Discovering the Atlantic Gyres

From Conquering The Pacific: An Unknown Mariner and the Final Great Voyage of the Age of Discovery, by Andrés Reséndez (HarperCollins, 2021), Kindle pp. 114-116:

The North Atlantic Gyre was a major find, but it turned out to be only half of the story. In the 1470s, the Portuguese crossed the equator and stumbled on a second gyre in the South Atlantic. Once again, it was necessity that prompted the discovery of this second great wheel of winds and currents. As the Portuguese sailors could not make any further progress in their Atlantic explorations by staying close to the African coast, on account of the contrary elements, they were forced again into the open Atlantic, this time venturing in a counterclockwise direction, away from the continent until practically crossing the entire ocean and nearing the coast of Brazil. This detour enabled Portuguese vessels finally to catch the southward-moving Brazil current and eventually to double back east toward the tip of Africa. This volta around the South Atlantic—a maneuver similar to the one in the North Atlantic but longer—could take up to three months of sailing without sight of land.

As early as 1500, Vasco da Gama, the great discoverer of the sea route from Portugal to India, penned a concise but unmistakable characterization of this second volta in the instructions that he left to his successor: “You should always go around the sea until reaching the Cape of Good Hope.” The recipient of such sound advice was Pedro Álvares Cabral, who followed da Gama’s words so closely that he drifted to the coast of Brazil, where he spent a few days before continuing eastward to India. Over the years, Portuguese seamen became familiar with the contours of the South Atlantic Gyre, as is evident in the so-called roteiros (derroteros in Spanish, rutters in English, routiers in French, and so on), or sailing instructions, occasionally penned by pilots to facilitate the task of future navigators. The South Atlantic roteiros alerted pilots to approach the coast of Brazil well to the south of Cabo de Santo Agostinho; otherwise they risked being knocked off course by the currents and pushed into the Caribbean, a disastrous turn of events that could delay the voyage by several months. Farther south along the Brazilian coast, pilots were warned to steer clear of the Abrolhos, a group of islands and reefs off the present-day state of Bahia. (“Abrolhos” comes from abre olhos, or “open your eyes” in Portuguese.) Once the fleets doubled back toward the tip of Africa, the only intervening land was Tristan da Cunha, a group of remote islands in the South Atlantic, first sighted in 1506, precisely during the early exploration of the South Atlantic Gyre.

Sixteenth-century navigators probably did not understand that Earth’s rotation is what causes the ocean gyres. It would not be until the early nineteenth century when Gaspard-Gustave de Coriolis worked out the mathematics of the forces in a rotating system. Yet five hundred years ago, Portuguese pilots clearly referred to the ventos gerais (general winds) to distinguish them from more localized and variable winds. They also knew that these ventos gerais formed two rotating systems on either side of the equator. “When you have passed the equator and reached the general winds, you need to go with them for as long as possible,” a pilot named Bernardo Fernandes counseled in 1550, “because with them you will reach the Cape of Good Hope latitude.” Evidently seamen like Fernandes had a clear mental image of the gyres.