Super East Asian monsoon Mei-yu in June and July 2020 tied to dissimilar-shifting upper-level westerlies

Extreme precipitation anomalies over the East Asia-western Pacific exhibited an unusually similar pattern in June and July 2020, with catastrophic Mei-yu floods in the Yangtze River Valley and severe droughts in southern China and the Bay of Bengal-the western North Pacific (BOB-WNP). However, the corresponding atmospheric circulation anomalies showed distinct characteristics, especially the meridional shifts of the East Asian westerly jet (EAWJ). In June 2020 the EAWJ displaced northward in response to a high-latitude Rossby wavetrain originated from the western North Atlantic, whereas in July 2020 the EAWJ shifted southward due to a robust Pacific-Japan (PJ) wavetrain. The linchpins of the PJ wavetrain are the rapid sea surface warming and the resultant strong convection over the Arabian Sea and the tropical eastern Indian Ocean in July, which considerably enhanced the divergence and sinking motion over the Philippine Sea and the convergence and rising motion over Korea and Japan, favorable for triggering the PJ wavetrain. Meanwhile, the enhanced WNP subtropical high was located further northward to Japan in July compared to June, transporting abundant moisture to Korea and Japan and further strengthening the Mei-yu/Baiu in July, which in turn maintained the PJ wavetrain.

Automated summary

Extreme precipitation anomalies occurred in June and July 2020 in the East Asia-western Pacific region, resulting in catastrophic floods in the Yangtze River Valley and severe droughts in southern China and the Bay of Bengal-western North Pacific (BOB-WNP). The atmospheric circulation anomalies were characterized by the meridional shifts of the East Asian westerly jet (EAWJ) which were influenced by high-latitude Rossby wavetrains and a robust Pacific-Japan (PJ) wavetrain.