The World Meteorological Organization (WMO) said La Nina "has developed and is expected to last into next year, affecting temperatures, precipitation and storm patterns in many parts of the world." 

It could also be contributing to an unusually active hurricane season, the agency said.

La Nina is considered the stormy sister of El Nino, which occurs every two to seven years when the prevailing trade winds that circulate surface water in the tropical Pacific start to weaken.

El Nino, which has a major influence on weather and climate patterns and associated hazards such as heavy rains, floods and drought, has a warming influence on global temperatures, whilst La Nina tends to have the opposite effect.

But global warming could worsen or distort the effects of such phenomena, WMO chief Petteri Taalas warned in a statement.

"All naturally occurring climate events now take place against a background of human-induced climate change which is exacerbating extreme weather and affecting the water cycle," he said.

"La Nina typically has a cooling effect on global temperatures, but this is more than offset by the heat trapped in our atmosphere by greenhouse gases," he pointed out.

"Therefore, 2020 remains on track to be one of the warmest years on record and 2016-2020 is expected to be the warmest five-year period on record," Taalas said.

"La Nina years now are warmer even than years with strong El Nino events of the past."

The last La Nina, which was brief and relatively weak, began developing in November 2017 and ended in April 2018, according to WMO.

This year's La Nina "is expected to be moderate to strong", WMO said, adding that the world had not seen a strong La Nina for a decade.

WMO stressed though that La Nina and El Nino were not the only factors driving global and regional climate patterns, and that "no two La Nina or El Nino events are the same". 

"Their effects on regional climates can vary depending on the time of year and other factors," it said, urging decision-makers to closely monitor seasonal forecasts and the latest data.

The UN agency pointed to fresh data indicating that this year's La Nina would among other things lead to below normal rainfall in the Horn of Africa region and Central Asia, while Southeast Asia, some Pacific islands and the northern part of South America would see more rain than usual.

Maxx Dilley, a WMO deputy director who head's the agency's climate program, said La Nina could also be contributing to this year's unusually active Atlantic hurricane season.

"There is a connection between La Nina and El Nino and hurricane frequency. El Nino tends to suppress frequency and La Nina tends to encourage them, so if we do have a strong hurricane season, La Nina could be contributing to that," he told reporters in a virtual briefing.

His comment came after Hurricane Zeta barrelled through the southern United States - the 27th storm of the season.

In September, meteorologists were forced to use the Greek alphabet to name Atlantic storms for only the second time ever, after the 2020 hurricane season blew through their usual list, ending on Tropical Storm Wilfred.

Dilley said Zeta was expected to be the last hurricane of the season, which typically runs from June through October, although the warming of the oceans, which provides more energy for hurricanes, has allowed storms to rage later into the year.

La Nina was instead expected to create drier than normal conditions in the southern United States and northern Mexico over the next three months, he said.

"So it may go from hurricanes and flooding to dry conditions fairly quickly."

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