Factors Affecting the Stagnation of the Baiu Front that Induced Heavy Rainfall in July 2020 over Kyushu, Japan

Abstract

In this paper, the influencing factors of the long-term stagnation of the Baiu front that induced heavy rainfall in July 2020 over Kyushu, Japan, were examined. The position of the Baiu front was determined from a weather chart at 130°E. The front remained stationary between 30 and 35°N for 20 days in July 2020. The number of total days of Baiu front stagnation was the largest from 2002–2021. By examining a certain index of the Yellow Sea High (YSH), which is a necessary condition for Baiu front stagnation near Kyushu, it was confirmed that in 2020, a positive high-pressure anomaly persisted over the Yellow Sea until the end of July. The observed YSH maintenance was potentially due to the negative sea surface temperature (SST) anomaly over the Yellow Sea throughout July 2020. In addition, the impact of extratropical cyclones passing at the end of June was suggested as a factor of the occurrence of significant negative SST anomalies in the Yellow Sea. With the passage of a strong extratropical cyclone through the central part of the Yellow Sea from June 29–30 in 2020, a rapid SST decrease ranging from 0.5–1.5°C/day was caused by ocean mixed layer deepening. This rapid SST decreasing process was repeated 3 times on 13, 19, and 24 July. As a result, a notable SST contrast was maintained at approximately 30°N because the SST difference between the Kuroshio Current and the Yellow Sea remained large throughout July 2020. The notable contrasts in the SST and surface air temperature (SAT) coincided well with YSH maintenance over the sea west of Kyushu and could contribute to long-term stagnation of the Baiu front.