This study examines the indirect effect of tropical cyclones (TCs) on cases of heavy rainfall during the Baiu season in Kyushu, Japan using data analyses and numerical experiments. A detailed analysis of the heavy rainfall event that occurred on 7 June 1999 (JST) is performed. This event was remotely affected by Typhoon Maggie (9903), which was located approximately 2000 km from Kyushu at the time. As Typhoon Maggie passed close to Taiwan, a high potential vorticity (PV) zone appeared to the north of Taiwan. A low PV region formed simultaneously to the east of Taiwan, corresponding to a northwestward extension of the Pacific high. These dynamical changes induced an enhanced southerly moisture flux between the high PV zone and the low PV region, leading to moisture convergence and heavy rain in the vicinity of Kyushu. During this time, Typhoon Maggie also caused the northward advection of a separate tropical disturbance.
The high PV zone to the north of Taiwan was produced by diabatic heating associated with interplay between the circulation of Typhoon Maggie and the topography of Taiwan. In contrast, the low PV region was formed through the advection of low-PV air from low latitudes by Typhoon Maggie. A piecewise PV inversion diagnostic shows that the low PV region was the largest contributor to the southerly moisture flux, although both Typhoon Maggie and the high PV zone also made positive contributions. Numerical experiments reveal that the precipitation in and around Kyushu was enhanced by both the topography of Taiwan and the northward advection of the additional tropical disturbance.
This study identifies a new mechanism as an indirect effect of TCs. The core element of this mechanism is a large moisture flux south of Kyushu, which is termed “moisture road,” and the difference from “atmospheric river” is discussed. This mechanism is not unique to Typhoon Maggie, as other cases of heavy rainfall in and around Kyushu are associated with similar situations.