Characterizing two types of transient intraseasonal oscillations in the Eastern Tibetan Plateau summer rainfall

During summer, the intraseasonal disturbances over the Eastern Tibetan Plateau (ETP) can initiate development of severe weather system downstream. Previous studies yielded inconsistent results on the intraseasonal variability (ISV) of summer rainfall in terms of periodicity, genesis process and propagation pathway over the EPT based on small samples. In this study, we detected two dominant peaks of transient ISV, centered on quasi-biweekly (12-24) days (QBW) and quasi-9 (8-11) days using daily rainfall data over the ETP during the period of 1992-2012. Composite analysis revealed that the two ISVs were predominantly associated with the non-stationary wave trains, which traveled along different pathways. For the QBW, its mid-latitude wave train featured a upper-level southeastward migration, originating in Northern Europe and traveling via the East European Plain, the Ural Mountains, Lake Balkhash-Lake Baikal, the Mongolian Plateau, and then continued southward to the ETP and South Asia; and its tropical wave train was characterized with a northwestward/northward migration in low-level, starting from the Philippine Sea (PS)-South China Sea (SCS) region, moving over northern Bay of Bengal-SCS region, and arriving over the southern fridge of TP and southern China. In contrast, for quasi-9-day, the most significant mid-latitudes variability featured an eastward propagating upper-level wave train, originating from Western Europe, passing across the Mediterranean, the Black and Caspian seas, arriving over the TP, and moving towards East Asia; and the most evident tropical variability exhibited a clear northwestward/westward migration of a low-level wave train, originating from PS, passing over Taiwan, and subsequently moving towards southeastern China. Their different spatiotemporal features of associated wave trains caused their distinct linkages with eastern China rainfall anomalies. A "meridional pattern" with a giant Ural Mountain Ridge was the most remarkable precursory signals in upstream region before the QBW wet phase occurrence, which was different from the quasi-9-day. The major processes generating the local wet spells for the two modes were commonly linked to the preceding boundary warming, the upper-level negative vorticity and low-level prevalent winds, which triggered the ascending anomaly and lower tropospheric pre-moistening. We also discussed the co-variation of the wave trains between mid-latitudes and low-latitudes. The distinct evolution related with the two ISVs over the ETP described here may provide a useful guidance for 2-3 weeks (extended range) forecasts over the ETP and its downstream regions.