Climate predictions shift


POSTED: Tuesday, March 16, 2010

Global warming will change ocean temperature patterns and rainfall rates in the tropics and subtropics, but not in the way most scientists have assumed, says a Hawaii meteorologist.

“;People have known for a long time that rainfall will change with global warming, but they didn't have a very good idea about what controls the rainfall changes,”; said Shang-Ping Xie, of the International Pacific Research Center at the University of Hawaii at Manoa.

Scientists assumed wet places would get wetter, based on another assumption that ocean warming is rather uniform, he said. “;But when we turned to real observations and model simulation projections for the next 50 to 100 years, immediately we saw ocean warming is not uniform,”; he said in an interview.

The picture is mixed for Hawaii, where models predict more rainfall as long as the tradewinds remain consistent. But there is a chance that trades could diminish, Xie said.

Xie heads a team that is analyzing global model warming projections used by the Intergovernmental Panel on Climate Change. Their study, published in the Journal of Climate, found significant changes in rainfall patterns will occur from changing ocean temperature patterns.


The global average ocean temperature will rise about 1.8 degrees Fahrenheit by the middle of the century “;if we continue to emit greenhouse gases and do business as usual,”; Xie said. But the increase could vary by up to 2.7 degrees Fahrenheit, depending on the region, he said.

“;In a sense it points to a different paradigm for rainfall change,”; the scientist said. “;Instead of a wet place getting wetter, we need to look for patterns of temperature warming.

“;The new rule is if local warming is greater than the tropical average, then you can expect to have more rain, whereas if your place is warming at a rate slower than the tropical average, you will see a drop in rainfall.”;

The maximum temperature rise in the Pacific is projected to be along a broad band at the equator where El Ninos already influence worldwide climate, Xie said.

This warming in the equatorial Pacific is due to “;an adjustment in the ocean current system”; that looks like an El Nino but is different physically, he said.

The Northern Hemisphere warms more than the Southern Hemisphere in the models, “;which is good news for rainfall for a region like Hawaii in the Northern Hemisphere subtropics,”; Xie said. “;We'll probably fare a little better than Tahiti across the equator.”;

But Hawaii's rainfall also depends on tradewinds, which bring a lot of moisture to the islands, Xie said. “;In that regard we have to watch not only how much ocean warming is around us, but how the trades are going to vary.”;

The models suggest tradewinds in the Northern Hemisphere will weaken, which would mean less rain for Hawaii, he said. “;So there's a competing effect.”;