''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 an El Nino and then compared that to Magellan's route.

Magellan's journals show that many of the crew had died or were sick with scurvey, so he may simply have chosen to sail with the existing winds and currents, reducing the number of crew needed to operate his ships, Fitzgerald said.

''It could have been an adept maneuver,'' the researchers wrote, allowing him to move west along the past of least resistance.

In his writings, Magellan said he chose the northerly route because of reports of a famine in the spice islands. This also could be accurate, Callaghan and Fitzpatrick say, as 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 may have known about a famine there, Fitzgerald said. But that cannot be determined for certain, because the correspondence was destroyed in the great Lisbon earthquake of 1755.

While the actual reasons for Magellan's choice of route remain uncertain, El Nino conditions ''may have been largely responsible for structuring the route and extent of what many consider the world's greatest voyage,'' the researchers wrote.

The trip, in fact, may be the earliest record of an El Nino, Fitzpatrick said.

Sir Francis Drake encountered mild conditions in the Strait of Magellan when he sailed through in 1578, but he then faced months of Pacific storms that scattered his ships, sinking one. Captain James Cook seems also to have benefited from El Nino conditions centered on 1769 during his Pacific exploration.