A combination of meteorological phenomena amplified by El Niño gave rise to the extreme weather events that struck the South and Southeast regions this week. It was a sum of factors, according to experts: the action of a cold front, low-pressure areas, a moisture corridor and a strong temperature contrast produced extreme rainfall in Rio Grande do Sul and gales that left people dead in São Paulo and Rio de Janeiro, according to meteorologists.
El Niño came into effect this year, but meteorologists consulted by Folha say that attributing the storms exclusively to it would be an oversimplification.
The warming of Pacific waters helps create a warmer and more humid environment, increasing the energy available in the atmosphere and favoring the occurrence of extreme events, but the intensity of each episode depends on the simultaneous action of different meteorological systems.
"It is very wrong to use El Niño to explain an isolated event. It creates a more favorable scenario, but extreme rainfall or a gale depends on the combination of several atmospheric factors," says meteorologist and climate consultant Francisco de Assis.
Assis explains that, in this week's episode, the cold front advanced over an atmosphere supplied by an intense transport of moisture coming from the Amazon. At the same time, a trough — an elongated area of low pressure — reinforced the instability, while the meeting of warm and cold air masses created a temperature contrast sufficient to intensify storms.
The front virtually stalled over part of Rio Grande do Sul. Continuously fed by the moisture corridor, storms formed successively over the same regions for several days, concentrating in a few hours rainfall volumes equivalent to an entire month.
"The most notable aspect was precisely the persistence. The front remained virtually stationary, associated with the trough and the continuous moisture transport by the Low-Level Jet [a fast current of winds at low altitudes], favoring the successive formation of storms over the same areas," says meteorologist Andrea Ramos.
According to the specialist, the severity of the flooding was not determined solely by the volume of rainfall. The terrain of the Serra and the river basins accelerated the runoff of water into rivers and streams, while soil already saturated by previous precipitation reduced infiltration and caused water to reach waterways more rapidly.
"The rain is the meteorological phenomenon; the flood is the basin's response. When there is intensity, persistence and repetition of storms over the headwaters, several tributaries respond at the same time and rivers rise very rapidly," she explains.
When the system moved into the Southeast, the effects changed. Instead of the highest rainfall accumulations, extremely strong winds predominated. In São Paulo, gusts reached 125 km/h, knocking down trees and causing deaths.
On an atmospheric scale, the episodes had the same origin. According to Andrea Ramos, the cold front advanced accompanied by a broad wind field associated with the pressure contrast. Inland, storms transported to the surface very strong winds present at higher levels of the atmosphere.
On the coast, the interaction between the terrain of the Serra do Mar and the atmospheric circulation helped to intensify gusts at certain points.
Gale was forecast
Despite the magnitude of the impacts, meteorologists say that the risk scenario had been flagged by numerical models days in advance. The main limitation of meteorology remains pinpointing exactly where the most extreme phenomena will occur.
"Models can predict that there will be extreme rainfall in a given region, but an individual storm may span only a few kilometers and shift position rapidly. Small differences in humidity, wind, or surface heating can move the heaviest accumulation from one municipality to another," says Andrea Ramos.
According to Francisco de Assis, the leading international models indicated intense rainfall several days ahead of time. "When a model forecasts between 80 and 100 millimeters and 120 or 150 millimeters falls, it is still correctly representing an extreme event. The problem would be forecasting 20 millimeters and getting 150," he says.
For meteorologist Willian Bini, of Metos Brasil, weather forecasting faces an additional challenge because extreme events are becoming more intense, frequent, and unpredictable on a warmer planet.
"Climate change has left the atmosphere with more energy. Events become more dynamic and explosive, which increases the difficulty of forecasting. Today, on top of being more intense and frequent, they are also becoming more unpredictable," he says. Bini emphasizes that meteorological models have evolved significantly in recent decades, but argues for greater investment in radars, weather stations, and observation networks.
According to him, the low density of equipment across various regions of the country limits data collection and hampers both real-time monitoring and the development of artificial intelligence-based tools capable of improving the forecasting of severe weather events.
