(openPR) In a new study, scientists propose a mechanism to explain the winter amplification of the Little Ice Age cooling in Europe by connecting it to the changes in the oceanic circulation of the subpolar North Atlantic.
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Our common conception of the Little Ice Age (LIA), the relatively cold climate period that climaxed in the 16th–18th centuries, is strongly influenced by painters like Brueghel the Elder or Avercamp and their famous genre scenes of winter hardships and pleasures in icy landscapes. These idealized representations of daily life are nonetheless consistent with documentary evidence and climate reconstructions that present the LIA as a largely winter phenomenon in Europe. In a new study [1], published in Scientific Reports, an open-access journal of the Nature Publishing Group, Drs. Eduardo Moreno-Chamarro, Katja Lohmann, and Johann Jungclaus from the Max Planck Institute for Meteorology, Dr. Davide Zanchetin from the University of Venice, and Prof. Juerg Luterbacher from the University of Giessen propose a new mechanism to explain the winter amplification of the LIA cooling in Europe by connecting it to the changes in the oceanic circulation of the subpolar North Atlantic.
Similar to (now quite rare) exceptionally cold winters in present times, the LIA winters were typically characterized by persistent high-pressure systems, or atmospheric blockings, over Scandinavia. These blockings not only carry colder air from the Eurasian continent to Europe via their associated easterly winds; they also “block off” the paths of the low-pressure systems that normally bring moisture and relatively warm air from the Atlantic to western Europe. But, how could these large-scale atmospheric circulation patterns be so prevailing and determining for winters throughout two centuries?
As part of his PhD project, Eduardo Moreno-Chamarro analysed climate model simulations covering the entire pre-industrial millennium in search for an explanation for those anomalous winter conditions. In agreement with reconstructions from climate archives, these simulations revealed not only that the LIA was characterized specifically by colder winter temperatures, but also that these were directly linked to a weaker-than-normal large-scale ocean circulation in the North Atlantic, in particular the Subpolar Gyre (SPG). The SPG is a counter-clockwise rotating basin-wide eddy that regulates the northward heat transport from the Atlantic Ocean to the Nordic Seas and the Arctic Ocean. Thus during the LIA, a weakened SPG caused a long-lasting cooling of the surface ocean in high northern latitudes that favoured the increase of the sea-ice covered area, especially in the Barents Sea. The changes in sea-ice cover in turn modified the exchange of heat between ocean and atmosphere effectively in winter, and so led to the persistent blocking weather regime over Europe.
“There were earlier speculations that the ocean could have played some role in these long-term changes,” points out Johann Jungclaus, project leader at the MPI-M. “However, the prevailing explanation was too simplified: it just said that the Gulf Stream, as a sort of warm-water-heating system for Europe, failed or weakened during the LIA, causing the cooling. Our investigations show that a general weakening of the Atlantic meridional overturning circulation was probably not involved, and that we need to look in detail at regional dynamical features, such as the SPG and the localized ocean–atmosphere feedbacks. This can only be done with complex climate models,” he adds. “Moreover, the mechanism we propose explains the particular seasonal characteristics of the LIA in Europe.”
The authors identified a number of volcanic eruptions at the end of the 16th century as the most likely cause of the changes in the SPG. “These events were much weaker than, for example, the prominent Tambora eruption in 1815, which led to the famous “year-without-summer” in 1816,” explains Eduardo Moreno-Chamarro. “However, in a previous study [2] that focused on the mechanisms behind the SPG changes in more detail, we found that the accumulative effect of these smaller eruptions initiated the relatively abrupt weakening of the gyre circulation”.
Publications:
[1] Moreno-Chamarro, E., D. Zanchettin, K. Lohmann, J. Luterbacher, and J. H. Jungclaus, 2017: Winter amplification of the European Little Ice Age cooling by the subpolar gyre. Sci. Rep., doi:10.1038/s41598-017-07969-0.
[2] Moreno-Chamarro, E., D. Zanchettin, K. Lohmann, and J. H. Jungclaus, 2017: An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes. Clim. Dyn., 48, 727-744.
Contact:
Dr. Johann Jungclaus
Max Planck Institute for Meteorology
Department The Ocean in the Earth System
Phone: +49 40 41173 109
Email:
Dr. Eduardo Moreno-Chamarro
Now at: Massachusetts Institute of Technology
Department of Earth, Atmospheric and Planetary Science
phone: +1 (617) 253 5458
e-mail:
Prof. Jürg Luterbacher
Institut für Geographie
Justus Liebig Universität Giessen
Tel.: 0641 99-36210
Email:
Weitere Informationen:
- https://mpimet.mpg.de/en/home/
Quelle: idw
