12
V. Barale
Atlantic Ocean. However, this current is heavily influenced by seasonal changes in
atmospheric forcing, and shifts direction with the monsoon winds. In summer, the
Somali Current is fed by the East African Coastal Current and flows to the northeast
along the Somali coast, eventually separating from the coastline and turning east
as it enters the Arabian Sea (and partly re-circulating in the Great Whirl, a large
gyre centered around 10
◦ N and 55
◦ E, present only during summer). As the summer
monsoon moves coastal waters northeastward, it induces coastal upwelling, i.e. the
only major upwelling system that occurs on a western boundary of any ocean. The
upwelled water within the current is carried offshore by Ekman transport and merges
with water that was brought to the surface by open-ocean upwelling, creating one of
the most productive ecosystems in the world’s oceans. In winter, the Somali Current
flows to the southwest along the Somali coast towards the equator, ultimately contributing with the East African Coastal Current to feed the Equatorial Countercurrent
flowing eastward. As the winter monsoon causes a reversal of the Somali current, it
damps coastal upwelling, while surface waters move southwestward. However, this
causes surface cooling and creates deep mixing, bringing abundant nutrients to the
surface.
The South Equatorial Current, flowing westward with the Trades north of 22
◦ S,
divides into a north-flowing stream that feeds the northward East Africa Coastal Current, and a south-flowing stream that feeds the East Madagascar Current. The East
African Coastal Current runs northward throughout the year between 11
◦ S and
3
◦ S. The comparison of transport figures with those in the boundary current north
of Madagascar suggest that most of the water of the South Equatorial Current northern branch goes into the East African Coastal Current. At depth, there is an excess
westward transport north of Madagascar, which probably goes into the Mozambique Channel
1 . In boreal summer, the East Africa Coastal Current merges at its
northern end into the northward Somali Current, while in boreal winter it converges
with the south-going Somali Current to form the Equatorial Countercurrent. The
south-flowing stream of the South Equatorial Current has long been assumed to feed
southward currents on both sides of Madagascar. That assumption can be challenged
by the alternate view of a recurrent train of large anti-cyclonic eddies, rather than
a western boundary current, propagating in the channel, largely dispelling the notion of a steady Mozambique Current within the channel itself. Nevertheless, it is
impossible to rule out completely the likelihood that a true Mozambique Current
may appear intermittently, for short durations
2 . This western boundary anomaly is
possibly due to the presence of Madagascar itself
3 . The Mozambique Channel
Eddies are large cyclonic and anti-cyclonic eddies that propagate southwards in the
1 The Mozambique Channel is a major feature of the African east coast, about 1,600 km long and
400–1,000 km wide, reaching depths of up to 3,000 m.
2 Numerical model simulations of the Mozambique Channel show the appearance of a current along
the Mozambican Coast, during periods between eddies.
3 The surface flow in the southeastern Indian Ocean, if there were no Madagascar, would be a
normal, continuous western boundary current, constituting a continuum with the Agulhas Current.
V. Barale
Atlantic Ocean. However, this current is heavily influenced by seasonal changes in
atmospheric forcing, and shifts direction with the monsoon winds. In summer, the
Somali Current is fed by the East African Coastal Current and flows to the northeast
along the Somali coast, eventually separating from the coastline and turning east
as it enters the Arabian Sea (and partly re-circulating in the Great Whirl, a large
gyre centered around 10
◦ N and 55
◦ E, present only during summer). As the summer
monsoon moves coastal waters northeastward, it induces coastal upwelling, i.e. the
only major upwelling system that occurs on a western boundary of any ocean. The
upwelled water within the current is carried offshore by Ekman transport and merges
with water that was brought to the surface by open-ocean upwelling, creating one of
the most productive ecosystems in the world’s oceans. In winter, the Somali Current
flows to the southwest along the Somali coast towards the equator, ultimately contributing with the East African Coastal Current to feed the Equatorial Countercurrent
flowing eastward. As the winter monsoon causes a reversal of the Somali current, it
damps coastal upwelling, while surface waters move southwestward. However, this
causes surface cooling and creates deep mixing, bringing abundant nutrients to the
surface.
The South Equatorial Current, flowing westward with the Trades north of 22
◦ S,
divides into a north-flowing stream that feeds the northward East Africa Coastal Current, and a south-flowing stream that feeds the East Madagascar Current. The East
African Coastal Current runs northward throughout the year between 11
◦ S and
3
◦ S. The comparison of transport figures with those in the boundary current north
of Madagascar suggest that most of the water of the South Equatorial Current northern branch goes into the East African Coastal Current. At depth, there is an excess
westward transport north of Madagascar, which probably goes into the Mozambique Channel
1 . In boreal summer, the East Africa Coastal Current merges at its
northern end into the northward Somali Current, while in boreal winter it converges
with the south-going Somali Current to form the Equatorial Countercurrent. The
south-flowing stream of the South Equatorial Current has long been assumed to feed
southward currents on both sides of Madagascar. That assumption can be challenged
by the alternate view of a recurrent train of large anti-cyclonic eddies, rather than
a western boundary current, propagating in the channel, largely dispelling the notion of a steady Mozambique Current within the channel itself. Nevertheless, it is
impossible to rule out completely the likelihood that a true Mozambique Current
may appear intermittently, for short durations
2 . This western boundary anomaly is
possibly due to the presence of Madagascar itself
3 . The Mozambique Channel
Eddies are large cyclonic and anti-cyclonic eddies that propagate southwards in the
1 The Mozambique Channel is a major feature of the African east coast, about 1,600 km long and
400–1,000 km wide, reaching depths of up to 3,000 m.
2 Numerical model simulations of the Mozambique Channel show the appearance of a current along
the Mozambican Coast, during periods between eddies.
3 The surface flow in the southeastern Indian Ocean, if there were no Madagascar, would be a
normal, continuous western boundary current, constituting a continuum with the Agulhas Current.