Effects of the Tibetan High and the North Pacific High on the Occurrence of Hot or Cool Summers in Japan

In this study, we investigated the effects of the Tibetan High near the tropopause and the North Pacific High in the troposphere on occurrences of hot or cool summers in Japan. We first classified Japan into six regions and identified hot and cool summer years in these regions from a 38-year sample (1980-2017) based on the monthly air temperature. To investigate the features of circulation fields over Asia during hot and cool summers in Japan, we calculated the composite differences (hot summer years minus cool summer years) of several variables such as geopotential height, which indicated significant high-pressure anomalies in the troposphere and lower stratosphere. These results suggest that both the North Pacific and the Tibetan Highs tend to extend to Japan during hot summer years, while cool summers seem to be associated with the weakening of these highs. We found that extension of the Tibetan High to the Japanese mainland can lead to hot summers in Northern, Eastern, and Western Japan. On the other hand, hot summers in the Southwestern Islands may be due to extension of the Tibetan High to the south. Similarly, the latitudinal direction of extension of the North Pacific High is profoundly connected with the summer climate in respective regions.

Automated summary

This study investigated the effects of the Tibetan High near the tropopause and the North Pacific High in the troposphere on hot or cool summers in Japan. Results showed that the extension of these highs to Japan is associated with hot summers, while their weakening is related to cool summers. The latitudinal direction of extension of these highs is critical in determining the summer climate in different regions of Japan.