Benguela Current
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The Benguela Current is the broad, northward flowing
ocean current An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, sh ...
that forms the eastern portion of the
South Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...
Ocean gyre. The current extends from roughly
Cape Point Cape Point () is a promontory at the southeast corner of the Cape Peninsula, a mountainous and scenic landform that runs north-south for about thirty kilometres at the extreme southwestern tip of the African continent in South Africa. Table M ...
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 depth in turn fuel high rates of
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
growth, and sustain the productive Benguela ecosystem.


Boundaries

Source waters for the Benguela include cold upwelled waters from the depths of the
Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...
close inshore, joined further off-shore by nutrient poor water that has crossed the Southern Atlantic from South America as part of South Atlantic Gyre. Eddies from the warm South Indian Ocean Agulhas current along South Africa's east coast come round the
Cape of Good Hope The Cape of Good Hope ( ) is a rocky headland on the Atlantic Ocean, Atlantic coast of the Cape Peninsula in South Africa. A List of common misconceptions#Geography, common misconception is that the Cape of Good Hope is the southern tip of Afri ...
from time to time to join the Benguela current. The Benguela current is 200 to 300 km wide and widens further as it flows north and northwest. Its western, seaward edge is well-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 eastward flowing Atlantic currents which are not deflected northward by the African continent. The icy Benguela and the warm, south-flowing Agulhas current do not meet off the Cape of Good Hope (see diagram on the right, above), but there is a body of water off the South African south coast, east and particularly west of
Cape Agulhas Cape Agulhas (; , "Cape of Needles") is a rocky headland in Western Cape, South Africa. It is the geographic southern tip of Africa and the beginning of the traditional dividing line between the Atlantic and Indian oceans according to the In ...
that consists of eddies from both currents, so that offshore water temperatures along the south coast of Africa vary chaotically.


Upwelling and primary production

Northward winds along the coast result in
Ekman transport Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman transport is a component of wind-driven ocean current. Ekman transport occurs w ...
offshore and
upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...
of nutrient-rich deep water to the euphotic zone. The intensity of the upwelling event is determined by wind strength. Variations in wind strength result in 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 A Kelvin wave is a wave in the ocean, a large lake or the 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-d ...
, 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 () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
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 are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...
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
biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
production in the Benguela system is 4.7× 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 Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Work ...
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 sulphur rather than oxygen, called sulphur-reducing bacteria, reside in the oxygen minimum zone. The most abundant fish in the Benguela system are '' Sardinops'' and '' Engraulis''. The Southern African pilchard (''S. s. ocellatus''), was intensely fished in the 1950s and peaked in 1968 with landings over 1.3 million tons. Since then, fishery of the ''Sardinops'' has declined and fishery of the Southern African anchovy (''Engraulis capensis'') has taken over.


Benguela Niño

Similar to the Pacific
El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...
, a thick slab of warm, nutrient-poor water enters the northern part of the Benguela upwelling system off the
Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...
coast about once per decade. During the Benguela Niño, warm, salty waters from the Angola Current move southward, between 15°S and 25°S. This slab of warm salty water extends to offshore and to 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 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.


See also

*
Cape Peninsula The Cape Peninsula () of South Africa is a generally mountainous peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At the southern end of the peninsula are Cape Point and the Cape of Good ...
*
Cape Point Cape Point () is a promontory at the southeast corner of the Cape Peninsula, a mountainous and scenic landform that runs north-south for about thirty kilometres at the extreme southwestern tip of the African continent in South Africa. Table M ...
* Humboldt Current, the Benguela's analogue in the South Pacific Ocean


References


External links


Benguela current

Hydrogen Sulfide and Dust Plumes along the Coast of Namibia - Earth Observatory
(NASA) August 10, 2010 {{Authority control Marine ecoregions Currents of the Atlantic Ocean Geography of the Southern Ocean Temperate Southern Africa