A new study relates projected changes in winter precipitation to rising levels of man-made greenhouse gases, making the Iberian region’s agriculture some of the most susceptible in Europe.
According to the article “Twentieth Century Azores High Expansion Unprecedented in the Last 1200 Years” published in Nature Geoscience, these variations in precipitation are related to a subtropical high-pressure system known as the Azores High that has been significantly larger more frequently in the industrial era (since 1850 CE) than in preindustrial times.
Extremely massive Azores Highs cause abnormally dry conditions to exist in the western Mediterranean, including the Iberian Peninsula, during the winter. These highs cover the eastern subtropical North Atlantic and Europe.
According to the paper, the Azores High “has changed significantly in the recent century” and that “these changes in the North Atlantic climate are exceptional within the last millennium.”
The development of the Azores High during the industrial age in a warming climate, according to the report, is due to an increase in atmospheric greenhouse gas concentrations brought on by human activity.
“What we are seeing here with the expansion of the Azores High is bad news for winter rainfall in the Iberian Peninsula. That has severe implications for agriculture and other sectors reliant on water resources,” says paper co-author Caroline Ummenhofer, associate scientist in the Physical Oceanography Department at the Woods Hole Oceanographic Institution (WHOI).
In order to extract specific effects of volcanic, ozone/aerosol, solar variability, orbital variability, and well-mixed atmospheric greenhouse gas forcings, researchers used state-of-the-art climate model simulations spanning the previous 1200 years. Only simulations with incorporated greenhouse gas concentrations were found to match the climate record.
Our work is exciting because it uses observations, ensemble modelling, and proxy methods to characterize climate trends. We leveraged advantages of each data type to provide new insights into how the North Atlantic climate is changing.
Cresswell-Clay
Researchers showed that unusually big Azores High areas occurred on average during 15 winters in the 20th century, compared to about 10 winters for all other 100-year periods over the preceding millennium, using a collection of numerical models known as the Last Millennium Ensemble.
“For the 20th century, this reflects a 50% increase in the frequency of winters with extremely large Azores High that are associated with dry conditions in the Iberian Peninsula an occurrence rate not seen at any time during the previous 1000 years,” says Ummenhofer.
The most recent 25-year period (1980–2005) had an average of 6.5 exceptionally big Azores High regions, compared to an average of 2.6 for earlier 25-year periods (from 1850). As a result, compared to the period between 1850 and 1980, it is currently 2-3 times more probable to encounter a winter with an exceptionally large Azores High.
The study relies on observations from the stalagmite carbon isotope record of the hydroclimate from Buraca Gloriosa cave, Portugal, in addition to climate model simulations.
“Paleoclimate archives, including speleothems, have provided evidence of unique hydroclimate conditions in Iberia during the last millennium, with relatively dry conditions in the Medieval Climate Anomaly, wet conditions in the Little Ice Age, and a trend toward dry conditions since about 1850 CE. Prior to this study, we hypothesized that the hydroclimate shifts were related to changes in dynamics of the Azores High system. The modeling aspect of this study corroborates that these unique hydroclimate conditions were likely related to the size, intensity, and mean location of the Azores High system,” says Alan Wanamaker, professor in the Department of Geological and Atmospheric Sciences at Iowa State University.
“Although our previous findings using speleothems hinted at large changes in hydroclimate over the last 1000 years, the ability to diagnose the most likely causes of these shifts is exciting. Recent drying in Portugal is primarily related to greenhouse gas forcing causing an expansion of the Azores High, whereas earlier changes were largely related to the non-stationary behavior and the relative intensity of the Azores High system.”
“Our work is exciting because it uses observations, ensemble modelling, and proxy methods to characterize climate trends,” lead author Nathaniel Cresswell-Clay says. Cresswell-Clay was a guest investigator at WHOI at the time of the research; he is currently a graduate student in atmospheric sciences at the University of Washington. “We leveraged advantages of each data type to provide new insights into how the North Atlantic climate is changing.”
The study was funded by the National Science Foundation, Cornell College, the WHOI Ocean Climate Change Institute, and the James E. and Barbara V. Moltz Fellowship for Climate-Related Research.
