The "El Nino" phenomenon, which sparks global climate extremes, is this year the worst in more than 15 years, the UN weather agency said Monday, warning it was already causing severe droughts and flooding.
The World Meteorological Organization said El Nino, which occurs every two to seven years, had resurfaced a few months ago, become "mature and strong", and was expected to become even more powerful by the end of the year.
"Severe droughts and devastating flooding being experienced throughout the tropics and sub-tropical zones bear the hallmarks of this El Nino, which is the strongest in more than 15 years," WMO chief Michel Jarraud said in a statement.
El Nino is triggered by a warming in sea surface temperatures in the Pacific Ocean. It can cause unusually heavy rains in some parts of the world and drought elsewhere.
The UN agency said this year's event was expected to push water surface temperatures in the east-central Pacific Ocean more than two degrees Celsius above normal, making it one of the four strongest El Ninos since 1950.
Previous particularly strong El Ninos occurred in 1972-73, 1982-83 and 1997-98.
Typically, El Nino events reach their maximum strength between October and January, but often continue to wreak havoc through the first quarter of the year.
- Climate change impact -
The phenomenon usually leaves countries like India, Indonesia and Australia drier, increasing chances of wildfires and lower crop production.
In recent months, bone-dry conditions caused by the El Nino have sparked some of the worst forest fires in Indonesia's history.
The phenomenon also often leads to heavier rainfall in the eastern Pacific and South American nations, raising the spectre of floods and landslides.
WMO linked this year's El Nino to the "very active tropical cyclone season" in the Pacific, including the record-breaking Hurricane Patricia that hit Mexico last month.
The UN meanwhile warned last week that El Nino could significantly increase the number of people going hungry, as countries like Sudan, Eritrea, Ethiopia and Djibouti are expected to see drier conditions, and others, including Kenya, Somalia and Uganda are at risk of floods.
While El Nino will certainly have severe impacts in many parts, Jarraud said the world was far better prepared for the event than in the past.
"The level of international, national and local mobilisation is truly unprecedented," he said, pointing out that a wide range of disaster management campaigns were expected to "save lives and minimise economic damage and disruption."
But while scientific understanding of the phenomenon has increased, Jarraud warned that due to climate change it could be "playing out in uncharted territory."
While scientists say climate patterns like El Nino are not caused by climate change, rising ocean temperatures caused by global warming is believed to impact their intensity and frequency.
"This naturally occurring El Nino event and human induced climate change may interact and modify each other in ways we have never before experienced," Jarraud said.
"Even before the onset of El Nino, global average surface temperatures had reached new records. El Nino is turning up the heat even further," he added.
WMO published its El Nino update ahead of a conference in New York Tuesday on the climate pattern, and just weeks before a global summit in Paris tasked with sealing a climate rescue pact.