Kazuyo Murazaki, Tosiyuki Nakaegawa
Received 15 June, 2025
Accepted 28 August, 2025
Published online 25 December, 2025

Kazuyo Murazaki¹⁾, Tosiyuki Nakaegawa²⁾

1) Department of Applied Meteorology Research, Meteorological Research Institute, Japan
2) Department of Climate and Geochemistry Research, Meteorological Research Institute, Japan

This study assesses long-term hydroclimatic variability in Japan using the Standardized Precipitation Index (SPI) derived from the JRA-3Q (Japanese Reanalysis for Three-quarter Century) dataset. Among various accumulation periods (SPI-3, SPI-6, SPI-12), the 12-month SPI (SPI-12) exhibited the strongest spatial correlation with observations from AMeDAS, Japan’s dense meteorological observation network, confirming its ability to capture interannual precipitation variability. A long-term trend analysis from 1949 to 2023 revealed a widespread wetting tendency across Japan. Composite analysis between the early (1949–1978) and recent (1994–2023) 30-year periods suggested potential dynamical and thermodynamical drivers, including enhanced water vapor flux at 850 hPa near Japan and the Philippines, and subtropical jet changes at 200 hPa – stronger winter flow and reduced northward migration in summer. These circulation shifts appear to contribute to the observed persistent wetting trend. Overall, the findings demonstrate that integrating SPI with JRA-3Q, a moderately resolved reanalysis dataset, enables effective detection of regional hydroclimatic trends. This approach offers a valuable framework for drought monitoring and climate risk assessment, even in regions with dense observation networks like Japan. Future studies should incorporate more advanced diagnostic or modeling approaches to better understand the underlying physical mechanisms.

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