Tuesday, July 10, 2012

Driven Forth Before the Wind – The Agulhas Current Southeast Africa

As some Book of Mormon historians and scholars have suggested, the Lehi Colony sailed down the east Coast of Africa, around Port Elizabeth and Cape Town, then across the Atlantic to Central America. However, there is a distinct problem sailing around the tip of Africa in a wind-driven vessel that has to be considered when thinking about such a route. And that is the Agulhas Current!

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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