South Brazil likely to see even more extreme rain in future

A mighty river is flowing out of the Amazon rainforest, and it’s not the one you’re thinking of. In the first kilometer above the forest canopy, a “flying river” transports moisture evaporated from Amazonian trees southwards along the Andes mountains towards Rio Grande do Sul, the southernmost state in Brazil.

Almost the entire state, an area roughly the size of Poland, is currently being affected by unprecedented floods. The flying river has acted like a firehose, dropping five months of rainfall in just two weeks, further enhanced by a strong jet stream located in a disadvantageous position above the region. And, based on future projections of climate change, this situation will likely get worse as temperatures rise.

While the southern part of the country is under water, a heatwave caused record-breaking temperatures in the states of São Paulo, Rio de Janeiro, and Minas Gerais. For scale, this is similar to all of northern France being flooded while Barcelona swelters in 40°C heat.

This is not the first time the southernmost part of Brazil has been affected by such large-scale disasters. Similar weather systems, featuring moisture from the Amazon near the surface and the jetstream crossing the Andes high above, were associated with floods between September and November 2023, as well as major floods in 1997 and 1983.

A combination of factors makes these floods more likely. 

For instance, warm ocean temperatures in the tropical Pacific (still the case even as El Niño starts to subside) are associated with these weather systems, as are abnormally warm tropical Atlantic temperatures, which add more moisture to the air brought south in the flying river.

Ominous expectations

As the atmosphere warms, it can carry more water, which means there is the potential to form massive clouds and heavy rains. This is a bit like buying a more absorbent sponge: it can hold more water, but when you squeeze it, more water falls out.

In fact, we are already observing this. Compared with the floods in 1941, the excessive rainfall was this time concentrated in a much shorter period, meaning water levels rose much faster. Future climate projections already indicate that a warmer atmosphere results in an intensification of the flying rivers from the Amazon into south Brazil and adjacent regions, and more precipitation.

We have analyzed results from state-of-the-art climate models that are able to simulate storms across South America in detail, just a few kilometers across. These indicate that extreme rainfall like that happening now is likely to become more frequent in the future, and such risks may in fact be underestimated by the previous generation of climate models.

These simulations, run under UK-Brazil and South America-U.S. partnerships, are being used to assess such risks in southern Brazil and right across South America. Early results suggest that, as in Africa, parts of Europe, North America, India, and elsewhere, short but very intense rainfall is likely to happen more often as the planet warms, irrespective of the unique weather systems that may affect particular regions.

Originally published by The Conversation

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