The weather in the Mediterranean and Pacific regions is influenced by the warming or cooling of the surface of the Atlantic, reveals a series of studies at the Barcelona Supercomputing Center (BSC). The work of the INADEC project also shed light on the Atlantic Multidecadal Variability (AMV), the fluctuations in the North Atlantic’s sea surface temperature spanning multiple decades. Researchers have revealed why current simulations are underestimating its effect on the climate in Europe and other regions. “All the studies highlight that to accurately simulate the impact of the AMV on the climate, the mean state of the climate models needs to be improved,” found research fellow Yohan Ruprich-Robert, who works in the BSC’s Climate Prediction Group. His research was supported by the Marie Skłodowska-Curie Actions. The North Atlantic’s considerable fluctuations in sea temperature over several decades are having an especially important impact on temperature and rain during the summer in the Mediterranean, says Ruprich-Robert. “In particular, the AMV modulates the occurrence of heatwaves around the Mediterranean basin by around 20 %,” he explains. He shared his findings in the article ‘Modulation of the occurrence of heatwaves over the Euro-Mediterranean region by the intensity of the Atlantic Multidecadal Variability’ in the ‘Journal of Climate’. Targeted climate simulations show that a warm phase of the AMV is driving an increase in tropospheric downward air motion over the Mediterranean region, concluded the research. However, the models vary in their ability to predict the AMV-Mediterranean teleconnection and further research is needed to ascertain why.
Accurate model estimates
The project also investigated the simulations of AMV teleconnections during winter, finding all climate models simulate a much weaker atmospheric circulation response than the observed one. “We revealed a link between model responses to AMV and errors in the models’ average climate conditions,” says Ruprich-Robert. “The models with the weakest errors simulated the strongest climate response to AMV.” These findings were set out in the paper ‘Atlantic Multidecadal Variability and North Atlantic jet: a multimodel view from the Decadal Climate Prediction Project’ in the ‘Journal of Climate’. “This result will help make our model estimate of the atmospheric circulation response to AMV more accurate,” adds Ruprich-Robert. “In the future, we will sub-select models with the weakest errors in average conditions to assess AMV’s climate impacts.”
The team initially set out to research the impact of AMV on the Mediterranean region but their findings led them to also include its impacts on the El Niño Southern Oscillation and the Pacific. They hope this will improve climate predictions for other regions around the world. They found that models differed by a factor 10 in their estimates of the amount the Equatorial Pacific cools down during a warm phase of the AMV. These differences were again linked to errors in the models’ average climate conditions, in particular in simulated precipitation. By correcting for those precipitation errors, the researchers concluded that a warming in the Atlantic of 0.26 °C resulted in an Equatorial Pacific cooling of 0.16 ºC. Ruprich-Robert believes his work shows current decadal prediction systems could be improved if they could better simulate the teleconnections associated with the Atlantic. “A better understanding of those mechanisms, to improve their representation in climate models, has the potential to increase our ability to predict the climate of the next few decades.”
INADEC, teleconnections, decadal prediction systems, Atlantic Multidecadal Variability, North Atlantic