Global warming: Middle East's vital wet winters are disappearing

Hoerling says he and his team were drawn to look at the Mediterranean region following the publication in 2007 of the latest snapshot of the state of climate science from the UN-sponsored Intergovernmental Panel on Climate Change.

Modeling studies included in the three main volumes pointed to the Mediterranean as one of several regions where the effects of global warming would be most pronounced as climate warmed during the 21st century. It became known as a global-warming hot spot, based on projections of an ever-drier regional climate.

The team looked at precipitation records gathered by four research groups based in the US, Britain, and Germany. The data the team used covers 110 years, beginning in 1901. While the records had gaps, they told a similar story. The team estimates that average precipitation from November through April in the region between 1971 and 2010 fell 6.8 percent below the average from 1902 to 1970.

The team began the hunt for causes.

The wetter north, drier south is similar to the pattern that sets up during one phase of a see-saw atmospheric shift over the North Atlantic called the North Atlantic Oscillation.

It's most pronounced in winter, and affects trans-Atlantic storm tracks. But when the team looked at records for the feature and ran modeling studies of 20th century climate, the team could only find a weak long-term shift in the feature that would reinforce the north-south rain and snowfall difference. The shift, however, wasn't nearly enough to account for the drying underway in the Mediterranean region.

At the same time, Hoerling says, he hearkened back to work he and colleagues had done some years before showing a link between sea-surface temperatures in the Indian Ocean and shifts in the North Atlantic Oscillation toward a phase that dries out the Mediterranean in winter.

As a consequence of global warming, ocean surface temperatures have warmed since 1900, he says. The team began using computer models to assess the effect of this ocean warming on the Mediterranean's winter precipitation.

The team found that warming all the oceans by a uniform 0.5 degrees Celsius (about 1 degree Fahrenheit) would dry out the eastern Mediterranean.

But the oceans haven't warmed uniformly. The greatest warming has come to tropical oceans. So the team focused next on warming the tropical oceans uniformly in their virtual world. The team got a Mediterranean-wide drying and a wetter northern Europe.

Still, neither of these experiments produced the strong positive North Atlantic Oscillation-like signal with enhanced drying to the south and heavier precipitation to the north.

But by adding another 0.5 degrees C to the Indian Ocean alone, model produced the strong north-south differences in precipitation associated with a positive phase of the North Atlantic Oscillation.

The group acknowledges that the work contains uncertainties, but the results appear to be strong enough to tie global warming's effect on the tropical oceans in general and the tropical Indian Ocean in particular to the enhanced drying trend the Mediterranean region has experienced, they say.

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