The study identifies where rainfall comes from for places such as northern Australia and the Kalahari Desert in southern Africa, where vegetation growth is mainly limited by how much rain falls in the growing season. They found that, when rainfall and vegetative growth is much more or less than usual, the reason is a mix of specific climatic forces.

In order to understand where wet and dry years come from in water-limited regions, "there needs to be an effort to understand where the rainfall that comes into these regions originated, and what their land cover and hydrological states are", according to Diego Miralles of Vrije Universiteit Amsterdam in the Netherlands. "Regions are interconnected through the atmosphere, as they are interconnected through rivers inland," he said.

Water-limited ecosystems are thought to be globally important in the conversion of carbon dioxide to vegetation, yet the reasons behind wet and dry years have been something of a mystery. At least in part this is because the mechanisms linking vegetative growth and rainfall have been poorly understood. Rainfall might result in more healthy vegetation, for instance, but more healthy vegetation might transpire more, and instigate extra rainfall.

To get a handle on these mechanisms, Miralles and colleagues – who are based at institutions in Australia, Austria, Belgium, Spain, the US and elsewhere in the Netherlands – turned to new global datasets on transpiration and vegetative water content. Together with a computer model, these datasets allowed the researchers to follow the trajectories of water molecules in the atmosphere. In other words, they could find out where any raindrop first evaporated from.

"For some regions, such as the Kalahari, a big chunk of the growing-season precipitation comes from water that was originally evaporated from the same region," said Miralles.

In the final step, the researchers explored why certain years have less rain, and thus less vegetative growth, than other years. The first reason, they found, was a change in atmospheric circulation, so that rain is not sourced from its usual region; the second was a change in evaporation in the rain’s source region; while the final reason was a change in atmospheric stability, which affects the overall chances of rainfall.

Some dry regions, such as the Kalahari and northern Australia, were often affected by a mixture of all three phenomena, the team found. Indeed, the phenomena themselves are not independent, and are driven both by large-scale climate patterns such as El Niño and by climate change. "As an example," said Miralles, "we find that for the Caatinga [in Brazil] and Kalahari Desert, the wettest years, in which vegetation is also healthier, always coincide with La Niña."

Miralles is now planning to explore how the mechanisms his group has uncovered allow droughts to self-propagate. "If a drought occurs in a region ‘B’ from which a region ‘A’ harvests its rainfall, the evaporation in ‘B’ will drop and less water vapour will be transported into ‘A’," he explained. "This would increase the chances of experiencing a second drought in ‘A’."

The study is published in Environmental Research Letters (ERL).

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