El Niño is one of the causes of the storm, but not the only factor, meteorologists say

El Niño came into play this year, but meteorologists interviewed by Folha say that attributing the storms exclusively to it would be a simplification.

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 rain or a gale depend 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 from the Amazon. At the same time, a trough - an elongated area of ??low pressure - reinforced instability, while the encounter between masses of hot and cold air created a temperature contrast sufficient to intensify storms.

The front practically parked over part of Rio Grande do Sul. Continuously fed by the humidity corridor, the storms formed successively over the same regions for several days, concentrating in a few hours volumes of rain equivalent to an entire month.

"The most striking aspect was precisely the persistence. The front remained practically stationary, associated with the trough and the continuous transport of moisture by the Low Level Jet [fast current of winds at low altitudes], favoring the successive formation of storms over the same areas", says meteorologist Andrea Ramos.

According to the expert, the severity of the floods was not determined only by the volume of rain. The relief of the mountains and the river basins accelerated the flow of water into rivers and streams, while the soil already saturated by previous rainfall reduced infiltration and meant that water reached the watercourses more quickly.

"Rain is the meteorological phenomenon; flooding 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 the rivers rise very quickly", he explains.

When the system advanced towards the Southeast, the effects changed. Instead of the greatest accumulations of rain, extremely strong winds prevailed. 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 field of winds associated with the pressure contrast. Inland, storms transported very strong winds present at higher levels of the atmosphere to the surface.

On the coast, the interaction between the relief of Serra do Mar and atmospheric circulation helped to intensify the gusts in some points.

SWELL WAS PREDICTED

Despite the magnitude of the impacts, meteorologists claim that the risk scenario had been pointed out by numerical models days before. The main limitation of meteorology continues to be indicating exactly where the most extreme phenomena will occur.

"The models can predict that there will be extreme rain in a given region, but an individual storm can be a few kilometers long and quickly change position. Small differences in humidity, wind or surface heating can shift the largest accumulation from one municipality to another", says Andrea Ramos.

According to Francisco de Assis, the main international models indicated intense rain several days in advance. "When the model predicts between 80 and 100 millimeters and it rains 120 or 150, it continues to correctly represent an extreme event. The problem would be predicting 20 millimeters and raining 150", he states.

For meteorologist Willian Bini, from Metos Brasil, weather forecasting faces an additional challenge because extreme events are becoming more intense, frequent and unpredictable on a hotter planet.

"Climate change has left the atmosphere with more energy. Events are becoming more dynamic and explosive, which increases the difficulty of forecasting. Today, in addition to being more intense and frequent, they are also more unpredictable," he says. Bini highlights that meteorological models have evolved significantly in recent decades, but advocates greater investment in radars, meteorological stations and observation networks.

According to him, the low density of equipment in several regions of the country limits data collection and hinders both real-time monitoring and the development of tools based on artificial intelligence capable of improving the prediction of severe phenomena.