WASHINGTON - The El Nino phenomenon that has puzzled climate scientists in recent decades may 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 battle through the 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 speculate in a new study.

When an 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 be worldwide, including drought in the western Pacific and more rain in Peru and the west coast of South America.

Tree ring data indicate that an El Nino was occurring in 1519 and 1520 and may even have begun in 1518.

After passing through the strait later named for him, Magellan sailed north along the South American coast and then turned northwest, crossing the equator and eventually arriving at 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 may have been dictated by mild conditions and favorable winds during an El Nino, anthropologists Scott M. Fitzpatrick of North Carolina State University and Richard Callaghan of the University of Calgary, Canada, propose in a new study of his trip.

Their research is summarized in Friday's edition of the journal Science and is scheduled 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 in a telephone interview.

"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.