Seasonal and secular variations of sunshine duration and natural seasons in Japan

Seasonal transitions of sunshine duration and their secular changes in Japan are investigated for the period 1951–2000, and the influence of climatic variations on seasonal weather tendencies is discussed. Two seasons with little sunshine in the Baiu and the Akisame seasons are recognized in central and western Japan (CW Japan). In northern Japan, however, only one season with little sunshine in summer is seen. Another season with little sunshine is recognized for the Sea of Japan side and the Southwest Islands during the winter monsoon season. The sunshine duration from spring to autumn in northern Japan decreased in the late 1980s and the 1990s, and the period of little sunshine in summer has become longer since the mid 1980s. In CW Japan, sunshine in late spring in the 1950s and the early 1960s was less than that in other years. A rapid increase in sunshine duration which corresponds with the end of the Baiu rainy season has been delayed since 1980. The period in midsummer with little sunshine has also appeared more frequently than it did before 1980. In the Southwest Islands, sunshine duration has been decreasing almost all through the year. When comparing the differences between the Sea of Japan side and the Pacific Ocean side in winter, the sunshine contrast between them has intensified recently. By using sunshine duration, an objective classification of six natural seasons in CW Japan is conducted. Then, the long‐term change of seasonal march in CW Japan is investigated. The end dates of the Baiu season after 1980 have been delayed by about 5 days compared with those before 1980. From the viewpoint of sunshine contrasts between the Sea of Japan side and the Pacific Ocean side, the winter monsoon season in Japan has become longer in recent years, even under the warming trend of winter temperature. Copyright © 2003 Royal Meteorological Society