Relationship of the Interannual Variation in Quasi-Stationary Heavy Precipitation Systems over Kyushu, Southwestern Japan with the Sea Level Pressure Pattern

This study classified atmospheric fields to investigate the distribution of, and the interannual variation in, heavy rainfall that occur over Kyushu, southwestern Japan, which have shown marked increase in recent years. Radar/rain gauge-analyzed precipitation data acquired during June–September (2006-2023) were used to identify quasi-stationary heavy precipitation systems (QSPSs), and the 6-hourly sea level pressure (SLP) pattern was classified to elucidate the impact of synoptic conditions on the interannual variation in the frequency of QSPSs. It was found that the occurrence location of QSPSs corresponded to the inflow side of lower-level water vapor transport. Specifically, under the SLP pattern representing a cyclonic circulation to the south of Kyushu, QSPSs occurred over the eastern mountain region in Kyushu Island. Conversely, QSPSs occurred over western Kyushu when the SLP pattern reflected the extension of the North Pacific high to the south of Japan. The interannual variation in the occurrence number of QSPSs was interpreted based on the frequency of the above mentioned favorable SLP patterns. The interannual variation in the occurrence number of QSPSs corresponded with changes in the appearance frequency of the favorable SLP patterns as evidenced by many QSPSs occurrences in 2020 under a prevailing North Pacific high, suggesting a possible, albeit not strong, influence of the synoptic circulation on QSPSs. Furthermore, this variation is strongly affected by dynamical and thermodynamical factors specific to each SLP pattern, which play a critical role in determining the probability of the QSPSs occurrence.