Data suggested the El Nino phenomenon had emerged, the Japan Meteorological Agency said, referring to conditions in the equatorial Pacific.
"The chances are high that the El Nino phenomenon will be maintained until the winter," the agency said in a statement.
Adding to worries, the U.N.'s Food and Agriculture Organization said on Thursday the world was closer to a repeat of a 2008 food crisis because of a spike in food costs.
The big unknown is how intense and how long the developing El Nino phenomenon will be. An intense El Nino can cause widespread drought in Australia, parts of Africa, Southeast Asia and India, but also bring rains to other parts of the globe.
While it can boost corn and soy crops in South America, wheat harvests can be devastated in Australia. Coffee, cocoa, rice and sugar output in Southeast Asia can also be hit.
Officials said El Nino could kick in at the end of the Indian monsoon in September, hurting winter wheat, rapeseed and chickpea crops.
Drier weather would be good for China's autumn grain growing period, mostly corn and soybean, which accounts for more than 70 percent of the country's total grain output, a senior Chinese meteorological official said.