Risk-Based Event Attribution Analysis of Anthropogenic Influence on the 2023 Baiu Extreme Rainfall in the Tohoku Region

Abstract

In July 2023, persistent rainfall associated with the Baiu front caused record-breaking rainfall and flooding on the Sea of Japan side of the Tohoku region, where heavy Baiu rainfall is usually less common. Using a risk-based event attribution approach with a 100-member climate simulation at 5-km grid spacing, our analysis revealed that anthropogenic global warming substantially increased the likelihood of such an extreme rainfall event under the July 2023 conditions. At the Akita weather station, the probability of a 72-h rainfall event similar to that observed in 2023 increased from approximately 0.4% in a non-warming climate to 7.2% in the historical climate. This indicates that anthropogenic warming increased the probability of a 2023-like extreme rainfall event by approximately 18 times. As observed, the ensemble experiment showed that westward extension of the western Pacific subtropical high facilitated moisture inflow toward the Sea of Japan side of the Tohoku region, and that this extension was forced primarily by the global sea surface temperature pattern. Under this synoptic condition, increase in lower-atmosphere equivalent potential temperature driven by global warming enhanced the latent instability, contributing to the persistent heavy rainfall similar to that observed during the 2023 event.