Large‐Scale Moisture Circulation Index Related to Socioeconomic Drought and Its Subseasonal Forecast Skill in the Asian Monsoon Rainy Season

Abstract Socioeconomic drought, which occurs when water demand exceeds available supply and often leads to water withdrawal restrictions, affects water resource management. However, the relationship between large‐scale moisture circulation and the occurrence of socioeconomic drought has not been well understood. In this study, we focused on the Yoshino River Basin in Japan, where water withdrawal restrictions occur during the rainy season. We showed that the occurrence of socioeconomic drought is strongly associated with a prolonged deficit in large‐scale meridional moisture transport related to the Asian monsoon. Using 11 historical water withdrawal restriction events from 1987 to 2019, lead–lag composite analysis revealed that the meridional component of vertically integrated vapor transport (IVT) remained suppressed for up to four weeks prior to the onset of socioeconomic drought. The suppression of moisture transport is associated with persistent westward or northward extensions of the western North Pacific subtropical high and eastward extensions of the Tibetan High (South Asian High) in midsummer, as well as recurrent ridge formation linked to meandering midlatitude westerlies in early summer. The area‐averaged cumulative meridional IVT near Shikoku was lower than climatology, which led to the development of a new index representing a prolonged moisture transport deficit. Based on a 1‐month forecast, this index may provide improved subseasonal (1‐month) forecast skills to predict socioeconomic drought compared to forecasts using the standardized precipitation index (SPI‐1). This index can serve as a monitoring metric for potential water withdrawal restrictions, especially when combined with precipitation‐based indicators such as SPI‐1.