Where two oceans collide
Where two oceans collide
The Indian Ocean and the Atlantic Ocean meet in only one place. That meeting is not without drama.
To the east of Africa is the Indian Ocean, to the west the Atlantic Ocean.
Together, these two vast bodies of water cover around 50% of the surface of our planet. They meet in one place only, a bit south of the coast of South Africa.
We'll start this story doing as Statsraad Lehmkuhl did, hitch a ride with the Agulhas Current after leaving Maputo on the African east coast.
The Agulhas Current
In the west of the Indian Ocean, from Madagascar and south along the coast of Africa, flows warm and salty water. The current has been named the Agulhas and is only about 30 kilometers wide, but is one of the strongest ocean currents in the world.
The Agulhas Current carries 70 million cubic meters of water per second, twice as much water as the Gulf Stream.
The current does not flow in a straight line along the coast, there are lots of bends. Some of these bends "break off" from the current and end up as eddies in the sea. The Agulhas Current stays close to the coast until it approaches the southern tip of Africa. There it "lets go" and continues out to sea.
A couple of times a year these eddies become so large that they have been given their own name, Natal pulses.
In the South Atlantic, between South America and Africa, there is a huge eddy. The water flows westward south of the equator, then south along South America, and eastward towards Africa. When the current meets the African coast, cold water is forced up from the depths.
A bit off the coast furthest south in Africa this cold water meets the warm Agulhas Current, with water five degrees warmer.
Below is a realtime model of the ocean currents around the southern part of Africa. The warmer the colour, the warmer is the water. The movement indicates the speed and direction of the currents. Map: Earth Nullschool.
The clash between cold and warm water is dramatic. Huge eddies are formed, both on the surface and down in the depths. In the eddies, 20-25% of the warm and salty water from the Indian Ocean mixes with the cold Atlantic water, and flows northward along the west coast of Africa. This current is called the Benguela, and some of this water eventually ends up in the Gulf Stream.
What is left of the Agulhas Current loses out to the cold Atlantic current and the wind that constantly blows from the west, and is forced back towards the east. Some of this water flows northward and back into the Indian Ocean, some becomes part of the ocean current that circulates around Antarctica.
These currents are part of what is called the "ocean conveyor belt", tying all the world oceans together. The engine driving the currents is differences in temperature and salinity.
Abundant marine life
When strong ocean currents meet like this, upwelling brings nutrient-rich deep-ocean water to the surface. Up there in the sunlight, there are good conditions for phytoplankton, which in turn become food for zooplankton, fish, and other animals.
In the Natal pulses, which are huge eddies of salt and warm water, there is little nutrients and less wildlife. But where these eddies hit shallower water, they also cause agitation and upwelling. The mechanism behind this is one of the things the researchers from NERSC and ESA try to find out.
That the Agulhas Current affects the wildlife was obvious to the scientists on board Statsraad Lehmkuhl.
- What surprised me was that when the ship sailed along the front of the current there were suddenly so many birds. There was so much more biological stuff happening than before. First of all it was amazingly beautiful, and it was also so cool to see that the stuff that your computer says will actually happen is happening, says Natacha Bourg, PhD student at the University of Toulon in France.
The eddies are so large that the heat they transfer to the air above affects the wind pattern, cloud formation and precipitation in the southern Indian Ocean.
The scientists on board are also looking into the effect of this in a warming world.
- It has been shown that the Agulhas Current has changed due to climate change, and that these changes in dynamics, speed etc has had an impact on the biology. Understanding this process is important, says Bourg.
Visible from space
The sensors in satellites are able to capture what is happening in the ocean off the coast of southern Africa. In the satellite images the Agulhas Current can be seen as a ribbon of warm water, and the eddies and Natal pulses as dark spots on the sea surface. It is also possible to see how the cold Benguela Current flows along the west coast of Africa.
The satellite images became available in 1978, and this is when the scientists first became aware of all the exciting things happening in the Agulhas Current.
But how detailed is this information and how accurate is it?
- One of the most interesting things about the current, is that it is not well understood yet, says Lennard Miller, PhD student at ENS Lyon in France, and on board Statsraad Lehmkuhl.
So NERSC and ESA want to learn more. The route between Maputo and Cape Town has been set so that the ship crosses the Agulhas Current several times.
- It's surprising to see how the models we’ve seen in class and the assumptions we have on depth and temperature and so on are not true. We’re trying to understand the difference between reality and what we’re being taught, Miller continues.
The scientists have taken plankton samples, and water samples both at the surface and down in the depths. They measure the speed of the current, and the direction, speed and size of the waves, and investigate the connection between the sea, and wind and precipitation.
The scientists also released wave-buoys. Small boxes with instruments that record how the box moves in the sea, and exactly where it is.
The boxes were launched when Statsraad Lehmkuhl was off Durban January 6. In the past two weeks, the sensors have drifted separate ways with the current.
- Fascinating to follow the wave buoys deployed during our leg from Maputo to Cape Town earlier this month. One on the shelf, two in the core of the Agulhas current, and two between two eddies, wrote Tore Furevik at NERSC on Twitter January 19.
The shelf is the shallow area close to land, where the white buoy is on the map below.
Looking at models, satellite images and the tracks of sensors is one thing. What really fascinated Sreelakshmi Sreenivasan was experiencing the Agulhas Current with her own eyes.
- Seeing the waves is something else than satellites. It helps me to understand more, giving more clarity. I think if I were to write a paper about waves now it would be much more clear, says Sreelakshmi Sreenivasan, research associate at INCOIS in India.