Two distinct intraseasonal modes of summer rainfall variation over the eastern Tibetan Plateau
Thursday, 18 December 2014
During summer, the weather disturbances over the eastern Tibetan Plateau (ETP) can initiate development of severe weather system downstream, causing flooding in East Asia. Previous studies yielded inconsistent results on the intraseasonal variability of summer rainfall in terms of periodicity, genesis process and propagation pathway, due to data paucity over the EPT. In this study, we detected two dominant peaks, centered on 10 (8–12) days and 19 (14–24) days using daily rainfall data over the ETP during the period of 1992–2007. Composite analysis revealed that the two modes with different periodicity were predominantly produced by the non-stationary wave trains in the upper troposphere, which traveled along different pathways. For the 19-day mode, the wave train featured a southeastward migration, originating in the Barents Sea and traveling via the East European Plain, the Ural Mountains, Lake Balkhash and Lake Baikal, the Mongolian Plateau, and then continued southward to the ETP and South Asia. In contrast, the 10-day wave train propagated eastward along the westerly jet, extending from the eastern North Atlantic via the Mediterranean, Black and Caspian seas towards the ETP, East Asia and Southeast China. These two transient wave trains took place on different large-scale background circulations: a meridional pattern with a “Giant Ural Mountain Ridge” for the 19-day mode and a zonal pattern with “Western Siberian Trough” for the 10-day mode. In terms of simultaneous downstream linkage, the ETP rainfall anomaly was in-phase with the rainfall anomaly over the Yangtze River Basin for the 19-day mode but out-of-phase with the 10-day mode. The major processes generating the local wet spells for the two modes were found to be commonly linked to topographic lift and mid-tropospheric latent heat release, but the moisture sources are somewhat different. The evolution of the intraseasonal variability over the ETP described here may provide a useful guidance for 2–3 week (extended range) forecasts over the ETP and its downstream regions.