Observational and Model Evidence for an Important Role for Volcanic Forcing Driving Atlantic Multidecadal Variability Over the Last 600 Years

Abstract

The modern history of North Atlantic sea surface temperature shows variability coinciding with changes in air temperature and rainfall over the Northern Hemisphere. There is a debate about this variability and, in particular, whether it is internal to the ocean-atmosphere system or is forced by external factors (natural and anthropogenic). Here we present a temperature record, obtained using the Sr/Ca ratio measured in a skeleton of a sclerosponge, that shows agreement with the instrumental record over the past 150 years as well as multidecadal temperature variability over the last 600 years. Comparison with climate simulations of the last millennium shows that large cooling events recorded, in the sclerosponge, are consistent with natural (primarily volcanic activity) and anthropogenic forcings. There are, however, multidecadal periods not connected to current estimates of external forcing over the last millennium allowing for alternative explanations, such as internally driven changes in ocean and atmospheric circulation.