Unusual Weathers

Japan experienced an unusually cool summer in 1993 after an interval of about 40 years. The mean temperature over northern and eastern Japan was about 2°C lower than normal. Agricultural products such as rice were severely damaged due to this cool summer climate. Figure 2 shows the surface weather map and the jet stream at about 5km height (red lines). Based on the analysis of the surface and upper air observations it can be understood that the extremely cool summer in 1993 may have resulted from (1) the Pacific High being very weak and (2) the Ohotsk High being stationed for a long period throughout the summer season to the north of Hokkaido. Recent studies indicate that the strength of the Pacific High is related to interannual variations of tropical convection and the sea surface temperature. The long duration of the Ohotsk High seems to be linked to the dominance of the blocking phenomena to the north of the Japan islands. In the Climate Analysis Section we intend to pinpoint the major causes of unusual weathers and investigate their mechanisms.