Benguela Current
Encyclopedia
The Benguela Current is the broad, northward flowing ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

 that forms the eastern portion of the South Atlantic Ocean gyre. The current extends from roughly Cape Point
Cape Point
Cape Point is a promontory at the southeast corner of the Cape Peninsula, which is a mountainous and scenic landform that runs north-south for about thirty kilometres at the extreme southwestern tip of the African continent in the Republic of South Africa. Table Mountain and the city of Cape Town...

 in the south, to the position of the Angola-Benguela Front in the north, at around 16°S. The current is driven by the prevailing South Easterly Trade winds. Inshore of the Benguela Current proper, the south easterly winds drive coastal upwelling, forming the Benguela Upwelling System. The cold, nutrient rich waters that upwell from around 200–300 m depth in turn fuel high rates of phytoplankton growth, and sustain the productive Benguela ecosystem.

Boundaries

Source waters for the Benguela include Indian and South Atlantic subtropical thermocline water; saline, low-oxygen tropical Atlantic water; and cooler, fresher deep water. The Benguela current is 200 to 300 km wide and widens further as it flows north and northwest. Its western, seaward edge is ill-defined, with many temporary and seasonal eddies and meanders. There is however a well-defined thermal front between the waters associated with the Benguela Upwelling System and those of the south east Atlantic.

Where the icy Benguela and the warm, south-flowing Agulhas current
Agulhas Current
The Agulhas Current is the Western Boundary Current of the southwest Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong...

 mix, there is a richly productive marine ecosystem off the Cape of Good Hope
Cape of Good Hope
The Cape of Good Hope is a rocky headland on the Atlantic coast of the Cape Peninsula, South Africa.There is a misconception that the Cape of Good Hope is the southern tip of Africa, because it was once believed to be the dividing point between the Atlantic and Indian Oceans. In fact, the...

 but storms and turbulence above.

Upwelling and Primary Production

Northward winds along the coast result in Ekman transport
Ekman transport
Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman , is the term given for the 90 degree net transport of the surface layer due to wind forcings...

 offshore and upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

 of nutrient rich deep water to the euphotic zone. The intensity of the upwelling event is determined by wind strength. Variations in wind strength cause pulses of upwelling, which propagate to the south along the coast with speeds of 5 to 8 m/s. The pulses are similar to a Kelvin wave
Kelvin wave
A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the...

, except on a scale of 30 to 60 km instead of 1000 km, and can propagate around the cape depending on wind systems.

Pulses of upwelling induce biological production. In the Benguela system, phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 growth requires a period of upwelling followed by a period of stratification and relatively calm waters. The phytoplankton bloom usually lags the upwelling event by 1 to 4 days and blooms for 4 to 10 days. In order for zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

 to have a continuous food supply, the phytoplankton blooms must not occur too far apart. Pulses of upwelling in the Benguela system regularly have a duration of 10 days, an optimal period for biological production. It is estimated that the annual new production in the Benguela system is 4.7 × 10^13 gC/y, making the Benguela system 30 to 65 times more productive per unit area than the global ocean average.

While upwelling promotes abundant primary and secondary production in the upper parts of the water column and near the coast, deeper waters with limited oxygen exchange create hypoxic areas called oxygen minimum zones at the coastal shelf and upper coastal slope. The Benguela oxygen minimum zone starts around a depth of 100 m and is a few hundred meters thick. Bacteria that use sulpher rather than oxygen reside in the oxygen minimum zone.

The most abundant fishes in the Benguela system are Sardinops and Engraulis. Sardinops ocelata (pilchard) was intensely fished beginning in the 1950s and peaking in 1968 with landings over 1.3 million tons. Since then, the Sardinops fishery has declined and the Engraulis capensis (anchovy
Anchovy
Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...

) fishery has taken over.

Benguela Niño

Similar to the Pacific El Niño, a thick slab of warm, nutrient poor water enters the northern part of the Benguela upwelling system off the Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...

 coast about once per decade. During the Benguela Niño, warm, salty waters from the Angola Current
Angola Current
Angola current is a temporary ocean surface current. It is an extension of the Guinea Current, flowing near western Africa's coast. It is known to have created similar effects in the upwelling as El Niño, though its effect is weaker....

 move southward, from 15°S to as far as 25°S. This slab of warm salty water extends to 150 km offshore and to 50 m depth. Heavy rains, changes in fish abundance, and temporal proximity to the Pacific El Niño have been observed; however, the causes and effects of the Benguela Niño are not well understood. One research team has shown that the Benguela Niño is caused by winds in the west-central equatorial Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

that propagate as subsurface sea temperature anomalies to the African coast.
A recent study has demonstrated the importance of local winds in the development of the Benguela Niño off the coast of Namibia and Angola. This local process together with the remote signal from the equatorial regions form the basis of the formation mechanism in which both processes sometimes reinforce each other.

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