Flash-flood phenomena found

New research has explained the drivers of extreme precipitation events around the globe.

Extreme precipitation events (EPEs) cause destructive flooding around the world each year. In the tropics, the atmospheric processes that cause them were well known – they’re mostly associated with cyclones. But the mechanisms that form EPEs in dry subtropical regions are less understood. While these events are rarer in dry regions, they’re also less expected and more damaging when they do occur – such as the 2015 flash flooding in the Atacama Desert.

Now, a paper published in Weather and Climate Dynamics sheds more light on the matter. Analysing data from 1979-2018, the study identifies two atmospheric phenomena that cause these events: Rossby waves and intense moisture transport.

Rossby waves (or planetary waves) are movements in water and air caused by the rotation of the Earth. They move latitudinally around the globe, shifting both warm and cold air, and they have a big effect on weather in midlatitude regions. Rossby waves are a fluid-dynamic process that has been observed on other planets as well – and even the Sun.

Intense moisture transport is the movement of large masses of water vapour in the atmosphere – often stretching for several thousand kilometres. It’s often referred to as an “atmospheric river” when it’s in an elongated shape.

“When Rossby waves amplify and break, cold-air masses intrude from high latitudes into lower latitudes, and vice versa,” says Andries-Jan De Vries, the paper’s author.

“This atmospheric process can drive intense moisture transport, destabilise the troposphere, and force air masses to ascend, which together favour the formation of extreme precipitation.”

The research found that these two phenomena combined had a significant impact on weather in the subtropics. “Importantly, the combined occurrence of these two atmospheric processes can explain up to 70 percent of extreme precipitation events in regions where one would expect them the least – the dry subtropics,” says De Vries. Intense moisture transport had a particularly strong association with these extreme events.

Atmospheric movements over South America from 22 to 27 March 2015, when there was flash flooding in several Chilean regions. Credit: Andries-Jan De Vries

These local events are affected by larger planetary movements. “Breaking [Rossby] waves that reach from the mid-latitudes unusually far towards the equator can draw moisture from the humid tropics into the dry subtropics, which feeds the heavy rainfall,” says De Vries.

The research also demonstrated that these processes had a key role in historic precipitation events – including the January 2011 rainfalls that contributed to the Queensland floods.

Atmospheric movements over South Africa, 25 to 30 September 1987. Credit: Andries-Jan De Vries

Climate change will create more frequent and intense rainfall events over the next few decades. This research could be useful for understanding how these events form.

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