VARIATIONS OF LARGE-SCALE CHARACTERISTICS ASSOCIATED WITH THE INCREMENT OF BAIU PRECIPITATION AROUND 1950

The characteristics of the inter-decadal variations of Baiu-precipitation after 1900, and their relationships to the large-scale characteristics were analyzed with the use of the data of precipitation amounts, surface temperature and sea-level pressure. The Baiu precipitation averaged over Japan tended to show small amounts from 1924 to 1944 (called ''Period I''). But it increased around 1950 and showed relatively large amounts from 1952 to 1972 (called ''Period II''). This inter-decadal variation had large amplitudes in the south-western part of Japan. The following meteorological components showed significant changes between the two periods; 1) the meridional temperature gradient on the western side of Japan (increased), 2) the sea-level pressure between 30-degrees-N and 40-degrees-N in Japan (decreased) and 3) the precipitation amount at Manila, Philippines (decreased). No significant changes were found in the meridional pressure gradient on the north-eastern side of Japan and in the first EOF component of the surface temperature in Japan, although they were correlated with year-to-year variations of Baiu precipitation. The results of the analysis suggest that the increment of Baiu precipitation from Period I to Period II was associated with long-term variations of sub-tropical circulation. An hypothetical process is proposed for the increment of Baiu precipitation around 1950.