A note from the late Johann Lutjeharms — author of the book “The Agulhas Current”

“I first experienced going to sea and the swift flow of the Agulhas Current as a teenager, during my year of voluntary service with the Navy in the 1960s. Several years later as a graduate oceanography student I went on my second cruise to investigate the Agulhas Current and I remember we swam in pools of slowly spinning warm water split from the Current, called eddies. We tried to work out how long it would take if you drowned for your body to reach the ocean bottom, some 4 km below us, but never found a definitive answer. Nowadays the ships’ Captains will not allow you to go swimming in the deep sea under any circumstances. A lot of other things have changed as well.

At that time technology was rather primitive compared to today. Navigation was still done by sextant, much as in the 17th century, by measuring the angles heavenly bodies made with the horizon. If the whole sky was covered with cloud navigation was impossible, since the sun and stars could not be seen. On this second cruise of mine we had a few days of continuous cloudiness and when eventually it became clear again and the officers could do some celestial navigation it turned out that the ship was hundreds of kilometres from where we were supposed to be! The Agulhas Current had carried us far to the south.

Nowadays there is satellite navigation that tells you exactly, within a metre, where you are at sea at any time of day or night. There are also science satellites that are able to sense the temperature of the sea surface and the amount of plankton (marine algae) in the water. All this information you can get via computer while at sea, so you know where the warm currents are (like the Agulhas), where most of the biological production is taking place, and you can guide the ship accordingly.

There are also satellites that measure sea surface height, which gives us information about surface currents — just like isobars on a weather map tell us about the strength and direction of the wind. Warm features, such as the eddy (anticyclone) we swam in when I was a student, have a slightly higher elevation than the ocean around them and cold eddies (cyclones) have a lower elevation, like shallow saucers. These elevation changes are only a few centimetres over tens of kilometres, but the satellite sensor is sensitive enough to measure these differences and depict the location and size of many oceanic features. Before these satellites went up in the 1990s, we had no idea there were so many eddies in the ocean.”