The large temperature variation in summer over the northeastern part of Japan (Tohoku District) is studied with special emphasis on the occurrence of anomalously cold summer. The result of EOF analysis of the monthly mean temperature for 1951-1980 and that of the daily mean temperature for 1976-1980 over Japan shows that the lst EOF component accounts for -70% of the total variance. The spatial function of the 1st EOF component indicates the same sign over the whole analyzed domain with small value in the southwestern Japan and the large value over the Pacific side of Tohoku District.
Some statistical relations between the time variation function of the 1st EOF component and some meteorological parameters expressing the large-scale pressure (height) field are examined. The large-scale situation in the anomalously cold summer is also compared with that in the anomalously hot summer. It is concluded that the cold summer in Tohoku District caused by the southwestward intrusion of the polar maritime airmass which is formed over the Okhotsk Sea, the Bering Sea and the northern Pacific. The high correla-tion among the surface air temperature, the surface sea water temperature and the sunshine duration are also pointed out.
It is the very interesting and important fact that the anomalously cold summer occurs over the localized area (the Pacific side of Tohoku District), despite the fact that the cold summer is caused by the large-scale easterly flow from the polar maritime airmass. The structure of the cold easterly flow which causes the localized Tohoku cold summer will be studied in the Part 2 of the present study.