Long-term Variations of Snowfall Ratio in the Sea of Japan Side Area of Tohoku Region since the Late 17th Century

Long-term variations of snowfall ratio during winter/early spring in the Sea of Japan side area of north-eastern Japan since 1665 are analyzed. Two long-term historical daily weather documents and instrumental meteorological data are used for this analysis. A time series of snowfall ratios from 1665 to 2005 is reconstructed. It is revealed that multi-decadal scale variations prevailed from the 17th to the early 20th century. In particular, the 1780s and the 1830s are characterized by high snowfall ratios, which coincide with periods of prolonged famines and unusually cool summers. Abrupt decreases of snowfall ratio are observed in the mid-1940s and from the 1970s to the 1990s. These abrupt decreases are unprecedented during the study period. It is revealed that these abrupt decreases were due to a recent rapid warming of the surface air temperature.