El Nino, the weather phenomenon that has puzzled climate scientists in recent decades, might have assisted the first trip around the world nearly 500 years ago.
Explorer Ferdinand Magellan encountered fair weather on Nov. 28, 1520, after days of battling rough waters south of South America.
From there his passage across the Pacific Ocean may have been eased by the calming effects of El Nino, researchers say in a new study.
When El Nino occurs, the waters of the equatorial Pacific become warmer than normal, creating rising air that changes wind and weather patterns.
The effects can occur worldwide, including drought in the western Pacific and increased rain in Peru and on the west coast of South America.
Tree-ring data indicate such an event in 1519 and 1520, and perhaps in 1518.
After passing through the strait later named for him, Magellan sailed north along South America's west coast and then turned northwest, crossing the equator and eventually arriving in the Philippines, where he was killed in a battle with natives.
Magellan was seeking the so-called Spice Islands, now part of Indonesia, and his course took him north of that goal.
But the route might have been dictated by mild El Nino conditions and favorable winds, anthropologists Scott M. Fitzpatrick of North Carolina State University and Richard Callaghan of the University of Calgary, Canada, propose in a new study of the trip.
Their research is summarized in Friday's edition of the journal Science and is to be published in full in the August edition of the Journal of Pacific History.
They were studying early exploration trips and were struck by the fact that Magellan sailed unusually far north, Fitzpatrick explained.
"We had not considered El Nino until afterward, when we were trying to account for why the winds were so calm when he came into the Pacific," he said.
"We knew it was unusual."
The researchers used a computer to model wind and weather conditions across the Pacific during El Nino and then compared that information with Magellan's route.
Magellan's journals show that many crew members died or were sick with scurvy, so he might simply have chosen to go with existing winds and currents to accommodate the loss in manpower, Fitzgerald said.
In his writings, Magellan said he chose the northerly route because of reports of a famine in the Spice Islands.
Callaghan and Fitzpatrick say El Nino conditions often result in drought in that region.
Magellan had received correspondence from a friend in the Spice Islands before setting out and so might have known about a famine there, Fitzgerald said.
But that can't be determined for certain, because the correspondence was destroyed in the great Lisbon earthquake of 1755.