The temperature of the equatorial Pacific reached 2.0°C above average on September 14, the threshold that classifies El Niño as "very strong", according to a bulletin released on Wednesday (16) by ClimaInfo, a Brazilian organization focused on climate, and reported by CNN Brasil. The U.S. National Oceanic and Atmospheric Administration (NOAA) estimates a probability above 90% that the phenomenon will stay at that level between September and January. Its effects are already splitting the country: the city of São Paulo is living through its rainiest September on record, while the Branco River in the northern state of Roraima runs below historical minimums.
Record rain in the South and Southeast
In 14 days, the Mirante de Santana station in the city of São Paulo recorded 253.8 mm of rain, the rainiest September since records began in 1943 and three times the average for the whole month, according to INMET, Brazil's national meteorology institute. Stations in Bragança Paulista, Registro and Iguape have already broken their monthly records. CNN Brasil reported two deaths and one missing person in São Paulo state, where the town of Eldorado, in the Ribeira Valley, was left completely flooded. The ClimaInfo bulletin, citing state civil defense agencies, counts at least eight deaths in Greater São Paulo and four in Paraná, where more than 21,000 people were affected across 76 municipalities. São Paulo's state civil defense attributes the storms to El Niño, and a new extratropical cyclone could bring heavy rain to nine states starting Sunday (20).
The outlook points to more rain in the same direction. The federal El Niño Panel, run by INPE, Brazil's space research agency, together with other government bodies, forecasts above-average rainfall in the South and part of São Paulo state through November. In Rio Grande do Sul, the southernmost state, soil moisture could run 150 mm above normal in October. In the rest of the Southeast, rain should stay above average in São Paulo, with isolated storms in northern Minas Gerais and Espírito Santo.
Drought in the North and Northeast
In the North, the effect is the opposite. The Branco River fell 22 cm in one week and reached 93 cm in Boa Vista, the capital of Roraima, below historical minimums for the period and 250 cm below its level on the same date in 2016. At Caracaraí, the river is also at its historical low for the season. The Geological Survey of Brazil sees little chance of recovery in the coming months: only 45% of expected rain fell over the basin in July and August. The state government of Roraima has declared an emergency for 180 days. Across most of the Amazon, the annual low-water season is still within its normal range, but falling fast: the Purus River dropped 120 cm at Beruri, the Solimões 111 cm at Manacapuru and the Negro 104 cm at Manaus.
According to INMET, the dry scenario should persist in the North and Northeast through November. In the Northeast, the soil water deficit is expected to reach 150 mm in Ceará, in central-northern Piauí and in the west of Rio Grande do Norte, Paraíba, Pernambuco and Alagoas.
El Niño is a natural cycle that warms the Pacific and disrupts wind and rain patterns, but this episode is playing out on a warmer planet. NOAA's index already discounts the background warming of the tropical ocean, so the 2 degrees measured come from the phenomenon itself. Scientists heard by ClimaInfo note that attribution studies still need to confirm El Niño's weight in each event, though a more humid atmosphere tends to intensify extreme rainfall. For now, the response is running on two tracks: Roraima has its 180-day emergency decree and the federal panel keeps monitoring river basins, even though federal records of affected populations, fed by municipalities, tend to arrive days or weeks after the events.