Bi-decadal variability excited in the coupled ocean–atmosphere system by strong tropical volcanic eruptions D. Zanchettin • C. Timmreck • H.-F. Graf • A. Rubino • S. Lorenz • K. Lohmann • K. Kru ¨ger • J. H. Jungclaus Received: 17 March 2011 / Accepted: 9 August 2011 / Published online: 24 August 2011 Ó Springer-Verlag 2011 Abstract Decadal and bi-decadal climate responses to tropical strong volcanic eruptions (SVEs) are inspected in an ensemble simulation covering the last millennium based on the Max Planck Institute—Earth system model. An unprecedentedly large collection of pre-industrial SVEs (up to 45) producing a peak annual-average top-of-atmo- sphere radiative perturbation larger than -1.5 Wm -2 is investigated by composite analysis. Post-eruption oceanic and atmospheric anomalies coherently describe a fluctua- tion in the coupled ocean–atmosphere system with an average length of 20–25 years. The study provides a new physically consistent theoretical framework to interpret decadal Northern Hemisphere (NH) regional winter cli- mates variability during the last millennium. The fluctua- tion particularly involves interactions between the Atlantic meridional overturning circulation and the North Atlantic gyre circulation closely linked to the state of the winter North Atlantic Oscillation. It is characterized by major distinctive details. Among them, the most prominent are: (a) a strong signal amplification in the Arctic region which allows for a sustained strengthened teleconnection between the North Pacific and the North Atlantic during the first post-eruption decade and which entails important impli- cations from oceanic heat transport and from post-eruption sea ice dynamics, and (b) an anomalous surface winter warming emerging over the Scandinavian/Western Russian region around 10–12 years after a major eruption. The simulated long-term climate response to SVEs depends, to some extent, on background conditions. Consequently, ensemble simulations spanning different phases of back- ground multidecadal and longer climate variability are necessary to constrain the range of possible post-eruption decadal evolution of NH regional winter climates. Keywords Strong tropical volcanic eruptions Á Coupled ocean–atmosphere Á North Atlantic Oscillation Á Meridional overturning circulation Á North Atlantic subpolar gyre Á Millennium simulations 1 Introduction Strong volcanic eruptions (SVEs) can eject large amounts of sulfur into the atmosphere that induce a major, though transient, impact on the global climate (see e.g., the reviews by Robock 2000; Cole-Dai 2010). This is essen- tially driven by the radiative perturbation due to increased concentration of stratospheric sulfate aerosols, which leads to temporary stratospheric warming and tropospheric (including surface) cooling. The first effect is due to the D. Zanchettin (&) Á S. Lorenz Á K. Lohmann Á J. H. Jungclaus Ocean in the Earth System Department, Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany e-mail: davide.zanchettin@zmaw.de C. Timmreck Atmosphere in the Earth System Department, Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany H.-F. Graf Centre for Atmospheric Science, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK A. Rubino Department of Environmental Sciences, Ca’ Foscari University, Dorsoduro, 2137 Venice, Italy K. Kru ¨ger IFM-GEOMAR, Leibniz-Institute of Marine Sciences, Du ¨rsternbrooker Weg 20, 24105 Kiel, Germany 123 Clim Dyn (2012) 39:419–444 DOI 10.1007/s00382-011-1167-1