At least 79 people have died in heavy rains in Brazil and the rains seem to continue. Rainfall is affected by declining surface temperatures in the Pacific and climate change.
It has rained heavily in the northeastern part of Brazil during the first part of the year. The floods and landslides brought by the rains have already killed hundreds of people.
The heavy rains are caused by the cooling of the Pacific surface water by several degrees around the equator. The phenomenon recurs irregularly every few years and is known as La Niña.
La Niña has a warm opposite, El Niño, and the Pacific surface temperature fluctuates slowly between these cool and warm phases – the La Niña and El Niño phases typically last more than half a year to a year.
Eastern winds normally blow in the Pacific Ocean known to the equator. They push warm surface water toward Southeast Asia, and at the same time on the coast of South America, cool water rises to the surface from the deeper layers of the sea.
During La Niña, the east winds are stronger than usual, with more cool water rising than normal.
La Niña and El Niño affect the weather around the world. In Southeast Asia, for example, La Niña usually brings heavy rainfall and drought to the eastern part of central Africa.
In South America, rainfall is increasing in the northern parts of the continent, but further south in Argentina and Uruguay, for example.
In Brazil, the rains intensified by La Niña have hit the easternmost state of Pernambuco and its capital, Recife, in particular, about 4,000 kilometers from the Pacific coast.
On a Finnish scale, a trip would mean, for example, that the cooling of the Gulf would cause heavy rains in Uusimaa.
Climate change may increase the rainfall that accompanies La Niña, as the warmer the air, the more water it can hold.
However, there is no precise information on the interactions between La Niña, El Niño and climate change, and this is a complex set of cross-cutting effects.
Sources: US Federal Organization for Meteorological and Oceanic Research (NOAA), United Nations Office for the Coordination of Humanitarian Affairs (OCHA)