OCEANOGRAPHY OF THE SOUTHERN AFRICAN REGION
B
c
Fig. 1 The changing position of the continents according to the theory of sea floor spreading (a) about 225 million years ago (b) 150 - 200 million years ago
(c) 80 - 120 million years ago.
served in water depths greater than 50 m on the continental
shelf between Lake Sibaya and Port Elizabeth. They can
have heights of up to 8 m and wavelengths of 200 m (as seen
off Port Alfred). Some of these dunes are apparently moving southwards, but near Durban they are moving northwards with the inshore countercurrent. A study of the bottom sediments can give information about the dynamics of
the currents that formed them.
South and west of Knysna is a large, flat, submerged
plain, known as the Agulhas Bank. The slope here is only
about 1: 1 000 and the distance from the shore to the 200 m
isobath can be as much as 300 km. Sea level has shown fluctuations on the long geological time scale, and at some
stages the mean sea level was considerably lower than at
present. A tidal range of two metres at a time of depressed
sea level would have meant that the intertidal zone on the
Agulhas Bank would have been some two kilometres wide.
On the west coast, the continental shelf is wider (30 - 180
km) and tends to be deeper at the shelfbreak (300 - 400 m).
The west and south continental shelf areas are of great
economic importance, as it is here that the major fishing as
well as the search for oil and other minerals (e. g. diamonds)
takes place.
In regions deeper than 1 000 m, one can see (Fig. 2) that
there is a variety of structural types. There are large flat
areas (known as abyssal plains) and high "mountainous" regions at the middle of the ocean ( mid-oceanic ridges). In
some places, there are ridges that lead out to the midoceanic ridge (e.g. the Walvis and the Mozambique
Ridges). In addition to these features, there are also isolated pinnacles (called seamounts) that rise thousands of
metres above the abyssal plain. Some 1 000 km WNW of
Cape Town is the Vema Seamount, which rises from a
depth of 5 000 m to within 50 m of the sea surface. For comparison, its base would extend from Cape Town to Cape
Point and it would be nearly 5 times higher than Table
Mountain. The top, slowly eroded by centuries of wave
action, is relatively flat. In 1965 the Vema Seamount was
found to be a crayfish haven, but such was the exploitation
that by 1978 divers could find not a single crayfish! There
were, however, still abundant yellowtail and wreckfish.
CURRENTS IN SOUTH AFRICAN WATERS
To understand the major current systems prevailing in
our waters, we must first discuss the large-scale oceanic
circulation. We may classify three main types of current
systems. (a) Density driven currents, where the indirect
effect of the wind on the surface of the ocean is to set up
different density levels within the ocean and so give rise to
large-scale currents (e.g. western boundary currents like
the Agulhas Current). (b) Directly wind-forced or winddriven current systems, where the surface waters respond to
direct coupling of the air-sea interface (e.g. the upwelling
system). (c) Oscillatory currents, which are generated by
wave-related phenomena (e.g. tidal currents resulting from
gravitational forces of the sun and moon).
Above the south Atlantic ocean the surface winds blow
towards the west near the equator (southeast trades) and towards the east at about 40
0
S latitude (westerlies), as a result
of the differential solar heating of the earth's atmosphere.
We thus find a general anticlockwise (anticyclonic) surface
wind circulation around the so-called semi-permanent Atlantic high-pressure air cell. This wind causes a stress on the
sea surface and indirectly results in surface currents rotating
in the same anti clockwise sense. However, due tq the
earth's curvature and rotation (giving rise to Coriolis
forces) the large-scale currents have an east-west asymmetry. This general argument applies to all major ocean
current systems.
In the Indian Ocean the deep, fast-flowing, narrow
Agulhas Current is one of the strongest in the world, comparable to either the Gulf Stream, Brazil or Kuroshio currents. High speeds are found in the Agulhas Current, the
maximum measured being 13 km/h and average values
about 4-7 km/h. The Agulhas Current is generally uninflu23
B
c
Fig. 1 The changing position of the continents according to the theory of sea floor spreading (a) about 225 million years ago (b) 150 - 200 million years ago
(c) 80 - 120 million years ago.
served in water depths greater than 50 m on the continental
shelf between Lake Sibaya and Port Elizabeth. They can
have heights of up to 8 m and wavelengths of 200 m (as seen
off Port Alfred). Some of these dunes are apparently moving southwards, but near Durban they are moving northwards with the inshore countercurrent. A study of the bottom sediments can give information about the dynamics of
the currents that formed them.
South and west of Knysna is a large, flat, submerged
plain, known as the Agulhas Bank. The slope here is only
about 1: 1 000 and the distance from the shore to the 200 m
isobath can be as much as 300 km. Sea level has shown fluctuations on the long geological time scale, and at some
stages the mean sea level was considerably lower than at
present. A tidal range of two metres at a time of depressed
sea level would have meant that the intertidal zone on the
Agulhas Bank would have been some two kilometres wide.
On the west coast, the continental shelf is wider (30 - 180
km) and tends to be deeper at the shelfbreak (300 - 400 m).
The west and south continental shelf areas are of great
economic importance, as it is here that the major fishing as
well as the search for oil and other minerals (e. g. diamonds)
takes place.
In regions deeper than 1 000 m, one can see (Fig. 2) that
there is a variety of structural types. There are large flat
areas (known as abyssal plains) and high "mountainous" regions at the middle of the ocean ( mid-oceanic ridges). In
some places, there are ridges that lead out to the midoceanic ridge (e.g. the Walvis and the Mozambique
Ridges). In addition to these features, there are also isolated pinnacles (called seamounts) that rise thousands of
metres above the abyssal plain. Some 1 000 km WNW of
Cape Town is the Vema Seamount, which rises from a
depth of 5 000 m to within 50 m of the sea surface. For comparison, its base would extend from Cape Town to Cape
Point and it would be nearly 5 times higher than Table
Mountain. The top, slowly eroded by centuries of wave
action, is relatively flat. In 1965 the Vema Seamount was
found to be a crayfish haven, but such was the exploitation
that by 1978 divers could find not a single crayfish! There
were, however, still abundant yellowtail and wreckfish.
CURRENTS IN SOUTH AFRICAN WATERS
To understand the major current systems prevailing in
our waters, we must first discuss the large-scale oceanic
circulation. We may classify three main types of current
systems. (a) Density driven currents, where the indirect
effect of the wind on the surface of the ocean is to set up
different density levels within the ocean and so give rise to
large-scale currents (e.g. western boundary currents like
the Agulhas Current). (b) Directly wind-forced or winddriven current systems, where the surface waters respond to
direct coupling of the air-sea interface (e.g. the upwelling
system). (c) Oscillatory currents, which are generated by
wave-related phenomena (e.g. tidal currents resulting from
gravitational forces of the sun and moon).
Above the south Atlantic ocean the surface winds blow
towards the west near the equator (southeast trades) and towards the east at about 40
0
S latitude (westerlies), as a result
of the differential solar heating of the earth's atmosphere.
We thus find a general anticlockwise (anticyclonic) surface
wind circulation around the so-called semi-permanent Atlantic high-pressure air cell. This wind causes a stress on the
sea surface and indirectly results in surface currents rotating
in the same anti clockwise sense. However, due tq the
earth's curvature and rotation (giving rise to Coriolis
forces) the large-scale currents have an east-west asymmetry. This general argument applies to all major ocean
current systems.
In the Indian Ocean the deep, fast-flowing, narrow
Agulhas Current is one of the strongest in the world, comparable to either the Gulf Stream, Brazil or Kuroshio currents. High speeds are found in the Agulhas Current, the
maximum measured being 13 km/h and average values
about 4-7 km/h. The Agulhas Current is generally uninflu23