Due to the lack of absolutely-dated oceanographic information prior to the modern instrumental period, there is currently significant debate as to the role played by North Atlantic Ocean dynamics in previous climate transitions (e.g., Medieval Climate Anomaly-Little Ice Age, MCA-LIA). Here we present analyses of the first millennial-length, annually-resolved and absolutely-dated marine 18O archive. We interpret our record of oxygen isotope ratios from the shells of the long-lived marine bivalve Arctica islandica (18O-shell), from the North Icelandic Shelf in relation to seawater density variability and demonstrate that solar and volcanic forcing coupled with ocean circulation dynamics are key drivers of climate variability over the last millennium. During the pre-industrial period (AD 1000-1800) variability in the sub-polar North Atlantic leads changes in Northern Hemisphere surface air temperatures at multi- decadal timescales indicating that North Atlantic Ocean dynamics played an active role in modulating the response of the atmosphere to solar and volcanic forcing.