How do eddies form and break away from the Agulhas Retroflection?
There is much research on how eddies in the Agulhas form and
effect the Agulhas retroflection, but I will try to explain in
a nut shell! Eddies are very common in the ocean and result from
instabilities in the circulation — the ocean is not a
“linear” system, where flows are smooth, but a
“turbulent” one, where flows tend to break up into
eddies and filaments. Even in a western boundary current like
the Agulhas (and the Gulf Stream) water parcels can leave and
join the current (we call it detrainment and entrainment) along
its course via eddies. In the Agulhas, eddies from the Mozambique
Channel and East Madagascar Current drift westward into the
current (all eddies like to drift west, because the Earth is
spinning!) and cause it to meander. The meander propagates
downstream with an anticlockwise eddy inshore and a clockwise
eddy offshore. As the meander approaches the Retroflection it
destabilizes the flow and an Agulhas Ring is formed, which is
basically when the retroflection forms a closed loop, temporarily
cut off from the current. This Agulhas Ring then drifts westward
away from the retroflection, leaking warm and salty Indian Ocean
water into the Atlantic.
What causes the Agulhas Retroflection?
You guys have some tough questions! You might remember from my
talk that the Agulhas is part of the Indian Ocean subtropical
gyre, which is similar to the North Atlantic subtropical gyre
where the Gulf Stream is the western boundary current. Subtropical
gyres are driven by Earth’s large wind systems — the Trades and
Westerlies — which cause a massive circulation of waters around
and around each ocean basin between about 15 degrees latitude
and 45 degrees latitude. The Agulhas Retroflection appears because
the African continent (ending at 36°S or so) does not reach as
far south as the gyre, so the Agulhas Current runs out of boundary
to flow along! At first the Current continues southwest under
its own inertia, until the Westerlies force it back to the east
to rejoin the Indian Ocean gyre circulation — hence the retroflection
is formed. But as you learned above, not all the waters from the
Agulhas are retroflected back into the Indian Ocean — some leak
into the Atlantic via Agulhas Rings and we call this Agulhas
Have you met Robert Ballard?
No! But I have visited his laboratory at the University of Rhode
Island and seen some of the artifacts he has brought up from
wrecks in the abyssal ocean.
Are the eddies a permanent feature of the South Atlantic or have
they grown more or less noticeable over time?
Eddies are a permanent feature of all oceans. The number of
Agulhas Rings can change, effected by what we call “coupled
ocean-atmosphere climate modes” like El Niño. An
analogy to this would be that the number of Atlantic hurricanes
each season varies dependent on large scale conditions and also
El Niño. There is also evidence from ocean and climate models
that the number of Rings, or the size of the Agulhas leakage,
may change in the future with climate change.
What kept your interest in the Agulhas Current?
The Agulhas is remote from developed nations with strong research
programs like the US and Europe and hence does not receive as
much attention as, for instance, the North Atlantic and Pacific.
I like to work on under-studied regions, where exploration and
discovery are still possible and the science is new and exciting!
I also like to travel and collaborate with researchers from other
countries. The community of scientists who work on the Agulhas
are from all over the world — Netherlands, UK, Japan,
Germany, France, and of course South Africa. Finally, the more
I discover about the Agulhas, the clearer it is that it plays
an important global role — it may be remote, but its impacts
are not. For instance, there is evidence that if Agulhas leakage
changes then the whole circulation of the Atlantic changes, which
has a profound effect on global climate (at time scales of decades
How do you distinguish the start and end of a current?
This is a good question and it is difficult to answer. Flow can
be concentrated into a strong current for a number of different
reasons, and then dissipate again. For instance, water feeds
into the Agulhas along its whole length, so that the current
gets stronger and stronger towards the tip of Africa. After the
current retroflects and becomes the Agulhas Return Current, the
flow gradually weakens again until it is so weak and broad that
it is hard to define as a current. Oceanographers might disagree
about the definition of the start and end of the current. I might
define it as where the velocity drops below 10 or 20 cm/s —
in other words where it is so weak that it cannot be discerned
from the background eddy field. However, deep currents are very
often weaker than this, but can still be discerned through their
water mass properties.
Do all Western Boundary Currents cause similar climactic effects
on their bordering continents?
Yes. All WBCs are strong and warm and so they tend to make bordering
coastal regions warmer and wetter. This is compared to Eastern
Boundary Currents, which are weak and cold (like the California
Current) and tend to influence the coastal regions with cooler,
Is there a specific instrument that measures Sverdrups?
No. A Sverdrup is a measure of volume transport — in meters
cubed per second — so we calculate it by measuring velocity
in the ocean (m/s) and then multiplying that by a cross-sectional
area (m2). Velocity is measured using an instrument
like the one I brought into class, an acoustic current meter.
For example, if you wanted to measure how many Sverdrups the
Gulf Stream is as it flows through the Florida Straits you would
first measure velocity across the Straits using an acoustic
current meter which profiles the ocean while the ship is underway,
called an ADCP (acoustic Doppler current profiler). The Florida
Straits is only 800 m deep, so an ADCP can “see” and measure all
the way to the bottom! Once you have the velocity field across
the Strait you multiply by the cross-sectional area of the Strait
to get the total transport in Sverdrups. I'll leave you to think
about how you might calculate transport in a deeper current, or
how you might calculate changes in transport over time.
Can climactic changes cause significant aberrations in ocean
Climatic changes can cause changes in ocean currents — and
vice versa — changes in ocean currents can cause changes
in climate! The ocean is an integral part of the climate system,
it is coupled with the atmosphere, meaning that they can effect
and feed back on one another. Imagine the ocean is warmer than
usual — like during an El Niño when the tropical
Pacific warms. More water evaporates from the warmer ocean, which
also heats the atmosphere more and causes more air to rise. These
effects result in more clouds and rain, which is why California
is wetter during El Niño.
Since the Agulhas Current directly affects the Gulf Stream, can
climactic changes in Europe be attributed to the Agulhas Current?
This is another example of the fact that the ocean is an integral
part of our climate system! Changes in Agulhas leakage can affect
the Atlantic overturning circulation: models have shown that if
the leakage of salty water from the Agulhas increases, then the
formation of deep water in the North Atlantic also increases,
because all that salt increases the density of the waters in the
Atlantic so that they sink more easily. It takes a few decades
for the signal to travel from the Agulhas into the North Atlantic
and affect the overturning. However, in reality it would be
difficult to attribute any change in European climate to the
Agulhas alone. The ocean-atmosphere system is so interconnected,
with affects and feedbacks on one another, that it is difficult
to say if any one process is more or less important than another.
This is why we need a lot more research!