北日本の冷夏とグローバル気候システム変動

Abstract

Mean summer temperatures in northern Japan appear to exhibit cyclical variations after the regime shift—global-scale climatic jump of oceanography and meteorology fields—around the late 1970s. Alternation between cool and hot summers has resulted in cool and hot weather damage to rice crops, respectively. The temperature fluctuations are caused by the Rossby wave propagation (Pacific-Japan pattern) from the tropical western Pacific (Nitta, 1987). This teleconnection also affects other regions, hence many agricultural districts in Asia may experience simultaneous meteorological effects. This paper focuses on the influence of the teleconnection pattern on three areas: northern Japan, northeastern China, and Java, Indonesia. These areas are important in terms of food supplies (mainly paddy rice production) for their countries. In northern Japan, variations of summer temperatures correlate strongly with the 500 hPa height field and SST east-west contrast around the western tropical Pacific. A positive correlation between the SST contrast and northern Japan summer temperatures has occurred since the 1980s, and it is clear that after the regime shift northern Japan summer temperatures have been affected by Rossby wave propagation from the tropical Pacific Ocean. The relations between summer temperatures in northern Japan and in Heilongjiang, northeastern China, and the global meteorology field were analyzed. Summer temperatures in northern Japan and Heilongjiang were found not to exhibit simultaneous variations. Northern Japan summer temperatures have a negative correlation with summer 500 hPa heights over a wide tropical area centered on the Indochina Peninsula and around eastern Siberia, which are the results of Rossby wave propagation and formation of Okhotsk high pressure in summer. In the case of Heilongjiang, there is not a strong correlation with the tropical area and there is no correlation around eastern Siberia. Hence, the teleconnection pattern does not strongly affect summer temperatures in northeastern China. Summer temperatures in northern Japan and spring surface pressure show a negative correlation around the Arabian Sea and the western tropical Pacific. This may signal the pre-Indian monsoon and pre-Asian monsoon pressure field. On the other hand, summer temperatures in Heilongjiang and the spring pressure field show a strong positive correlation over the Tibetan Plateau. This result raises interesting issues about their relationship, such as whether a spring high pressure indicates a dry surface over the plateau and whether an increase of sensible heat from the land affects any feedbacks in the next season. In summary, the factors that cause summer temperature variations over northern Japan and Heilongjiang, respectively, are different. Precipitation in JJA in Java and global surface pressure show a positive correlation around northern Japan. This implies that high precipitation over Java is related to a high-pressure anomaly over northern Japan. This result may be due to the simultaneous influence of the Rossby wave on both areas. In conclusion, the factors affecting summer temperature variations in northern Japan and Heilongjiang in China differ, and summer climates in northern Japan and Java, Indonesia are closely connected through Rossby wave propagation. This study improves our understanding of simultaneous variations of climate via teleconnection patterns for these agriculturally important locations, which is of importance to understand variations of food supplies under possible future climatic variability.