Future Rainfall Uncertain In South Pacific
October 29, 2012

Rainfall In The South Pacific Could Become Difficult To Predict

redOrbit Staff & Wire Reports - Your Universe Online

Two different competing climatic effects -- increasing temperatures and changes in atmospheric water transport -- will determine how much (or how little) rainfall the South Pacific islands will receive in the future, claims a new study published in Sunday's online issue of the journal Nature Climate Change.

According to the study, which was written by Matthew Widlansky and Axel Timmermann of the International Pacific Research Center and the University of Hawaii at Manoa and an international team of colleagues, those two phenomenons occasionally counteract each other, meaning that future rainfall projections will be extremely difficult to predict.

The island nations in the South Pacific rely upon the 8,000km long South Pacific Convergence Zone (SPCZ), the biggest rainband in the Southern Hemisphere, for the bulk of their precipitation, the researchers said. Any changes that occur in that cloud and precipitation formation would have "severe consequences" for the "vulnerable island nations already having to adapt to accelerating sea level rise."

However, researchers know little about how the SPCZ will react to climate change created as a result of increasing greenhouse gas emissions.

The reason for that, according to Widlansky, a postdoctoral fellow at the International Pacific Research Center, is that many existing climate models are "notoriously poor" when it comes to simulating the band.

In order to try and make South Pacific climate simulations more accurate, he and his colleagues removed some deviations in observed sea surface temperature. In doing so, they were able to pinpoint a pair of competing climatic mechanisms which impacted precipitation trends in the region.

"We have known for some time that rising tropical temperatures will lead to more water vapor in the atmosphere," Timmermann, the professor of oceanography at University of Hawaii at Manoa, explained. "Abundant moisture tends to bring about heavier rainfall in regions of converging winds such as the SPCZ."


The two competing mechanisms are dubbed the "wet gets wetter" and the "warmest gets wetter" climate change mechanisms, respectively, the scientists said, and they are said to be the cause of the uncertainty in SPCZ rainfall predictions.

"The scientists found that depending upon the degree of tropical warming expected this century, one or the other mechanism is more likely to win out," the university said. "With moderate warming, weaker sea surface temperature gradients are likely to shift the rainband towards the equator, potentially causing drying during summer for most Southwest Pacific island nations. For much higher warming possible by the end of this century, the net effect of the opposing mechanisms is likely a shift towards more rainfall for the South Pacific islands."

"To be more definite in our projections, however, we need more extensive observations in the South Pacific of how clouds and rainfall form and how they respond to such climate phenomena as El NiÃ±o," Timmerman added. "Before we have more confidence in our calculations of the delicate balance between the two climate change mechanisms, we need to be able to simulate cloud formations more realistically."