A La Niña usually means a more active Atlantic hurricane season with potentially stronger storms.
In the eastern Pacific Ocean, La Niña is expected to be moderate to strong this season, according to the World Meteorological Organization.
The global declaration of a La Nina event is used by governments to help plan responses in vulnerable sectors like agriculture, health, water resources and disaster management, the WMO said.
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This year's La Nina is expected to bring drier than usual conditions to East Africa during the planting season, which the U.N. weather agency called "a further worrying development which may add to the food security challenges in the region."
In North America, La Nina typically brings more rain to the north of the continent and less to the south.
Elsewhere, it is expected to bring wet conditions across large parts of South East Asia and Australia.
South America is expected to see above average rainfall on the northern part of the continent and below average on much of the south.
Meanwhile, the National Weather Service said Thursday marked the 192nd consecutive day that downtown Los Angeles had a high temperature of at least 70 degrees. That marks the longest streak in that area's recorded history.
The Associated Press contributed to this report.