Between the fifteenth and seventeenth century, Europe and North America experienced a Little Ice Age (LIA) which became part of our collective imagination thanks to the snowy landscapes painted by Flemish masters since the Renaissance. These everyday live scenes are consistent with what scientists reconstructed: during the Little Ice Age era the temperatures anomalous drop in Europe was amplified in Winter time.
Neither painters nor scientists have yet understood how to explain such a dramatic drop. An international group of researchers has just discovered the mechanism behind such a tendency: a large scale block to the oceanic circulation in the Subpolar North Atlantic. The research was published yesterday on the journal Scientific Reports.
Winters during LIA typically featured stable atmospheric conditions with high pressure above Scandinavia. Such systems are associated with blocked atmospheric phenomena which advect cold polar air masses from Western Russia toward Europe and divert the flow of warm air masses of Atlantic origin to Europe. The research investigates how these meteorological conditions have prevailed for over two centuries.
This project is the result of the doctoral dissertation of Eduardo Moreno-Chamarro, now post-doc fellow at MIT, under the supervision of Johann Jungclaus, researcher at the Hamburg Max Planck institute of Meteorology and of Davide Zanchettin, currently post-doc fellow at Ca’ Foscari University of Venice as part of the European project “PREFACE” in professor Angelo Rubino’s research group. The research was based on the analysis of numerical simulations of the climatic history of the last thousand years.
These simulations have revealed that the cold winters of the LIA were probably linked to weak oceanic heat transport in the Subpolar gyre (SPG). The SPG is a high-pressure vortex in the subpolar latitudes of the North Atlantic ocean which regulates heat transport from the Atlantic Ocean to the North and to the Arctic Ocean.
The SPG weakening during LIA could have caused a long-lasting cooling of the surface of the ocean in the higher latitudes, favoring ice expansion in the Nordic and Barents seas. These changes in sea ice cover would have modified the heat fluxes between the ocean and the atmosphere more efficiently during the winter than during the Summer, increasing the frequency of blocking events.
“There were speculations on the role that the Ocean played in those long lasting changes linked to the Little Ice Age - explains Johann Jungclaus - but without convincing explanations, at most linked to a weakening stream in the Gulf. Our research indicates that the key to understand those harsh winters must be sought in atmospheric phenomena, regional oceans and in the details of local dynamics of the subpolar Atlantic region. That can only be carried out with complex climatic models”.
The authors have identified strong volcanic eruptions at the end of the sixteenth century as the natural cause of those changes in temperatures. “These eruptions were much weaker compared to the eruption of Mount Tambora in the “year without a summer 1816 CE”, for example - explains Eduardo Moreno-Chamarro - but their cumulated effect was sufficient to start weakening the SPG”.
As Davide Zanchettin states, the research has an impact that goes beyond paleoclimatology: “This research highlights the importance of the SPG for temperature changes in Europe through decades and centuries. This discovery can help us better understand the current European climate and to foresee its evolution in the medium term.
The research is part of an international collaboration between the Hamburg Max Planck Institute for Meteorology, Ca’ Foscari University of Venice and the Justus Liebig-Universität Gießen.
Eduardo Moreno-Chamarro, Davide Zanchettin, Katja Lohmann, Jürg Luterbacher & Johann H. Jungclaus (2017) Winter amplification of the European Little Ice Age cooling by the subpolar gyre. Scientific Reports, 7, doi:10.1038/s41598-017-07969-0