316
Chapter 10: The Indian Ocean
meanders are persistent in location, as off Cape St. Francis at 34
S, where an episodic
meander may form and attain a wavelength 100–150 km. This then propagates downstream at ∼15 km per day (Swart and Gonzalves, 1983). Some mesoscale eddies may be
shed by meanders at the seaward boundary of the current at any latitude, but they are
most frequent in the terminal area south of South Africa.
Agulhas Bank and Retroflection Retroflection occurs where the western boundary
current runs out of topography at the southern extremity of Africa and encounters the
eastward flow of the circumpolar zonal currents. This region is difficult to place in our
system of provinces and is located here simply because its ecological effects are continuous
with those on the Agulhas Bank. Otherwise, it might be better considered as part of the
South Subtropical Convergence (SSTC) Province because the flow from the retroflection
(see Color plate 13) feeds directly into the easterly jet current flowing along 40
S within
the core of the SSTC as the South Indian Ocean Current into the zonal circumpolar
current system (Stramma, 1992).
Eddies, originating in the “Natal pulse” at about 30
S, are shed as they pass around the
retroflection loop and normally follow the general flow and pass eastward to the south
of the Agulhas Return Current. However, some of the eddies shed from meanders on the
western flank of the Agulhas pass into the South Atlantic at the origin of the Benguela
Current system flow (Lutjeharms and van Ballegooyen, 1988). These are warm-core and
anticyclonic in form, and their frequency is about four to five cycles per year, the location
of shedding varying between years.
Although local forcing is different in each region of this province, the seasonal changes
in the depth of the mixed layer follow two patterns: (i) in the northern part of the
province, during the monsoon period of June–September (note, here this is a Southeast
Monsoon), the pycnocline of the Indian Ocean tilts down westward, a motion that
results in deepening from about 40–60 m to about 80–100 m along to the coast north
of 25
S. Of course, this is also the austral winter so that, (ii) further to the south, wind
stress and heat loss at the surface combine to deepen the mixed layer to about 100 m.
Consequently, over the Agulhas Bank in winter, mixing penetrates to 65–75 m above a
weak thermocline/nitracline, whereas in summer, stratification is established at around
30–35 m coincident with a strong nitracline (McMurray et al., 1993). There is evidence
(see later discussion) of episodic occurrences of coastal upwelling along the coast between
Port Elizabeth and Mossel Bay, perhaps especially during summer.
Regional Response of the Pelagic Ecosystem
A few ecological observations, made at 5–6
S, which should be typical of the whole of
the East Africa Bight (0–10
S) were reviewed by McClanahan (1988). During the period
of boreal summer monsoon winds (here largely from the southeast) of May–September,
irradiance at the sea surface falls from 520 to ∼380 langleys, surface temperature falls
from 29.5 to 26
C, and wave height and rainfall are much increased. Salinity is minimal
(34‰) in the same period, corresponding to the discharge cycle of the Tana and Sabaki
rivers. The nutrient regime over the shelf is largely controlled by wind mixing, which
delivers nutrients to the surface layers (Newell, 1959). Sediments and deep shelf waters are
relatively nutrient-rich, although this is a new phenomenon caused by current farming
practices in adjacent watersheds and consequent massive erosion.
In this region, water transparency is highest and chlorophyll biomass minimal just
prior to the Northeast Monsoon, when primary production is maximal and large-celled
algae are most diverse. Stability is strongest in the water column at this time, although the
seasonal changes are not large. A reduction in chlorophyll and numbers of larger algal cells,
and generally lower abundance of zooplankton, is associated with the deepening of the
Chapter 10: The Indian Ocean
meanders are persistent in location, as off Cape St. Francis at 34
S, where an episodic
meander may form and attain a wavelength 100–150 km. This then propagates downstream at ∼15 km per day (Swart and Gonzalves, 1983). Some mesoscale eddies may be
shed by meanders at the seaward boundary of the current at any latitude, but they are
most frequent in the terminal area south of South Africa.
Agulhas Bank and Retroflection Retroflection occurs where the western boundary
current runs out of topography at the southern extremity of Africa and encounters the
eastward flow of the circumpolar zonal currents. This region is difficult to place in our
system of provinces and is located here simply because its ecological effects are continuous
with those on the Agulhas Bank. Otherwise, it might be better considered as part of the
South Subtropical Convergence (SSTC) Province because the flow from the retroflection
(see Color plate 13) feeds directly into the easterly jet current flowing along 40
S within
the core of the SSTC as the South Indian Ocean Current into the zonal circumpolar
current system (Stramma, 1992).
Eddies, originating in the “Natal pulse” at about 30
S, are shed as they pass around the
retroflection loop and normally follow the general flow and pass eastward to the south
of the Agulhas Return Current. However, some of the eddies shed from meanders on the
western flank of the Agulhas pass into the South Atlantic at the origin of the Benguela
Current system flow (Lutjeharms and van Ballegooyen, 1988). These are warm-core and
anticyclonic in form, and their frequency is about four to five cycles per year, the location
of shedding varying between years.
Although local forcing is different in each region of this province, the seasonal changes
in the depth of the mixed layer follow two patterns: (i) in the northern part of the
province, during the monsoon period of June–September (note, here this is a Southeast
Monsoon), the pycnocline of the Indian Ocean tilts down westward, a motion that
results in deepening from about 40–60 m to about 80–100 m along to the coast north
of 25
S. Of course, this is also the austral winter so that, (ii) further to the south, wind
stress and heat loss at the surface combine to deepen the mixed layer to about 100 m.
Consequently, over the Agulhas Bank in winter, mixing penetrates to 65–75 m above a
weak thermocline/nitracline, whereas in summer, stratification is established at around
30–35 m coincident with a strong nitracline (McMurray et al., 1993). There is evidence
(see later discussion) of episodic occurrences of coastal upwelling along the coast between
Port Elizabeth and Mossel Bay, perhaps especially during summer.
Regional Response of the Pelagic Ecosystem
A few ecological observations, made at 5–6
S, which should be typical of the whole of
the East Africa Bight (0–10
S) were reviewed by McClanahan (1988). During the period
of boreal summer monsoon winds (here largely from the southeast) of May–September,
irradiance at the sea surface falls from 520 to ∼380 langleys, surface temperature falls
from 29.5 to 26
C, and wave height and rainfall are much increased. Salinity is minimal
(34‰) in the same period, corresponding to the discharge cycle of the Tana and Sabaki
rivers. The nutrient regime over the shelf is largely controlled by wind mixing, which
delivers nutrients to the surface layers (Newell, 1959). Sediments and deep shelf waters are
relatively nutrient-rich, although this is a new phenomenon caused by current farming
practices in adjacent watersheds and consequent massive erosion.
In this region, water transparency is highest and chlorophyll biomass minimal just
prior to the Northeast Monsoon, when primary production is maximal and large-celled
algae are most diverse. Stability is strongest in the water column at this time, although the
seasonal changes are not large. A reduction in chlorophyll and numbers of larger algal cells,
and generally lower abundance of zooplankton, is associated with the deepening of the