The Agulhas Current, off the east coast of South Africa, has some truly humongous waves, not really surfable, but nonetheless interesting. Here’s how it works.
Off the coast of South Africa you get some giant swells. I’ve spent the past 17 winters in Cape Town, surfing big waves at spots like Sunset Reef and Dungeons and I can tell you that it gets huge. Sometimes even too big to surf. That’s on the southwest tip of Africa, where swells from the Southern Ocean hit square-on. But I’ve also heard of humungous waves off the East Coast, just offshore from Durban – perhaps not surfable waves, but ones that have caused more than a few problems with shipping.
The most famous case of a ship being lost off that coast was the S.S. Waratah, a 150-m long cargo liner steamship. On 26th July 1909 the Waratah left Durban on her way to Cape Town with 211 people on board. She was last spotted 180 miles from Durban and, to this day, no trace has ever been recovered.
Over the last few decades, more than 50 large ships have been severely damaged by gigantic waves in that area. This has prompted scientists to have a closer look at what is going on. Dr Marten Grundlingh and colleagues from the South African Council for Scientific & Industrial Research plotted on a chart the locations of all the accidents over a 20-year period. When they overlaid this onto an infrared image of the sea surface, they noticed that all the disasters coincided with a line of warmer water. This was the notorious Agulhas Current.
The Agulhas Current flows from northeast to southwest, almost hugging the east coast of South Africa, and transports warm water from the Indian down into the Southern Ocean. With maximum velocities of around 8.5 km/h, it is one of the fastest and strongest flowing currents in the world. The amount of water being transported by the Agulhas Current is around 70 million cubic metres per second – twice as much as the Gulf Stream.
Now, those same giant swells that hit Cape Town from the Southern Ocean also propagate up the east coast of South Africa, from southwest to northeast. That is, in the opposite direction from the Agulhas Current. The giant swells and the giant current meet face-to-face, and, as a result, the waves are magnified to gargantuan proportions. This is the reason why so many ships have come to grief in that area.
How does it work? Well, there are two ways of thinking about it, neither of them rocket science. Firstly, you could think of the swells and the current as two energy sources propagating in opposite directions on the ocean surface. When they meet, the Agulhas current pumps extra energy into the swells, making them bigger.
The other way of thinking about it is as follows. The current slows down part of the swell as it propagates through the middle of that swell. As a result, the waves are focused towards the centre, concentrating the energy and making the waves higher. This is just like the focusing that occurs when a swell hits a finger of shallow reef at spots such as Pe'ahi. The slowing down also causes the swells to squash together, piling them up steeper and higher; again very similar to what happens when swells experience a sudden transformation from deep to shallow water, at spots such as Maverick's.