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THE AGULHAS

geography

Geography

The Agulhas Current flows south along the east coast of South Africa in the southwest Indian Ocean. It is one of the world’s great Western Boundary Currents, like the Gulf Stream, flowing swift and narrow on the western side of the ocean basin. In this figure, the background colors are sea surface temperature and it is clear that the Agulhas carries warm waters southwestward.

Unique to the Agulhas Current, south of the African continent, the current loops around to flow back eastward and this is called the Agulhas Retroflection. As you'll see in the movie below, this retroflection is unstable and periodically sheds Agulhas rings, eddies, and filaments of Indian Ocean water into the South Atlantic. This exchange of waters, called Agulhas leakage, is important to the global ocean circulation and ultimately to climate.

The position of the Agulhas Current Time-series (ACT) array of moored current meters is shown on this figure. We deployed the array during our first ACT cruise in April 2010. Also shown is the sediment core underneath the Agulhas leakage in the southeast Atlantic. This core, through so called paleo-proxy techniques, shows us how Agulhas leakage has varied over the past 500,000 years.

transport

Transport

As a western boundary current, the Agulhas is one of the fastest currents in the global ocean, with peak surface speeds of 2 m/s, or 7.2 km/hr, or 4.5 mph. This is about as fast as a brisk walking pace or steady jog.

The Agulhas is also one of the largest currents in the world, next to the Antarctic Circumpolar Current. The unit of measurement for volume transport of seawater is called a Svedrup, named after a famous Norwegian oceanographer. For reference, 1 Sv = 1 million m3/s, or 3.6 km3/hr. The Agulhas Current has a volume transport of about 70 Sv. If this transport were put in terms of numbers of soda cans, it would be around 2 billion cans of soda per second! Another frame of reference would be the largest river in the world, the Amazon River, which has a flow of 0.2 Sv.

This figure is a cross-section of the Current, as measured by an instrument called a Lowered Acoustic Doppler Current Profiler, or LADCP. These velocity measurements were collected in 1995. Everything in red, yellow, and green is part of the main current flowing southwestward beside the continental slope of Africa, which is shown in black. Everything in blue is water flowing in the opposite direction, within the Agulhas Undercurrent. The numbers give velocity in centimetres per second.

properties

Properties

Water masses in the Agulhas come from all over the Indian Ocean and even from the Atlantic and Antarctic Oceans! We can trace water masses from their regional origins at the sea surface by examining their properties. Measures of salinity, temperature, and oxygen content are particularly useful for identifying water masses.

In the Agulhas, the order of water masses is as follows from top to bottom:

  1. Tropical Surface Water (TSW)
  2. Subtropical Surface Water (STSW)
  3. SubAntarctic Mode Waters (SAMW and SouthEast Indian SAMW)
  4. Arabian Sea Low Oxygen Water (ASLOW)
  5. Red Sea Water (RSW) down at depths of about 1000 m
  6. Antarctic Intermediate Water (AAIW)
  7. North Atlantic Deep Water (NADW)
retroflection
Click on the image to play a movie

Retroflection & Leakage

When the Agulhas Current flows south of the African continent, it loops back on itself, sending most of the warm and salty water meandering eastward back into the Indian Ocean. This phenomenon is called the Agulhas Retroflection.

Five or six times per year a pocket of warm, salty water spins off the retroflection and drifts into the Atlantic Ocean as an Agulhas Ring. This event is effectively a leakage of warm and salty water from the Indian Ocean into the Atlantic, and is thus called Agulhas Leakage.

This movie is a state-of-the-art climate system model, from NOAA’s Geophysical Fluid Dynamics Laboratory. It shows sea surface temperature (SST), which warms and cools on a broad scale with the changing seasons. The Agulhas Current shows up as a stream of warm (red and yellow) water flowing along the east coast of South Africa and retroflecting south of the continent.

The movie will take you through a year in the life of the Agulhas system before moving on to the North Brazil Current in the western Atlantic. Look closely and you will see several Agulhas rings spin off the retroflection and drift northwestward into the Atlantic Ocean. This is Agulhas Leakage! Notice how there are many more vortices or ocean eddies, besides Agulhas rings. The ocean is turbulent and full of eddies spinning water around and around.

History

There was a time when the Agulhas Current was better known than the Gulf Stream, because of its geographic position on one of the spice routes from Europe to India.

Vasco de Gama

In 1497 Vasco da Gama was the first mariner to document the existence of the Agulhas Current. He headed an expedition, backed by the King of Portugal, to find a sea route to India for the trading of silks and spices. It wasn’t until three centuries later that English naval officer and surveyor Major James Rennell (a contemporary of Benjamin Franklin) prepared the first charts of the Agulhas Current in 1778.

James Rennell

Two centuries later, scientists invented an acoustic current meter that could be lowered to the sea floor. This invention enabled further ocean research. In 1995 Dr. Beal worked on her first cruise as a student, taking the first direct measurements of the full-depth velocity structure of the Agulhas Current.