Changes of Atlantic Multidecadal Variability During the Medieval Climate Anomaly and Little Ice Age and Associated Causes

Abstract This study investigates changes in the period, amplitude, and spatial pattern of Atlantic Multidecadal Variability (AMV) during the Medieval Climate Anomaly (MCA; 950–1250) and Little Ice Age (LIA; 1450–1850) by analyzing simulations from the Community Earth System Model‐Last Millennium Ensemble. During the MCA, the AMV indices from ensemble members display periods of 30–50 years and their amplitudes decrease by 16%–26% for the 25th–75th percentile range relative to last millennium. During the LIA, periods of 25 and 35–50 years and increase of amplitudes (5%–13%) are found in ensemble members. The AMV shows horseshoe patterns during both periods, with a stronger subtropical warming center during the MCA than during the LIA. Further analysis indicates that AMV changes are mainly derived from their externally forced components, especially the volcanic forcing induced ones. However, internally generated parts of AMVs change little during the two periods.