But adding all that energy to the upper atmosphere can also introduce a ripple in a jet stream that may affect weather halfway around the world. “It’s like waving a paddle back and forth in the stream and generating planetary-scale atmospheric waves,” said Michael McPhaden, a senior scientist with the National Oceanic and Atmospheric Administration.

That leads to patterns of precipitation, or lack of it, that can pop up in far-flung regions at different times — heavy rains in south-central South America from September to January, increased dryness in Central America for much of the year and a reduced summer monsoon in India, among other effects.

Because these patterns often recur in different El Niño years, the effects can be predictable. Nonetheless, they can still test the ability of governments and aid agencies to respond.

El Niño often affects parts of Ethiopia, for example, and this time was no exception. It is among the countries worst hit by drought, Dr. Bonifacio said, with as many as 10 million people in need of food assistance. Yet Ethiopia is handling the problems largely on its own. “They made a decided effort to deal with the situation,” he said.

But as the lack, so far, of prolonged rains in Southern California this winter shows, the effects of El Niño can still be difficult to forecast.

Dr. Bonifacio noted, for instance, that the Sahel in Africa often suffers drought in El Niño summers, but last year, after a dry June, rains picked up. “From July onward, things just flipped over completely,” he said.

El Niño does not just affect people. The National Oceanic and Atmospheric Administration said this month that the concentration of carbon dioxide in the atmosphere — an important climate-change measurement — had the greatest year-to-year increase in 56 years, and that the rise was partly because of the effect of El Niño-related weather on vegetation. More drought, for example, means less growth of plants that absorb carbon dioxide from the air.