Snow anomaly events from historical documents in eastern China during the past two millennia and implication for low-frequency variability of AO/NAO and PDO

Models and instrumental data indicate that the spatial and temporal variations of snow cover are significantly related to atmospheric circulation (e. g. the AO/NAO). Here, we present historical snow anomaly events during the past two millennia that provide a unique temporal window to studying long-term AO/NAO, a prominent phenomenon in wintertime. Direct descriptions such as "no snow during the winter'' and "pray God for snow'' are interpreted as convincing evidence for snow anomalies. The variations of positive/negative snow abnormal events show clear decadal to century variations during the past two millennia. Based on the previous instrumental research and comparison with other reconstruction data, we suggest the Index of Abnormal Snow (IAS) may be an AO-like atmospheric variability. The winter during the Medieval Warm Period (MWP) (AD 900-1300) might be strongly influenced by a predominantly positive AO with less snow condition, whereas the Little Ice Age (LIA) (AD 1300-1900) by negative AO concomitant with heavier snowfalls in East Asia. Our data show that a warm climate period (the MWP)/a cold period (the LIA) can be perturbed by a cold spell/a warm spell which are linked with a change in atmospheric circulation. Low-frequency variability of snow records may be intrinsic to the natural climate system. Although the dynamic mechanisms linking snow anomalies with atmospheric circulation (the AO/NAO, the PDO) is unclear on the decadal to century time scales, Pacific Ocean may play an important role in regulating atmospheric circulations since the IAS is highly correlated with the reconstruction of PDO.