The Positive Pacific–Japan Pattern Drives Compound Heat and Dry Extremes in Summer over Taiwan

This study investigates the role of the Pacific–Japan (PJ) pattern, a dominant mode of the East Asian summer monsoon, in driving compound heat wave–drought events in Taiwan, with a particular focus on land–atmosphere interactions. While the PJ pattern is known for modulating regional rainfall and circulation, its use as a physically interpretable framework to examine dry and warm atmospheric forcing at the land surface remains underexplored for Taiwan. Using a high-resolution 2-km regional reanalysis dataset [Taiwan Reanalysis Downscaling data (TReAD)] for Taiwan from 1980 to 2019, we show that the positive phase of the PJ pattern is associated with suppressed rainfall, depleted soil moisture, and more frequent heat waves. To understand the processes linking these anomalies, we analyze land–atmosphere energy and water exchanges, revealing that soil moisture deficits reduce latent heat flux and increase sensible heat flux—amplifying near-surface temperatures and exacerbating heat extremes. These findings highlight the importance of land–atmosphere coupling in modulating surface climate responses under large-scale meteorological forcing. By connecting predictable large-scale climate modes with local surface processes, this study provides a process-based foundation for using the PJ pattern as a seasonal-scale indicator of compound hot and dry risk in Taiwan. The framework has potential application in early warning and climate risk management, particularly when combined with subseasonal to seasonal (S2S) forecast products. Significance Statement The research investigates how the Pacific–Japan (PJ) pattern influences extreme summer heat and drought events in Taiwan, with a focus on land–atmosphere interactions and soil moisture responses. The study reveals that positive phases of the PJ pattern reduce rainfall and soil moisture, intensifying heat extremes by increasing sensible heat flux. This underscores the role of soil moisture in moderating surface temperatures and highlights how large-scale atmospheric patterns can exacerbate local climate extremes. These insights are essential for developing adaptation strategies to mitigate the social and economic impacts of these compound heat–drought events in Taiwan.