Impact of anomalous Eurasian blocking activities on the East Asian Meiyu rainfall

Abstract

An extreme East Asian Meiyu rainfall in both amount and duration occurred along Yangtze River valley during June–July of 2020, however, possible mid-high latitude signatures causing this super Meiyu have not been well identified. This study explores the cause of the Meiyu rainfall from the aspect of anomalous Eurasian blocking activities with a two-dimensional blocking index, using the Japanese 55-yr Reanalysis for 1979–2020. The major findings are as follows. Variabilities of the Eurasian blocking activities are primarily characterized by a tripole pattern with three centers over the Baltic Sea, the Ural Mountains, and the Sea of Okhotsk, respectively. The tripole pattern is associated with two zonally-oriented Rossby wavetrains which may originate from the rainfall anomaly in central Europe. Corresponding to a positive phase of the tripole pattern, the northern wavetrain through energy dispersion tends to induce an anomalous anticyclone (i.e., enhanced blocking) over the Sea of Okhotsk, while the southern wavetrain tends to induce an anomalous cyclone over the Sea of Japan which is conducive to a southward intrusion of more high-latitude cold airs. Consequently, a meridional dipole anomalous circulation pattern over the northeastern Asia is formed, favoring more East Asian Meiyu rainfall. In 2020, the Eurasian blocking activities exhibit a significantly positive phase of the tripole pattern and considerably contribute to the super Meiyu. The results identified in this study highlight the importance of the Eurasian blocking activities in the East Asian Meiyu and provide a new perspective for the prediction of the Meiyu rainfall with mid-high latitude signatures.