Long-Term Variability of the Western Edge of the North Pacific Subtropical High and Its Relation to Summer Temperatures over Japan, 1901-2000

The long-term variability of the western edge of the North Pacific subtropical high (NPSH) and its relationship to summer temperature variability at 17 weather stations in Japan were investigated for the period from 1901 to 2000. Using the Hadley Center sea level pressure dataset (HadSLP2), we developed NPSH indices (NPSHIs) to describe both the zonal and meridional variability of the western edge of the NPSH.The NPSHIs revealed that the western edge of the NPSH had shifted southwestward over the past 100 years. This study contrasts the correlation of temperatures over Japan with NPSHIs between 1901-1950(pre50) and 1951-2000(post50). In the post50, a significant positive correlation between temperature andthe meridional displacement of the western edge of the NPSH was observed at the most stations, which indicates that temperatures over Japan increase (decrease) when the western edge of the NPSH shifts northward (southward). In the pre50, in contrast, a significant positive correlation between temperature and the meridional displacement of the western edge of the NPSH was found at only two stations. In the post50, a strong relationship between the meridional displacement of the western edge of the NPSH and the east-west gradient of summertime sea surface temperature (SST) between the South China Sea and the tropical western Pacific east of the Philippines was observed, while there was no significant connection between them in the pre50. The Pacific-Japan(PJ) pattern, which affects meridional displacement of the western edge of the NPSH, is closely related to the east-west gradient of summertime SST across the Philippines. It is thus considered that the PJ pattern is excited frequently in the post50, which may affect the relationship between the meridional displacement of the western edge of the NPSH and summer temperatures over Japan