Friday, November 2, 2012

South Pacific rainfall in an era of warming: it's complicated

If complexity is the enemy of understanding, then oceanic rainfall patterns are plenty confusing.

A new study suggests that in a time of climate change, warming conditions can create conditions that both increase and decrease rainfall over Pacific Islands. 

(Image: Storms in the South Pacific. Credit: Digital Typhoon, National Institute of Informatics.)

If oceans are warmer, they pump more moisture into the atmosphere, creating the potential for more rain.

On the other hand, warming can reduce the difference in temperatures as you go deeper—and that can create drier weather—at least in certain areas of the southwest Pacific—by moving the rainy South Pacific Convergence Zone to the northeast.

Researchers  who built a model to try to make sense of this include Matthew J. Widlansky, Axel Timmermann, Karl Stein, Shayne McGregor, Niklas Schneider, Matthew H. England, Matthieu Lengaigne and Wenju Cai. Widlansky, Timmermann, Schneider and Stein are from the University of Hawai`i—the International Pacific Research Center and/or the Department of Oceanography.

What their model suggests is that which of the more-rain/less-rain paradigms wins out depends on how much warming there is. A couple of degrees of warming yields a slight drying trend; more than 3 degrees and the convergence zone gets wetter.

The South Pacific Convergence Zone is a band of clouds that bring moisture to a vast region of the South Pacific from the Solomon Islands to Tonga, Fiji and Samoa.

Timmermann, in a University of Hawai`i press release, describes the phenomenon of warming and rainfall like this:

“We have known for some time that rising tropical temperatures will lead to more water vapor in the atmosphere. Abundant moisture tends to bring about heavier rainfall in regions of converging winds such as the SPCZ.

“Nearly all climate change model simulations, however, suggest the equatorial Pacific will warm faster than the SPCZ region. This uneven warming is likely to pull the rainband away from its normal position, causing drying in the Southwest Pacific and more equatorial rainfall.”

The abstract for the researchers’ paper describes their result: “On the basis of a multi-model ensemble of 76 greenhouse warming experiments and for moderate tropical warming of 1–2 °C we estimate a 6% decrease of SPCZ rainfall with a multi-model uncertainty exceeding ±20%. For stronger tropical warming exceeding 3 °C, a tendency for a wetter SPCZ region is identified.”

For the islands of the South Pacific, who have complained bitterly about world inaction on climate change, the results are problematic. They are once again at the whim of the climate, with little ability to impact their own fate.

© Jan TenBruggencate 2012

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