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Chapter 10: The Indian Ocean
Eastern Africa Coastal Province (EAFR)
Extent of the Province
EAFR comprises the coastal boundary of the Indian Ocean from Zanzibar (5
S) to
the Cape of Good Hope, including the Mozambique Channel and the east coast of
Madagascar. The seaward, eastern limit of the province is the edge of significant flow in
the western boundary currents (Colborn: 33 and parts of 27 and 30–32). For convenience,
I include the Agulhas Retroflection, south of Africa. Were this province to be subdivided
for other purposes into more natural regions, the following would be a suitable primary
classification: (i) the equatorial East African coastline, (ii) the subtropical Madagascan
region, (iii) the temperate Agulhas region, and (iv) the somewhat offshore Agulhas
Retroflection. Sufficient differences could be found in their ecologies to support such an
arrangement.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelves along most of the eastern coast of Africa are rather narrow and
the slope is steep. Where there are fringing reefs, as in the bight between Tanga and
Dar-es-Salaam (4–7
S) and the adjacent islands of Pemba and Zanzibar, and also along
parts of the NE and SE coasts of Madagascar, the coral fronts stand above deep water
and rather close to the shoreline.
However, in the wide bight (15–24
S) along the coast of Mozambique, the shelf is
wider so that at Beira the break of slope is almost 150 km offshore. On the east coast
of Madagascar, it lies about 100 km offshore. But the greatest area of shelf is the flat,
triangular Agulhas Bank which lies between the Cape Peninsula (19
E) and Port Elizabeth
(26
E) and reaches 180-km width south of Cape Agulhas. Several major rivers drain onto
the shelf of this province, including the Zambezi just north of Beira at 18
S, the fourth
river of Africa, and Kipling’s “great, green, greasy Limpopo” at the southern end of the
Mozambique Channel near Lorenço Marques at 25
S.
The tidal regime is dominated by the fact that the amphidromal point for the M 2
tide lies south of Cape Town; tidal amplitudes and the velocity of tidal streams increase,
therefore, northward along the coastline. At Beira, the tidal amplitude may attain 6 m,
and here the small-scale topography of the sediments suggests that tidal currents are more
important than currents forced by processes over the deep ocean. As elsewhere, coastal
processes determine the sedimentary regime in each locality. Major river discharges of
organic material lead to a substrate with a high silt content, the presence of reef corals
leads to white, highly calcareous sediments and, otherwise, sands together with shell
fragments will dominate.
Defining Characteristics of Regional Oceanography
Processes over continental shelves are everywhere directly influenced by local patterns
of weather and wind stress at the sea surface. In this province, these factors respond to
the seasonal, latitudinal progression of the atmospheric intertropical convergence zone
(ITCZ) of generally easterly flow that lies across the northern Madagascar Channel in
January, whereas during austral winter the ITCZ shifts into the northern hemisphere.
Thus, during austral summer the Northeast Monsoon winds of the Arabian Sea penetrate
down to at least 15
S over Mozambique and northern Madagascar. South of this latitude
during austral summer, and over the entire province in winter, winds coming off the
southern Indian Ocean predominate, because of a persistent high-pressure cell that lies
southeast of Madagascar. In austral winter/boreal summer, this wind is continuous with
the Southwest Monsoon of the Arabian Sea. Rainfall is associated with this situation and
Chapter 10: The Indian Ocean
Eastern Africa Coastal Province (EAFR)
Extent of the Province
EAFR comprises the coastal boundary of the Indian Ocean from Zanzibar (5
S) to
the Cape of Good Hope, including the Mozambique Channel and the east coast of
Madagascar. The seaward, eastern limit of the province is the edge of significant flow in
the western boundary currents (Colborn: 33 and parts of 27 and 30–32). For convenience,
I include the Agulhas Retroflection, south of Africa. Were this province to be subdivided
for other purposes into more natural regions, the following would be a suitable primary
classification: (i) the equatorial East African coastline, (ii) the subtropical Madagascan
region, (iii) the temperate Agulhas region, and (iv) the somewhat offshore Agulhas
Retroflection. Sufficient differences could be found in their ecologies to support such an
arrangement.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelves along most of the eastern coast of Africa are rather narrow and
the slope is steep. Where there are fringing reefs, as in the bight between Tanga and
Dar-es-Salaam (4–7
S) and the adjacent islands of Pemba and Zanzibar, and also along
parts of the NE and SE coasts of Madagascar, the coral fronts stand above deep water
and rather close to the shoreline.
However, in the wide bight (15–24
S) along the coast of Mozambique, the shelf is
wider so that at Beira the break of slope is almost 150 km offshore. On the east coast
of Madagascar, it lies about 100 km offshore. But the greatest area of shelf is the flat,
triangular Agulhas Bank which lies between the Cape Peninsula (19
E) and Port Elizabeth
(26
E) and reaches 180-km width south of Cape Agulhas. Several major rivers drain onto
the shelf of this province, including the Zambezi just north of Beira at 18
S, the fourth
river of Africa, and Kipling’s “great, green, greasy Limpopo” at the southern end of the
Mozambique Channel near Lorenço Marques at 25
S.
The tidal regime is dominated by the fact that the amphidromal point for the M 2
tide lies south of Cape Town; tidal amplitudes and the velocity of tidal streams increase,
therefore, northward along the coastline. At Beira, the tidal amplitude may attain 6 m,
and here the small-scale topography of the sediments suggests that tidal currents are more
important than currents forced by processes over the deep ocean. As elsewhere, coastal
processes determine the sedimentary regime in each locality. Major river discharges of
organic material lead to a substrate with a high silt content, the presence of reef corals
leads to white, highly calcareous sediments and, otherwise, sands together with shell
fragments will dominate.
Defining Characteristics of Regional Oceanography
Processes over continental shelves are everywhere directly influenced by local patterns
of weather and wind stress at the sea surface. In this province, these factors respond to
the seasonal, latitudinal progression of the atmospheric intertropical convergence zone
(ITCZ) of generally easterly flow that lies across the northern Madagascar Channel in
January, whereas during austral winter the ITCZ shifts into the northern hemisphere.
Thus, during austral summer the Northeast Monsoon winds of the Arabian Sea penetrate
down to at least 15
S over Mozambique and northern Madagascar. South of this latitude
during austral summer, and over the entire province in winter, winds coming off the
southern Indian Ocean predominate, because of a persistent high-pressure cell that lies
southeast of Madagascar. In austral winter/boreal summer, this wind is continuous with
the Southwest Monsoon of the Arabian Sea. Rainfall is associated with this situation and