Experts say the Pacific ocean phenomenon known as La Niña is partly to blame for the drought ravaging the Horn of Africa.
But while the latest La Niña episode has ended, climate scientists are concerned about what the next few months will bring and the intensifying effects of a changing global climate.
La Niña begins when eastern Pacific waters near the equator turn cooler than normal. A cascade of changes in ocean temperatures and wind currents follows, and the consequences are global.
The latest episode began in July of last year.
In the United States, altered winds pushed moisture away from the south, causing severe droughts that persist today, and toward the north, causing floods.
Weather patterns half the world away are affected too, said meteorologist Wassila Thiaw with the US National Weather Service.
"The easterly winds that are supposed to bring moisture into East Africa [were] reduced. There was less moisture coming into East Africa and therefore rainfall is reduced" Thiaw says, "That would start happen late last year. Rains that were supposed to fall over Somalia, southern Ethiopia and northern Kenya failed."
Thiaw runs the Africa desk at the Climate Prediction Center. He says there is a second rainy season in that part of the Horn of Africa, from March through May. That would fail too, but for different reasons, he says.
"What played out here during the March-April-May season, we don't think that is really La Niña, but it probably mostly due to the atmospheric conditions that prevailed during that time."
He says La Niña's influence on the March-through-May rainy season typically is not as strong as on the October-through-December season. Natural variations in atmospheric conditions were enough to prolong the drought.
This La Niña episode ended in May. But Thiaw says there is a chance another one may begin by the end of this year.
"If that happens before the October-through-December rainfall season, then there's a high chance for us to be in another dryer-than-normal season."
That could be catastrophic for drought-stricken parts of East Africa, he says. But he cautions it is too soon to say whether it will happen, or how severe it would be.
La Niña and the related phenomenon called El Niño are natural cycles that happen every three to five years or so. Man-made contributions to global warming - auto and industrial greenhouse-gas emissions, for example -- are a concern, too. But Columbia University climate scientist Simon Mason says the human factor in the Horn's climate woes is less clear.
"East Africa is actually one of our big perplexing areas for the moment."
Mason says several groups around the world have developed computer models to predict how increasing greenhouse gases will change the climate.
"Most of the models are actually suggesting that East Africa will become wetter," "However, if we look at what's been happening in East Africa at least for the last decade or so, it's actually been getting quite a lot drier."
Mason says that drying trend is at least partly due to global warming, which is contributing to rising temperatures in the Indian Ocean. That creates conditions that draw moisture away from East Africa.
Current computer models do not capture this effect very well, Mason says. "So we need to do a bit of work there to work out what our models are possibly doing wrong."
Others say that decade-long drought trend could just be natural variation, but in terms of food production, it does not make that much difference whether it gets wetter or drier because it is sure to get hotter. Experts may not be sure about rainfall, but they are confident temperatures in East Africa will continue to go up.
Agriculture economist Claudia Ringler at the International Food Policy Research Institute says by Skype that hotter weather means thirstier crops. "So you actually need more water to get the same out of the crop at a higher temperature."
And Ringler notes that much of the land in drought-stricken East Africa is not very productive in the best of times. "it will not get any better. Even if we have a bit more rainfall, the general potential for food production in the region is not expected to improve dramatically in the region."
She adds that when the rain falls is in some ways as important as how much. Climate change tends to promote more heavy downpours with longer dry stretches in between. That is not good for raising crops, either, she says.
So Ringer adds that farmers and herders in East Africa need help becoming more resilient in the face oflooming climate changes, whatever they turn out to be.
La Niña: 拉尼娜现象
atmospheric conditions: 大气条件
looming: to become imminent; to impend（即将到来的）
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