El Nino, a warming of water surface temperatures in the eastern and central Pacific Ocean, is linked to extreme weather conditions from tropical cyclones to heavy rainfall to severe droughts.
The world's hottest year on record, 2016, coincided with a strong El Nino - though experts says climate change has fuelled extreme temperatures even in years without the phenomenon.
The World Health Organization said last month it was preparing for an increased spread of viral diseases such as dengue, Zika and chikungunya linked to El Nino.
During El Nino, winds blowing west along the equator slow down, and warm water is pushed east, creating warmer surface ocean temperatures.
In the past, it has caused severe droughts in Australia, Indonesia, parts of southern Asia, Central America and northern South America.
Persons:
El, Maria Neira, Gabrielle Tétrault, Farber, Andrew Heavens, Alex Richardson, Christina Fincher
Organizations:
WHO, El, World Meteorological Organization, El Nino, WMO, Prediction Service, World Health Organization, Environment, Health, Thomson
Locations:
GENEVA, Pacific, Geneva, South America, United States, of Africa, Central Asia, Australia, Indonesia, Asia, Central America