The Mediterranean region will be significantly drier in coming decades, potentially seeing 40 percent less precipitation during the winter rainy season, say researchers. A new analysis could help refine the models and add certainty to projections, which have significant implications for the management of water resources and agriculture in the region.
An analysis by researchers at MIT has found the underlying mechanisms that explain the anomalous climate effects in the Mediterranean region, especially in the Middle East and in northwest Africa. The study was carried out by MIT graduate student Alexandre Tuel and professor of civil and environmental engineering Elfatih Eltahir.
The different global circulation models of the Earth’s changing climate agree that temperatures virtually everywhere will increase, and in most places so will rainfall, in part because warmer air can carry more water vapor, explains a statement. However, “There is one major exception, and that is the Mediterranean area,” Eltahir says, which shows the greatest decline of projected rainfall of any landmass on Earth.
Tuel and Eltahir found that this projected drying of the Mediterranean region is a result of the confluence of two different effects of a warming climate: a change in the dynamics of upper atmosphere circulation and a reduction in the temperature difference between land and sea.
The effect of two factors
Neither factor by itself would be sufficient to account for the anomalous reduction in rainfall, but in combination the two phenomena can fully account for the unique drying trend seen in the models.
The first effect is a large-scale phenomenon, related to powerful high-altitude winds called the midlatitude jet stream, which drive a strong, steady west-to-east weather pattern across Europe, Asia, and North America.
But in the Northern Hemisphere, those winds run into obstacles, with mountain ranges including the Rockies, Alps, and Himalayas, and these collectively impart a kind of wave pattern onto this steady circulation, resulting in alternating zones of higher and lower air pressure. High pressure is associated with clear, dry air, and low pressure with wetter air and storm systems. But as the air gets warmer, this wave pattern gets altered.
However, that effect alone can’t account for the projected Mediterranean drying. That requires the addition of a second mechanism, the reduction of the temperature difference between land and sea. That difference, which helps to drive winds, will also be greatly reduced by climate change, because the land is warming up much faster than the seas.
“What’s really different about the Mediterranean compared to other regions is the geography,” Tuel says. “Basically, you have a big sea enclosed by continents, which doesn’t really occur anywhere else in the world.” While models show the surrounding landmasses warming by 3 to 4 degrees Celsius over the coming century, the sea itself will only warm by about 2 degrees or so. “Basically, the difference between the water and the land becomes a smaller with time,” he says.
That, in turn, amplifies the pressure differential, adding to the high-pressure area that drives a clockwise circulation pattern of winds surrounding the Mediterranean basin. And because of the specifics of local topography, projections show the two areas hardest hit by the drying trend will be the northwest Africa, including Morocco, and the eastern Mediterranean region, including Turkey and the Levant.
That trend is not just a projection, but has already become apparent in recent climate trends across the Middle East and western North Africa, according to the researchers.