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 November 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 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 today’s edition of the journal Science and is scheduled to be published in full in the August edition of the Journal of Pacific History.