Secular and seasonal variations of winter monsoon weather patterns in Japan since the early 20th century

Winter weather conditions in Japan are characterized by a clear contrast between the regions on the Japan Sea side and those on the Pacific Ocean side of the Japanese Islands. Weather conditions in the Japan Sea side are characterized by snowfall or rainfall under winter monsoon outbreak conditions. However, the Pacific Ocean side has fine weather because the central ridges of mountains block snow clouds coming from the Japan Sea. This particular weather pattern can easily be recognized from the daily precipitation distributions. In this study, we used the daily precipitation data recorded by the Japan Meteorological Agency since 1901 to define winter monsoon weather pattern based on the contrast in the spatial distribution of daily precipitation between the regions on the Japan Sea side and those on the Pacific Ocean side. On the basis of seasonal changes in the occurrence frequencies of winter monsoon weather pattern, we delimited the beginning and ending pentads of the winter monsoon season for each year. We aimed to clarify the characteristics of long‐term variations in the lengths of the winter monsoon season and in the seasonal march of winter weather conditions in Japan since the early 20th century. The results revealed a significant decreasing trend in the lengths of the winter monsoon season since the early 20th century. We also found that the occurrence frequencies of winter monsoon weather pattern have been decreasing significantly since the early 20th century. In addition to the long‐term decreasing trend, we detected an abrupt decrease in the occurrence frequencies of winter monsoon weather pattern in the mid‐1980s, which suggests a weakening of the winter monsoon after the mid‐1980s. Copyright © 2010 Royal Meteorological Society