Development of late spring-early summer rainstorms in southern East Asia from the eastward propagating mid-tropospheric cyclones of South Asia

From mid-May to late June, the eastward propagation of some mid-tropospheric cyclones (MTCs) originating in the Bay of Bengal (BOB)-Burma (BUM) region across northern Indochina and southwest China to East Asia are observed. From 1979 to 2016, 137 MTCs were identified for the BOB-BUM region. Eighty-eight MTCs were trapped over northern Indochina, but 49 were propagated by the southwesterly flow of the BOB mid-tropospheric trough to reach East Asia. Thirty-one moved across Taiwan, but 18 did not. Eventually, all these MTCs moved to Okinawa. The former group of rainstorms produced rainfall >= 100 mm.day(-1) over Taiwan, while 44 rainstorms generated rainfall >= 50 mm.day(- 1) over Okinawa. The 49 eastward propagating through-Indochina MTCs exhibited the following salient features: (a) initial formation mostly occurred in the mid-troposphere ahead/within the BOB trough in the morning aver the BOB, but in the afternoon-evening over BUM. (b) The eastward propagating BOB-BUM MTCs underwent two descending processes: descending one mandatory level 6 hr after formation, and descending again from 700 to 925 hPa over southeastern China to interact with a low-level cyclonic shear flow. (c) Intensified (weakened) by dipole anomalous circulation cells, the confluent monsoon southwesterly flow west (east) of Taiwan facilitated the merger of the MTC with a local front. (d) Rainfall produced by rainstorms over Taiwan/ Okinawa was maintained by the intensified convergence of water vapour flux toward Taiwan/Okinawa. This hydrological process was stronger toward Taiwan than Okinawa and generated more rainfall in the former region. The impact of the BOB-BUM MTC on East Asian weather is confirmed by this study.