The United States Climate Prediction Center, part of the National Oceanic and Atmospheric Administration, said in its monthly diagnostic discussion on 13 August that El Niño is strengthening in the tropical Pacific, and that the chance of a very strong event through the northern hemisphere autumn and winter is greater than ninety per cent. The centre put the odds at sixty-nine per cent that the three-month average of the Niño-3.4 index reaches +2.5°C or higher between October and December — a threshold no El Niño has crossed in the record that begins in 1950.
The measurements behind that forecast are, for now, more modest than the language suggests. The Niño-3.4 index averaged +1.4°C in July. The Niño-1+2 region, which hugs the coasts of Ecuador and Peru, averaged +2.9°C. What appears to persuade forecasters is what sits beneath the surface: a deep reservoir of unusually warm water strung along the equator, and an atmosphere that has begun to answer it. The Southern Oscillation Index ran about two standard deviations negative through July, trade winds slackened, and the belt of heaviest rainfall has been sliding east towards the central Pacific.
El Niño does not so much make weather as move it. The World Meteorological Organization’s seasonal outlook, issued on 31 July, expects drier-than-normal months across the Indian subcontinent, southern and eastern Australia, parts of Central America and the Caribbean and northern Europe, and wetter ones across the Greater Horn of Africa, southern Europe and south-eastern South America. For Southeast Asia the practical worry is a dry season that runs long and a fire season that runs with it. India’s meteorological department, forecasting back in May, put this year’s monsoon at ninety per cent of its long-period average and raised the probability of a deficient season.
“Forecasts in themselves do not prevent hazards, people do. This El Niño provides a window of opportunity to anticipate risks and act before impacts unfold,” said Celeste Saulo, secretary-general of the World Meteorological Organization.
The Pacific has given several months of notice. What is made of them is a separate matter, decided on land.