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

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. The Atlantic Multidecadal Variability (AMV) is the dominant mode of North Atlantic sea surface temperature variability at multidecadal scales, which has great impacts on global and regional climates, such as Sahel rainfall, Atlantic hurricanes, and European climate. Considering instrumental data are short for fully addressing multidecadal issues, it is of high interest to understand historical AMV changes. There exist two typical periods in last millennium, namely, the warm Medieval Climate Anomaly (MCA) and cold Little Ice Age (LIA). Both periods provide us with an opportunity to investigate the AMV features. Our study shows that the externally forced components of the AMV vary obviously during the MCA and LIA, while the internally generated parts of the AMV are little modified. Particularly, volcanic forcing plays a vital role in the AMV changes during both periods. Atlantic Multidecadal Variability (AMV) displays a weak power spectrum of 30-50 years during the Medieval Climate Anomaly (MCA) and a strong one of 25 and 35-50 years during the Little Ice Age (LIA)AMV amplitude decreases by 16%-26% during the MCA and increases by 5%-13% during the LIA relative to last millenniumAMV changes during the MCA and LIA are mainly derived from their externally forced components