April 6, 2010: Into the Current

By Dallas Murphy

On Saturday 16 December 1497, the Portuguese explorer Vasco da Gama wrote this in his log:

That night we laid up off the Rio do Infante [present Kei River, South Africa?]. The following day we set off with following winds until evening when we set an offshore tack. We tacked seawards and landwards until Tuesday evening when the westerly wind returned and we laid up offshore for the night to check our position. In the morning, we closed on land and at ten o’clock found ourselves...60 leagues back from our last land fix. This was due to a very strong current.

This is the first mention in Western history of the Agulhas Current. Ten years earlier, his countryman Bartholomeu Dias had returned from his voyage with history-changing news: Africa had a bottom. No European knew that before. It meant that a ship from Western Europe could sail around Africa into the Indian Ocean, and by that route reach the East Indies where riches beyond dreams literally grew on trees—nutmeg, mace, cloves, cinnamon, and pepper.

Agulhas current Click image to view animation of current float. Courtesy of Jenny Veitch

Spices had heretofore arrived in Europe via the Mediterranean Sea after the tortuous overland trip across the length of Asia, and by the time they reached Italy after passing from one middleman after another, only kings could afford spices. It was spices, in fact, that had been funding the flowering of the Italian Renaissance. Dias reached the Cape of Good Hope—which he named the Cape of Storms, reflecting his experience of the region—but he had not actually rounded Africa. He had intended to after discovering its bottom, but his crew informed him that if he tried to press any father into the unknown, they would mutiny and throw him overboard.

Vasco da Gama dropped his hook off Calicut, India on 22 May 1498 after a 12,000-mile passage from Lisbon, a voyage four times as long and far more difficult than Columbus’s (who had sailed west six years earlier in order to reach the East, but of course there was this irksome New World in his way). Gama, strolling the market place, couldn’t help but notice that a bag of pepper purchased in India would bring something around a 3,000 per cent profit when sold in Lisbon. Within a decade, the Portuguese had taken control of the Indian Ocean. Then the Dutch took over, then the English, each in turn dominating the spice trade and the local people with the point of their ships’ cannons. A fascinating, violent, and troubling three-century history ensued, but that’s not our story except insofar as it demonstrates that ships and the ways of the ocean and the winds sometimes determine the tide of human history.

I just came down from the bridge where modern electronics show that Knorr is making 12 knots through the water, but only ten knots over the bottom. This indicates that she’s driving into the teeth of the Agulhas Current flowing against her bow at two-knots. That’s fairly light for the Agulhas; we’ll see twice that and more by the time our work is done. Gama was probably being set by about five knots of foul Agulhas current.

We’re almost there now, at the inshore start of Dr. Lisa’s study line, but before we get to work on deck, let’s take a look upstream of the origin of the Agulhas Current to see how it forms. Remembering that the Agulhas is part—the western boundary—of the large-scale circulation in the southern Indian Ocean, let’s “begin” with the South Equatorial Current. That current is created by a permanent belt of wind that girdles the entire globe. The Indian Ocean portion of the southeast trade winds blow from Australia westward to the east side of Africa. And let’s simplify a fairly complex physical process to say that when strong wind blows constantly over an ocean, the wind pulls the surface of the ocean along with it. So now we have a huge mass of water—that South Equatorial Current—flowing across the tropical reaches of the Indian Ocean from east to west.

At the western side of the ocean the current hits the big island of Madagascar (at about 25 degrees South latitude) as if it were a rock in a country stream. You’ve probably seen how when flowing water encounters an obstruction, the water swirls and eddies as it flows around the obstruction. The same thing happens here, though of course on a vastly greater scale. So now the South Equatorial Current, though broken and disturbed as it tries to get around the island, cannot simply stop at the Madagascar obstruction, because the wind continues to blow and the South Equatorial Current to flow. And it can’t reform on the downstream side of the island and carry on because the continent of Africa is in its way. But it has to go so somewhere. It can only go one of two ways, north or south. Which?

Now another force in addition to the wind enters the picture. It’s the planet we live on. Earth rotates at high speed, around 800 miles per hour at these latitudes. We don’t feel the speed because the atmosphere rotates along with Earth. And that Earth rotates makes little difference to things fixed firmly to its surface, including us attached here by gravity. But to things less firmly attached—such as ocean water—the rotation matters very much indeed. It exerts an invisible force, called Coriolis after the French mathematician who first described it, that determines which way all that water must flow. To simplify complex physics, Coriolis insists that the water goes south.

And so it does—eventually in the form of what we call the Agulhas Current, sprinting fast, hot, and narrow toward the bottom of Africa—right across Dr. Lisa’s study line. It’s now 4:30, 1630 in nautical time, and we’ve arrived at the line. Knorr has done what no other ship does except those dedicated to ocean research; it has stopped in the open ocean so the science staff and the technicians can test their instruments and practice their deployment. She will stop time and again along Dr. Lisa’s study line. So why don’t we stop as well? Anyway, it’s almost dinnertime.

We’ll talk tomorrow about the ship and how she performs her work. Good night for now.