The difficulty lies in the
several ocean currents that converge around the tip of Africa. These include
Mosambique Current, South Equatorial Current, East Madagascar Current, and the
Agulhas Current—all along the southeast coastal area of Africa. These currents
slam into the Angola Current, creating the subtropical convergence and the
Aguilhas Retroflection Region.
The sources of this strong
Agulhas Current are the East Madagascar Current, creating 25 Sverdrups (1
Sverdrup is a flow rate of 1 million cubic meters of water per second), and the
Mozambique Current, creating 5 Sverdrups, and a reticulated part of the Agulhas
Current, itself creating 35 Sverdrups. Thus, the net transport of the Agulhas
Current is estimated at 100 SV. Sverdrups, or SV, is a measurement of the
transport of ocean currents (the entire global input of fresh water from rivers
to the ocean is equal to about 1 sverdrup).
This Agulhas Current
follows the east African subcontinental shelf from Maputo to the tip of the
Agulhas Bank, or Cabo das Agulhas (Cape of Needles), where the swiftly south
flowing current slams into the western African currents stemming from the
Angola current, a strong current running south along the western African
coast—this latter cuirrent was the one early Portuguese sailors used to reach the
southern tip of Africa. Of course these early sailors, in their coastal
vessels, lost many ships trying to pass through the turbulent Agulhas
Retroflection Region off the coast of Cape Town as far east as Port Elizabeth.
At this point, the momentum
of the current overcomes the vorticity balance (spin, or the angular rate of
rotation) holding the current to the topography and the current leaves the
shelf. In this region of the southeast Atlantic Ocean the current retroflects
(turns back on itself) in this Agulhas Retroflection due to shear interactions
with the strong Antarctic Circumpolar Current (often called the West Wind Drift
driven by the Prevailing Westerlies). This, in turn, becomes the Agulhas Return
Current, which rejoins the Indian Ocean Gyre. It is estimated that up to 85 SV
of the net transport is returned to the Indian Ocean through the retroflection.
This means that a
wind-driven vessel sailing on these currents southward along the east coast of
Africa would be retroflected (turned back) into either the South Equatorial
Current or the Circumpolar Current, or the West Wind Drift, moving eastward
across the Southern Ocean, away from an Atlantic Crossing.
Although no mention of the
Agulhas Current survives from his first voyage, during the second attempt at
rounding South Africa in 1497, Vasco de Gama wrote into the ship’s logs about a
southward current near Algoa Bay (Port Elizabeth) of such strength that the
flotilla was set steadily back for three days. It took nearly fifty years, but
by the mid 1500s, the Portuguese knew enough about the Agulhas Current to
remain far out to sea as they rounded the African Horn on the way to India,
picking up the Agulhas Return Current, which rejoins the Indian Ocean Gyre.
The sharp rocks and reefs
offshore of the cape were often described as needles by early Portuguese
seafarers, which combined with the treacherous currents to claim many ships,
becoming known among these sailors as the “Graveyard of Ships.” This Agulhas
Current is the western boundary current of the South Indian Ocean. Its greatest
source of water is recirculation in the southwest Indian Ocean sub-gyre,
causing temporal and latitudinal variations in the depth, path, and transport
of the current, and is considered around the tip of South Africa one of the
major Rogue Wave Zone where freak monstrous waves occur with regularity, making any east to west rounding of the Cape not only treacherous, but almost impossible for early Portuguese sailors two thousand years after Lehi sailed.
(See the next post, “Driven Forth Before the Wind – The Rogue Wave Zone off South Africa,” where early sailors often whispered of monster waves when ships mysteriously sunk but, until now, no one quite believed them)
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