Indian Ocean Coastal Biome
315
is maximal in the central and northern regions of the province from October to March.
Beyond 30
S, the climate and wind-stress pattern is dominated by the westerlies of the
southern oceans, and the passage of cold fronts. Compared to many coastal regions,
seasonal changes in SST are here relatively small, because of the presence close to the
coast from Madagascar to the Cape peninsula of subtropical surface water in the Agulhas
Current.
It will be convenient to discuss the coastal circulation in relation to the four subregions
of this province discussed earlier. The recent review of the coastal oceanography of
southeast Africa of Schumann (1998) has been helpful, but excludes the first and last of
these subregions.
East African Current This is the partially reversing flow along the coast from northern
Kenya to the Madagascar Channel fed by the South Equatorial Current, which meets
the African continent at about 10
S and then diverges north and south throughout the
year. During the boreal summer, the northward flow is continuous with, and enters, the
deep flow of the Somali Current, as this is spun up by the strong Southwest Monsoon
winds (see Arabian Sea Upwelling Region). In boreal winter, the northward coastal flow
of water from the SEC encounters the southward flow of the reversed, slower Somali
Current; the combined flow diverges from the coast at 2–3
S off Malindi and enters the
South Equatorial Countercurrent to pass eastward across the ocean. Some upwelling may
occur at the coast during this process.
Mozambique Channel and Madagascar The flow in the Mozambique Channel is
complex because southward transport along the African coast along the shelf edge is
influenced by a persistent field of mesoscale gyres that lies further east (Saetre and
Jorge da Silva, 1984): much of the flow through the channel is recirculated through
these eddies. Along the remarkably linear east coast of Madagascar, southward flow
occurs during the Northeast Monsoon and feeds into the Agulhas Current through a
persistent field of mesoscale eddies (Lutjeharms, 1988). Part of this may pass directly
into the subtropical gyre by retroflection south of Madagascar. At the southeast tip of
Madagascar, upwelling is induced by the presence of a persistent cyclonic eddy, locked
to shelf topography, and by favorable wind conditions (Lutjeharms et al., 1981). This
feature is intermittent and apparently associated with anomalously high wind stress, as
occurred in February–March 2000, when a surface cold anomaly of 3–5
C was observed
by DiMarco et al. (2000).
Agulhas Current There is no seasonal variation in the strength of the Agulhas Current
for the same reasons as for its larger analogue, the Gulf Stream of the North Atlantic
(Pearce and Gruendlingh, 1982): at the latitudes occupied by these two western boundary
currents, wind stress is transformed into mixing rather than momentum (see Chapter 4).
Agulhas flow originates in the variable circulation at the southern end of the Mozambique
Channel but is an essentially continuous jet to the southern extremity of Africa. About
four eddies each year propagate southward through the Mozambique Channel and pass
into the upstream region of the Agulhas Current (Schouten et al., 2003). As in other
boundary currents, three zones can be distinguished: a coastal zone of cyclonic shear,
a jet of maximum velocities (>25 m sec
−1 ) at the shelf edge, and an offshore zone of
anticyclonic shear. Gradients of velocity and temperature resemble those of the Florida
Current and the Kuroshio. An inshore countercurrent, or recirculation flow, may be
induced in the larger coastal bights (Schumann, 1982). Such flows may be associated
with pulses of vortex shedding (Lutjeharms and Connell, 1989).
The Agulhas Current becomes progressively wider, and the dimensions of associated
plumes and eddies progressively increase, southward (Lutjeharms et al., 1989). Some
315
is maximal in the central and northern regions of the province from October to March.
Beyond 30
S, the climate and wind-stress pattern is dominated by the westerlies of the
southern oceans, and the passage of cold fronts. Compared to many coastal regions,
seasonal changes in SST are here relatively small, because of the presence close to the
coast from Madagascar to the Cape peninsula of subtropical surface water in the Agulhas
Current.
It will be convenient to discuss the coastal circulation in relation to the four subregions
of this province discussed earlier. The recent review of the coastal oceanography of
southeast Africa of Schumann (1998) has been helpful, but excludes the first and last of
these subregions.
East African Current This is the partially reversing flow along the coast from northern
Kenya to the Madagascar Channel fed by the South Equatorial Current, which meets
the African continent at about 10
S and then diverges north and south throughout the
year. During the boreal summer, the northward flow is continuous with, and enters, the
deep flow of the Somali Current, as this is spun up by the strong Southwest Monsoon
winds (see Arabian Sea Upwelling Region). In boreal winter, the northward coastal flow
of water from the SEC encounters the southward flow of the reversed, slower Somali
Current; the combined flow diverges from the coast at 2–3
S off Malindi and enters the
South Equatorial Countercurrent to pass eastward across the ocean. Some upwelling may
occur at the coast during this process.
Mozambique Channel and Madagascar The flow in the Mozambique Channel is
complex because southward transport along the African coast along the shelf edge is
influenced by a persistent field of mesoscale gyres that lies further east (Saetre and
Jorge da Silva, 1984): much of the flow through the channel is recirculated through
these eddies. Along the remarkably linear east coast of Madagascar, southward flow
occurs during the Northeast Monsoon and feeds into the Agulhas Current through a
persistent field of mesoscale eddies (Lutjeharms, 1988). Part of this may pass directly
into the subtropical gyre by retroflection south of Madagascar. At the southeast tip of
Madagascar, upwelling is induced by the presence of a persistent cyclonic eddy, locked
to shelf topography, and by favorable wind conditions (Lutjeharms et al., 1981). This
feature is intermittent and apparently associated with anomalously high wind stress, as
occurred in February–March 2000, when a surface cold anomaly of 3–5
C was observed
by DiMarco et al. (2000).
Agulhas Current There is no seasonal variation in the strength of the Agulhas Current
for the same reasons as for its larger analogue, the Gulf Stream of the North Atlantic
(Pearce and Gruendlingh, 1982): at the latitudes occupied by these two western boundary
currents, wind stress is transformed into mixing rather than momentum (see Chapter 4).
Agulhas flow originates in the variable circulation at the southern end of the Mozambique
Channel but is an essentially continuous jet to the southern extremity of Africa. About
four eddies each year propagate southward through the Mozambique Channel and pass
into the upstream region of the Agulhas Current (Schouten et al., 2003). As in other
boundary currents, three zones can be distinguished: a coastal zone of cyclonic shear,
a jet of maximum velocities (>25 m sec
−1 ) at the shelf edge, and an offshore zone of
anticyclonic shear. Gradients of velocity and temperature resemble those of the Florida
Current and the Kuroshio. An inshore countercurrent, or recirculation flow, may be
induced in the larger coastal bights (Schumann, 1982). Such flows may be associated
with pulses of vortex shedding (Lutjeharms and Connell, 1989).
The Agulhas Current becomes progressively wider, and the dimensions of associated
plumes and eddies progressively increase, southward (Lutjeharms et al., 1989). Some