The Benguela Current is a cold, northward-flowing current along the Atlantic coast of southern Africa, running from the Cape of Good Hope up past Namibia to about the latitude of southern Angola. It is the eastern boundary current of the South Atlantic subtropical gyre, and it drives what is generally regarded as the most intense coastal upwelling on Earth: the upwelling cell off Luderitz in southern Namibia is stronger than any comparable feature in the Humboldt, Canary or California systems. That upwelling supports large sardine, anchovy and hake fisheries, though the Namibian sardine stock collapsed in the 1970s and has never recovered. The cold water is also responsible for the Namib Desert immediately inland — one of the oldest deserts in the world — and for the dense coastal fog that sustains the Namib's remarkable specialised fauna and gave the Skeleton Coast its name and its shipwrecks. Uniquely among the four great upwelling systems, the Benguela periodically releases enormous plumes of hydrogen sulphide from the seabed, visible from orbit and lethal to fish.
Cold Water Against a Desert Shore
The configuration is the familiar one for eastern boundary upwelling: a subtropical high pressure system offshore, persistent equatorward winds along a north-south coast, and Coriolis deflection driving surface water away from land so that deep water rises to replace it. In the Southern Hemisphere the deflection is to the left of the wind, which for a southerly wind on a west-facing coast means offshore.
What makes the Benguela extreme is the strength and persistence of the wind. The South Atlantic high sits reliably offshore, and the pressure gradient against the heated interior of southern Africa keeps the alongshore wind blowing hard for most of the year. Around Luderitz the coastline orientation and the wind align almost perfectly, and upwelling there runs essentially continuously rather than seasonally.
The result is water at 12 to 16 °C along a tropical and subtropical coast, and a surface layer stuffed with nitrate and phosphate. Sea surface temperatures off Walvis Bay in summer are lower than off Cape Town further south, which is the sort of inversion that only an upwelling system produces.
The Most Productive of the Four
Measured as carbon fixed per unit area, the Benguela outperforms the other eastern boundary systems, and the fisheries built on it have been correspondingly large — and correspondingly volatile.
The system supports two distinct sub-systems separated by a permanent upwelling cell at Luderitz that acts as an environmental barrier. The northern Benguela, off Namibia, and the southern Benguela, off South Africa, behave almost as separate ecosystems with separate stocks and separate management.
The northern Benguela carries the cautionary tale. Namibian sardine landings peaked at well over a million tonnes in the late 1960s under heavy and largely unregulated distant-water fishing, then collapsed. Half a century later the stock has not returned. What has happened instead is an ecosystem reorganisation: the niche the sardines occupied has been substantially taken over by jellyfish and by the bearded goby, a small fish that tolerates very low oxygen and can shelter in water that excludes competitors. That combination supports far less commercially valuable biomass and far fewer seabirds, and it appears to be a stable alternative state rather than a stage on the way back. The southern Benguela retained more of its structure but has seen its sardine and anchovy stocks shift progressively eastward around the Cape, away from the west coast fishing towns that were built to catch them.
Sulphur Eruptions and the Dead Layer
Extreme productivity has an extreme consequence here, more so than in any of the comparable systems.
The organic matter produced at the surface sinks, and on the broad Namibian shelf it accumulates in sediments that are among the richest in organic carbon anywhere on the seafloor. Decomposition consumes all available oxygen, and once oxygen is gone, sulphate-reducing bacteria take over and produce hydrogen sulphide. Over time, enormous quantities of it build up in the sediment and in the water immediately above.
Periodically it is released. The eruptions can be triggered by methane bubbling up from below, by disturbance, or simply by accumulation exceeding what the sediment can hold, and they can be very large: plumes tens of kilometres across, turning the water milky turquoise as the sulphide oxidises to elemental sulphur, and clearly visible in satellite imagery. Where such a plume reaches the coast the smell is severe, and fish and crustaceans in its path are killed outright. Mass strandings of rock lobster driven ashore trying to escape low-oxygen water have wiped out significant fractions of Namibia's commercial stock in single events.
The same low-oxygen conditions compress the habitable water column, as they do in the Humboldt, and they are part of why the bearded goby has done so well: it can survive excursions into water almost nothing else tolerates.
Fog, Shipwrecks and the Namib
Warm desert air meeting cold sea produces advection fog on a spectacular scale, and along this coast that fog is the difference between life and no life.
The Namib receives almost no rain — parts of it under 20 mm a year — but fog rolls inland on well over a hundred nights a year, penetrating tens of kilometres. An extraordinary set of organisms has evolved to harvest it. The fog-basking beetle stands head-down on dune crests at dawn so condensation runs down its body to its mouth. The dune grass Stipagrostis and the bizarre living-fossil plant Welwitschia mirabilis, which can live over a thousand years, both take fog moisture directly. Web-footed geckos, sidewinding adders and dune-dwelling spiders all depend on it. The desert's fauna is not surviving despite the fog; it exists because of it.
The same fog wrecked ships. The Skeleton Coast combines dense fog, a heavy southwesterly swell breaking on a shallow shelving beach, a strong northward current, and — behind the beach — several hundred kilometres of waterless dune. Vessels driven ashore were rarely recoverable, and crews who reached the beach faced a desert. The name refers both to the whale bones left by nineteenth-century whalers and to the wrecks and the people who did not get out. The Portuguese reportedly called it the sands of hell.
The Current That Sorts Diamonds
The Benguela does one thing no other upwelling current is known for: it grades and concentrates gemstones.
Diamonds eroded from kimberlite pipes in the South African interior have been carried west by the Orange River for tens of millions of years and delivered to the Atlantic at its mouth. Once in the sea, they entered a conveyor. The northward-flowing Benguela and the heavy southwesterly swell moved the sediment up the coast, and the process acted as an enormous natural sorting machine: the surf and the current winnowed out lighter material and left the dense, extremely hard diamonds behind, concentrated in gravel traps, beach terraces and gullies in the bedrock.
The consequence is that the coastline north of the Orange River mouth holds some of the richest marine diamond deposits in the world — and, crucially, deposits of unusually high quality, because the journey destroyed anything flawed. Only the toughest stones survive several hundred kilometres of surf, so what remains is disproportionately gem-grade.
This produced one of the more unusual mining geographies anywhere. The stretch of Namibian coast south of Luderitz was closed off as the Sperrgebiet, the forbidden territory, from 1908, and remained one of the most tightly restricted areas on Earth for most of a century, with the diamond town of Kolmanskop eventually abandoned to the dunes. Recovery today includes onshore beach mining behind sea walls built to hold the Atlantic back, and offshore vessels working the seabed in water tens of metres deep. The Namib and its coast are, in this respect, a product of the current twice over: the fog that sustains its life and the sorting that concentrated its wealth come from the same cold water.
Benguela Ninos and the Angolan Front
At its northern end the Benguela runs into warm tropical water flowing south from Angola, and the boundary between them — the Angola-Benguela Frontal Zone, around 15 to 17°S — is one of the sharpest thermal fronts in the Atlantic.
That front moves. Every few years it shifts unusually far south, pushing warm, nutrient-poor, low-oxygen tropical water down over the productive northern Benguela in an event known as a Benguela Nino. The effects mirror a Pacific El Nino on a smaller scale: upwelling delivers warm water instead of cold, productivity crashes, fish stocks disperse southward or die, and seabird colonies fail. Major events in 1984 and 1995 caused severe losses to the Namibian fishery.
The mechanism is partly remote — equatorial Atlantic wind anomalies propagating east and then south along the coast as waves trapped against the continental margin — and partly local. It makes the northern Benguela one of the more difficult systems in the world to manage, because the stock assessment can be invalidated within a season by an event that originated a thousand kilometres away. On its southern flank, meanwhile, the system is bounded by an entirely different kind of current: the warm, fast Agulhas, which sweeps down the Indian Ocean side of Africa and occasionally leaks around the Cape into the Atlantic.