Decadal climate variability of the North Sea during the last millennium reconstructed from bivalve shells (Arctica islandica)

Uninterrupted, annually resolved paleoclimate records are crucial to contextualize the current global change. Such information is particularly relevant for the Europe realm for which weather and climate projections are still very challenging if not virtually impossible. This study presents the first precisely dated, annually resolved, multiregional Arctica islandica chronologies from the North Sea which cover the time interval ad 1040-2010 and contain important information on supra-regional climatic conditions (sea surface temperature (SST), ocean productivity, wind stress). Shell growth varied periodically on timescales of 3-8, 12-16, 28-36, 50-80, and 120-240 years, possibly indicating a close association with the North Atlantic Oscillation, ocean-internal cycles of the North Atlantic controlled by ocean-atmosphere couplings, and the Atlantic Multi-Decadal Oscillation. Increased climatic instability, that is, stronger quasi-decadal variability, seems to be linked to the predominance of atmospheric forcings and some significantly decreased insolation phases (e. g. Sporer and Maunder Minima). Increased climatic variability of shorter timescales was also observed during some particularly warm phases or regime shifts (e. g. during the 'Medieval Climate Anomaly' and since c. 1970). More stable climatic conditions, that is, extended warm or cold periods ('Medieval Climate Anomaly', 'Little Ice Age'), however, fell together with a predominance of multi-decadal oceanic cycles. Whether the sunspot number and the higher frequency climate variability are causally linked and which processes and mechanisms are required lie beyond this study.