Climate science: Iberian Peninsula drying out by Azores High expansion

The expansion of a high pressure system over the Atlantic — the Azores High — is leading to the driest conditions on the Iberian Peninsula in the last thousand years, according to a modelling study published in Nature Geoscience.

Weather and longer-term climate patterns in Western Europe are strongly impacted by atmospheric circulation associated with a persistent, clockwise-rotating area of high pressure known as the Azores High. Dry air descending towards the surface within the system is a leading cause of hot, arid summers in much of Portugal and Spain, as well as the western Mediterranean more broadly. During the characteristically wetter winter months, shifts in the position of the Azores High are responsible for westerly winds along its’ southern edge, moving moisture towards the Iberian Peninsula. This wintertime precipitation has, however, been decreasing in recent decades.

Caroline Ummenhofer and colleagues modelled how the size and spatial extent of the Azores High has changed over the last 1,200 years. The authors reveal that the Azores High began to, on average, cover a greater area starting around 200 years ago when human greenhouse gas emissions began to substantially increase, with this spatial expansion becoming more pronounced in the twentieth century. Comparison of their models with geochemical indicators of past precipitation levels preserved in Portuguese stalagmites extending back to 850 CE, suggests that the expansion of the Azores High is linked to the onset of drier winters in the western Mediterranean.

The authors conclude that the Azores High will continue to expand in to the twenty-first century as greenhouse gas levels continue to rise, leading to elevated risk of drought on the Iberian Peninsula.