Portugal and Spain are struggling for the umpteenth year with major drought. In the spring, especially in February, there was significantly less rain than usual. A month ago, almost all of Portugal was declared a ‘severe drought’ after the country experienced the warmest month of May since 1931.
Earlier this week, scientists identified a cause of the increasing drought in the Iberian Peninsula. The large, near-permanent high-pressure area located in the Atlantic Ocean south of the Azores, which greatly influences the weather in Portugal and Spain, has been expanding in a northeasterly direction since about 1850. The researchers reconstructed the development of this high-pressure area, the Azores High, since 850 AD. The expansion that has taken place over the past 170 years is “unparalleled,” they write in their study, published in the journal Nature Geoscience†
High pressure areas are characterized by descending, dry air, a calm wind (which turns clockwise in the Northern Hemisphere), and a cloudless sky. In the Iberian Peninsula, the average annual rainfall of 732 mm – 40 percent of which falls in winter – has decreased by 5 to 10 mm per decade in the second half of the last century. Already there are olive and grape growers moving their orchards, and this is expected to happen more often in the future.
Climate scientists have been discussing the increasing drought in large parts of Portugal and Spain for years. Is it a consequence of climate change, and is it structural? Or is it an expression of natural variation, and will the trend reverse at some point? Before that, a large weather system in the Atlantic Ocean, the North Atlantic Oscillation, abbreviated NAO, was often looked at. The Azores High is the southern part, the northern part is formed by a low pressure area near Iceland.
The location of the two areas, and their difference in air pressure, strongly influences the strength and direction of westerly winds that blow across the Atlantic to mainland Europe and have a major impact on the weather there. There is a lot of natural variation in the system.
“But studying the NAO is actually not very useful at all to explain the drought in Portugal and Spain,” says climate scientist Rein Haarsma of the KNMI. “Because the system is so large, and because it has so much natural variation.” That makes it difficult to find a climate signal. In the recently published study, the researchers limit themselves to the Azores high. “That’s very smart,” says Haarsma.
In their study, the American researchers reconstruct the outline of the Azores high. For this they use data on precipitation and air pressure from 1850. As a check, they compared these data with data from the growth of stalagmites in the cave Buraca Gloriosa, in western Portugal – this growth is related to precipitation. Based on all that data, they then used a series of climate models to analyze the development of the Azores high since 850 AD. reconstructed.
They focused specifically on the years when the Azores high is extremely large – half the size of the average. Such years have become increasingly common since 1850. While the average over the last 170 years is about once every 9 to 10 years, the Azores high is now extremely high almost every 4 years. Haarsma: “The role of climate change in drought trends is becoming increasingly clear.”