Spain, Portugal and nearby places will see a 10-20 per cent drop in winter rainfall over the coming century
An extremely large ‘Azores High’ (a subtropical weather phenomenon) has resulted in abnormally dry conditions across the western Mediterranean, including the Iberian Peninsula, primarily occupied by Spain and Portugal, according to a new study.
Azores High is a subtropical high pressure system that extends over the eastern subtropical North Atlantic and western Europe during winter, the researchers explained. It is associated with anticyclonic winds in the subtropical North Atlantic. It is formed by dry air aloft descending the subtropics and coincides with the downward branch of the Hadley Circulation.
An annual drying of 5-10 millimetres per year per decade has been recorded in the Iberian Peninsula throughout the second half of the 20th century, according to the record published in the journal Nature Geoscience. A further 10-20 per cent drop in winter precipitation is expected by the end of the 21st century, it added.
These projected changes make agriculture of the Iberian region some of the most vulnerable in Europe. The study projected:
The researchers showed that the Azores High expansion is driven by external climate forces and that the only external forcing that produces this signal in the industrial era is atmospheric greenhouse gas concentrations.
The researchers explored the changing atmospheric conditions since the onset of the industrial era that contributed to these regional hydroclimatic changes by assessing how the characteristics of the Azores High varied over the past 1,200 years.
Observations and climate model simulations were used to show that winters with an extremely large Azores High were significantly more common in the industrial era (since 1850) than in pre-industrial times.
Simulations of the past millennium indicated that the industrial-era expansion of the Azores High is unprecedented throughout the past millennium (since 850) “consistent with precipitation proxy evidence from Portugal”.
The study identified a robust increase in the frequency of extremely large Azores High Area (AHA) events and an overall expansion signal in the winter Azores High over the past 100 years.
Azores High expansion emerged after 1850 and strengthened in the twentieth century, consistent with anthropogenically driven warming, according to the report.
The study showed that the Azores High has changed dramatically in the past century and that these changes in North Atlantic climate are unprecedented within the past millennium. The report added:
An expanding Azores High agrees with reports of a poleward shift in the North Atlantic storm track and an increase in anticyclone frequency at the Azores High’s poleward edge as well as a positive trend in regional subtropical indicators.
The findings have important implications for projected changes in the western Mediterranean hydroclimate throughout the twenty-first century.
The study is important to understand the future climate risks posed to productive agricultural sectors such as viticulture and olive plantations across the Iberian Peninsula.
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