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Lorenz (1968, 1976) stated that regime transition in almost-intransitivity of nonlinear climatic system may play an important role in climatic change, and he suggested that climatic change associated with the transition may appear in interannual variabilities. Referring to Lorenz's suggestion, we will treat abrupt changes of time mean, designated as climatic jumps. A quantitative definition of jump and simple method of its detection are presented, noting that the time of jump appearance can be specified within a margin of several years. Some jumps are detected in time series of seasonal mean data of surface air temperature, sea level pressure, precipitation, sunshine duration and maximum depth of snow-cover averaged spatially over Japan. The fact that jumps appear commonly in various climatic elements around 1950 suggests an association of these jumps with some abrupt changes of the atmospheric general circulation.Concerning the cause of the jumps around 1950, we survey some change in external forcings. Big explosions of several volcanoes over the world occurred almost simultaneously with the jumps around 1950 after a pause of about 30 years. It is inadequate to assume that this volcanic activity would directly cause the jump in transitive system, because the possible climatic effect of volcanic eruption is mainly cooling and the jump of temperature is warming over Japan. However, further studies are needed for any definite conclusion on problem whether this reopening of volcanic activity would be a triggering action of the regime transition or not.

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