Conditions oscillate between El Nino and its opposite La Nina, with neutral conditions in between.

The WMO's monthly Global Seasonal Climate Update points towards "a rapid development into a strong El Nino event during July-September".

The UN agency classifies El Nino events as weak, moderate, strong or very strong, meaning it is set to reach the third-highest level out of four.

"El Nino conditions have developed in the tropical Pacific and are forecast to strengthen rapidly over the coming months, increasing the likelihood of... extreme weather events in many parts of the world," the WMO said.

Heatwave risks

The Geneva-based agency said that forecasts produced by leading global climate centres, using different models, indicate a consistent and significant warming of ocean temperatures across the central and eastern equatorial Pacific.

"Seasonal-average sea-surface temperature anomalies expected to exceed 2C in key monitoring regions," it said.

The models show "remarkable agreement, providing high confidence in the outlook", the WMO said.

"El Nino is expected to continue strengthening during the Northern Hemisphere autumn, with its influence extending across many regions of the globe.

"Meanwhile, the equatorial Atlantic basin is expected to remain generally warmer than average."

The last El Nino contributed to making 2023 the second-hottest year on record and 2024 the all-time high at around 1.55C above the 1850-1900 pre-industrial average.

While El Nino usually peaks between November and February, the resulting spike in temperatures typically comes later down the line.

"El Nino conditions are already under way and are forecast to strengthen rapidly into a strong event," said WMO chief Celeste Saulo.

"This will intensify the chances of drought and heavy rainfall and the risk of heatwaves on land and marine heatwaves in many regions."

The WMO said it was stepping up early warning support to help guide preparedness, especially in climate-sensitive sectors like agriculture and health.

"Advanced seasonal forecasts and early warnings are vital to save lives and cushion the impact on our economies and our communities," said Saulo.

Temperature impact

The update predicts an overwhelming likelihood of above-average temperatures across most land areas between 60 degrees south and 60 degrees north - covering nearly all populated areas outside the polar regions.

And the July to September rainfall outlook is consistent with a strengthening El Nino, with above-normal rainfall forecast in some areas such as portions of the southwestern United States, and below normal forecast across the Indian subcontinent and much of Australia.

The WMO says there is no evidence that climate change increases the frequency or intensity of El Nino events.

However, the agency believes it can amplify the associated effects, because a warmer ocean and atmosphere increase the availability of energy and moisture for extreme weather events, such as heatwaves and heavy rainfall.

During the northern hemisphere summer, warm waters associated with El Nino can fuel hurricanes in the central and eastern Pacific, while hindering their development in the Atlantic Ocean.

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