Search

During June–July 2020, the record‐breaking flooding in the recent four decades struck the plum rain belt over China, Japan, and Korea. Concurrent with this persistent heavy rainfall, pronounced Indian Ocean basin warming (IOBW) was observed, following the previous El Niño event in the transition to La Niña‐like mean state this summer. This tropical Indo‐Pacific large‐scale thermal condition provided favorable conditions for rainfall surpluses over the plum rain belt via the western North Pacific anticyclone. Superimposed on the tropical Indo‐Pacific large‐scale thermal condition, an extraordinary long‐lasting and quasistationary Madden‐Julian Oscillation (MJO) active phase persisted in the Indian Ocean throughout June–July 2020, lasting for 59 days. The MJO‐associated teleconnection was mainly responsible for the extreme rainfall over the plum rain belt, which was facilitated by the conducive large‐scale Indo‐Pacific oceanic‐atmospheric condition. Plain Language Summary: The East Asian rainy season (commonly called the plum rain, also known as Meiyu in China, Baiu in Japan, and Jangma in Korea), is caused by rainfall along a persistent stationary front. The plum rain season usually starts in June and persists into the middle of July. During June–July 2020, the region of the plum rain belt over China, Japan, and Korea, was ravaged by the strongest flooding in decades with catastrophic losses. The physical mechanisms driving this extreme flooding remains unclear. Concurrent with this persistent rainfall surpluses, the Indo‐Pacific oceans featured a developing La Niña‐like state in the eastern tropical Pacific and pronounced sea surface temperature warming in the Indian Ocean basin. Although these Indo‐Pacific thermal conditions can provide a favorable large‐scale background for the rainfall surpluses over East Asia, they cannot explain the extreme rainfall amount over the plum rain belt during 2020. We show that an exceptionally long‐lasting and quasistationary Madden‐Julian Oscillation activity over the Indian Ocean was the key reason for this extreme event. Key Points: The rainfall amount over the East Asian plum‐rain belt during summer 2020 was the largest in the recent four decadesThe intense flooding coincided with an exceptionally persistent Madden‐Julian Oscillation (MJO) active phase in the Indian Ocean throughout June and JulyThis extraordinary MJO activity was facilitated by the conducive pantropic background oceanic‐atmospheric conditions [ABSTRACT FROM AUTHOR]

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p') at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (su) values are almost the same when the density values are less than 1.14 g/cm3, and the su values increase linearly with an increase in density when the density values are in the range of 1.14–1.2 g/cm3. [ABSTRACT FROM AUTHOR]