New research suggests global warming could either lead to either drying significant increases in rainfall in the South Pacific.

Warming world creates uncertainty for Pacific rainfall

Alvin Stone
20 November 2012

Global warming has increased the uncertainty around future rainfall patterns over some of the most vulnerable nations in the South Pacific Islands, according to a new paper in Nature Climate Change.

The paper, Changes in South Pacific rainfall bands in a warming climate, has revealed two competing effects generated by global warming that could directly affect the future of a vital rainfall band, known as the South Pacific Convergence Zone, which brings important seasonal rain to the region.

As a result, the future rainfall patterns over islands in this region are now clouded in doubt.

The research was led by a team of scientists at the International Pacific Research Centre (IPRC) in collaboration with researchers from the Climate Change Research Centre (CCRC) and ARC Centre of Excellence in Climate System Science (CoECSS). 

“The South Pacific Convergence Zone is the main source of rainfall for many South Pacific Nations,” said one of the co-authors of the paper, Chief Investigator Prof Matt England.

 “This new research suggests we could see two completely opposite effects impact these islands that are directly linked to global warming. These effects could either lead to drying of this region or generate significant increases in rainfall.”

The cause of these apparently opposite effects is directly related to the warming world.

The first effect is caused by rising tropical temperatures, which introduce more water vapour into the atmosphere. This increases the size and distribution of heavy rainfall events around areas of converging winds – such as the South Pacific Convergence Zone.

The second effect is caused by the fact that the Equatorial region is likely to heat faster than the convergence region. This uneven heating can cause the rain band across the South Pacific Convergence Zone to move northwards, bringing more rain to Equatorial regions while taking it away from the current area under the convergence zone. 

Previously, climate models were notoriously poor in simulating this rainband, even under present day conditions. The researchers overcame this lack of clarity by adjusting their models to match the sea surface temperatures observed in the region.

With the resulting improvements in climate model performance from this approach, the researchers could then clearly identify the two competing mechanisms affecting rainfall trends in the Pacific and see how they responded to different rates of global warming.

“The key is how quickly the tropics warm compared to the region around the South Pacific Convergence Zone,” said Dr. Shayne McGregor, a co-author of the study.

“With moderate warming the altered temperature gradients between the tropics and South Pacific act to pull the rainband northwards, potentially drying the Southwest Pacific nations underlying the current convergence zone.

“But if we get much higher warming, which is possible by the end of the century, models suggest the increase in atmospheric water vapour will overcome the temperature gradient changes, leading to more rainfall in the South Pacific Convergence Zone.”

However, before scientists can make more definite projections of the delicate balance between the two climate change mechanisms, more extensive observations in the South Pacific are needed to determine how clouds and rainfall form and how they respond to climate phenomena such as El Niño along with improvements in the simulation of cloud formation physics in the models.

Source: ARC Centre of Excellence for Climate System Science web site

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