudes. Although this
area is seldom visited by the average fisherman, it may well
become an important fishing ground if the vast stocks of
Antarctic krill (Euphausia superba) become a major protein source.
North of Durban southwards to Port Elizabeth we can define coastal currents as those that occur between the inner
edge of the Agulhas Current and the coastline. There is a
region of high shear between the fast -flowing Agulhas Current and the coastline, and clockwise eddies are generated
here, occasionally giving rise to north-setting currents close
to the shore. There is also considerable onshore-offshore
water movement. This effect is well pronounced near Durban, where there is a widening of the continental shelf.
South of Durban and particularly off the Transkei coast,
the continental shelf is narrow and the coastal currents 5-10
km offshore are almost exclusively south-going, with high
speeds that can be a danger to ships and boats. There have
been numerous reports of high waves off the east coast.
Wave heights as large as 20 m can occur when gale-force
southwesterly winds drive huge seas against the south-flowing Agulhas Current.
The region from Port Elizabeth to Cape Agulhas, where
the Agulhas Current slowly leaves the coast, is one of large
variability. Since the Agulhas Current steers away from the
coastline and follows the edge of the Agulhas Bank, the
water movements on the Agulhas Bank are less affected by
the Agulhas Current than areas close to shore such as near
East London. Consequently, currents on the Agulhas Bank
tend to be much slower (about 0.5 m/s) and are affected by
the wind and tidal forces. Wind-driven currents can generally attain a speed of about 2 percent of the windspeed, thus
a 50 knot wind might drive a current at about 1 knot.
25
is usually found approximately above the 200 m depth contour and thus close to the shore in places. As the Agulhas
Current flows south and west of Port Elizabeth, a portion of
it tends to leave the coast and turn back eastwards in a Uturn known as a retroflection. Only small amounts of warm,
high-salinity Agulhas water round the Cape and possibly
proceed up the west coast. After the U-turn, some of the
Agulhas Current flows eastwards just north ofthe Subtropical Convergence zone, and executes a number of north- and
southward meanders, largely due to the changing depth of
the sea in this area. The main inputs to the Agulhas Current
are the Mozambique Current (flowing southwards from the
Mozambique Channel) and the East Madagascar Current,
which join just north of Durban. Because the Agulhas Current originates in warm tropical regions and is low in nutrients and oxygen, it does not support as much fish life as
does the Benguela Current system. There is, however, a
greater diversity of species in these warm east -coast waters.
The large-scale circulation in the Indian Ocean is important to marine biologists wishing to understand the drift of
larvae of various marine species. Drift cards and drifting
buoys indicate that it takes about two years for a complete
circulation of the Indian Ocean surface water from Durban past Port Elizabeth and around towards western Australia. Recently, a lifeboat left to the mercy of the currents
near Port Elizabeth arrived 15 months later on an Australian beach. A similar circuit was executed by one of the
satellite-tracked free-drifting buoys. However, if the water
parcel were to be caught up in the Agulhas Return Current,
it would be recycled to Durban in a few months. Countercurrents can also develop inshore of the Agulhas Current
and so transport animals northwards, as happens during the
Natal sardine run.
The cold, sluggish, nutrient-rich Benguela Current off
the western coast of southern African has normal surface
speeds of only 0.15 m/s or about 15 km per day. This eastern
(Atlantic) boundary current differs from the Agulhas Current in that local winds, blowing equatorwards, have a considerable effect on the upper coastal waters. The net effect
at a coastal boundary of these southerly and southeasterly
winds, coupled with the earth's rotational effect (Coriolis
acceleration), is to drive the surface and upper waters
offshore. Colder water from intermediate depths wells up
vertically to adjust the sea level at the coast, replacing the
warm surface water. These deep waters are rich in nitrate
and phosphate nutrients, and the areas of upwelling often
support massive blooms of phytoplankton. These blooms
start the biological food chain with zooplankton feeding on
the phytoplankton, and pelagic filter-feeding fish preying
on the zooplankton. Blooms of phytoplankton can sometimes be seen as brown murky water in the surf zone, especially off the Cape south coast, and occasionally as red
OCEANOGRAPHY OF THE SOUTHERN AFRICAN REGION
water (when there is a little wind or wave activity).
Off the Cape Peninsula the cold upwelled surface water
forms medium-sized plumes in summer, with extremely
sharp temperature increases at the oceanic boundary some
16-30 km offshore. Often warm-water species of fish like
tuna feed at the open ocean cold/warm-water interface.
Cold surface temperatures (as low as 9° or 10 0c) of the west
Cape coastal waters are found soon after severe southeasterly or southerly winds. As these winds are at their
strongest in summer, the coldest surface waters are found in
this region at that time. In winter, the prevailing north-westerly wind causes downwelling and warmer surface water
(15° -16°C) moves inshore. However, off Namibia the prevailing southeasterly winds are steadier in winter than in
summer, resulting in a regular upwelling of cold, nutrientrich water during winter. This, together with the wide continental shelf, is partly responsible for the extremely rich fishing grounds off the west coast of southern Africa.
The West Wind Drift or Antarctic Circumpolar Current,
a slow-moving, deep current, flowing from west to east
around the Antarctic continent, is caused by the atmospheric circulation at these high latitudes. Although this
area is seldom visited by the average fisherman, it may well
become an important fishing ground if the vast stocks of
Antarctic krill (Euphausia superba) become a major protein source.
North of Durban southwards to Port Elizabeth we can define coastal currents as those that occur between the inner
edge of the Agulhas Current and the coastline. There is a
region of high shear between the fast -flowing Agulhas Current and the coastline, and clockwise eddies are generated
here, occasionally giving rise to north-setting currents close
to the shore. There is also considerable onshore-offshore
water movement. This effect is well pronounced near Durban, where there is a widening of the continental shelf.
South of Durban and particularly off the Transkei coast,
the continental shelf is narrow and the coastal currents 5-10
km offshore are almost exclusively south-going, with high
speeds that can be a danger to ships and boats. There have
been numerous reports of high waves off the east coast.
Wave heights as large as 20 m can occur when gale-force
southwesterly winds drive huge seas against the south-flowing Agulhas Current.
The region from Port Elizabeth to Cape Agulhas, where
the Agulhas Current slowly leaves the coast, is one of large
variability. Since the Agulhas Current steers away from the
coastline and follows the edge of the Agulhas Bank, the
water movements on the Agulhas Bank are less affected by
the Agulhas Current than areas close to shore such as near
East London. Consequently, currents on the Agulhas Bank
tend to be much slower (about 0.5 m/s) and are affected by
the wind and tidal forces. Wind-driven currents can generally attain a speed of about 2 percent of the windspeed, thus
a 50 knot wind might drive a current at about 1 knot.
25