They say that human-caused global warming has reduced vital winter rains, with “severe implications for farming and tourism”, including the production of olive oil.
A new modelling study publishing in the Nature Geoscience journal shows that the Azores High, an atmospheric high-pressure system referred to as a “gatekeeper” for European rainfall, has expanded as the planet has warmed.
Researchers said the Azores High, which influences the weather and climatic patterns of vast areas of North Africa and Southern and Western Europe, had “changed dramatically in the past century” to create conditions “unprecedented within the past millennium".
Using climate model simulations over the past 1,200 years, the study found that the high-pressure system, typically found south of the Portuguese Azores archipelago in the Atlantic Ocean, started to grow to cover a greater area about 200 years ago, as human greenhouse gas pollution began to increase.
It extended its reach even further in the 20th century in step with global warming.
“The number of extremely large Azores highs in the last 100 years is really unprecedented when you look at the previous 1,000 years,” said Dr Caroline Ummenhofer, of Woods Hole Oceanographic Institution in the US, part of the research team.
“That has big implications because an extremely large Azores high means relatively dry conditions for the Iberian Peninsula and the Mediterranean.”
Those regions have been hit by an increasing number of heatwaves and droughts in recent years. This May was the hottest on record in Spain and Portugal is bracing for another heatwave as much of the country experiences severe drought.
The study cites projections that the level of precipitation could fall by another 10 per cent to 20 per cent by the end of this century, which the authors say would make Iberian agriculture "some of the most vulnerable in Europe".
A study cited in the latest research estimates that the area suitable for growing grapes in the Iberian Peninsula could shrink by at least a quarter and potentially vanish completely by 2050 because of severe water shortages.
Meanwhile, researchers have predicted a 30 per cent drop in production for olive regions in southern Spain by the year 2100.
Last year, scientists found that a severe spring frost that ravaged grape vines in France was made more likely by climate change, with plants budding earlier and therefore being more susceptible to damage from the colder weather.