Interannual Variations in Summer Extreme Precipitation Frequency over Northern Asia and Related Atmospheric Circulation Patterns

This study investigates the spatial-temporal variations in summer extreme precipitation event (EPE) frequency over northern Asia and related atmospheric circulations. The division analysis indicates that three subregions of western Siberia (WS), eastern Siberia (ES), and eastern Mongolia-northeastern China can be identified, and the EPE variations over WS and ES are focused on here. On an interannual time scale, higher EPE frequencies are related to a similar dipole pattern in the upper troposphere [anomalous cyclone (anticyclone) to the west (southeast) of these two subregions] and a local anomalous cyclone in the lower troposphere. The dipole pattern leads to anomalous air divergence in the upper troposphere and compensating ascending motion over the subregions; the local anomalous cyclone in the lower troposphere leads to water vapor convergence. These anomalous atmospheric circulations therefore provide favorable dynamic and moisture conditions for higher EPE frequencies. Further analysis indicates that the WS EPE frequency is influenced by the combination of polar-Eurasian (POL) and North Atlantic Oscillation (NAO) patterns, while the ES EPE frequency is influenced by Scandinavian (SCAND) [British-Baikal Corridor (BBC)] pattern over 1987-2004 (2005-15). The alternate influence on the ES EPE frequency may result from the interdecadal change in the structure of SCAND and BBC patterns. In addition, the East Asian summer monsoon (EASM) shows enhanced influence on ES EPE frequency after the late 1990s, which could be due to interdecadal strengthening and extending of the anomalous cyclone around Lake Baikal. This cyclone is concurrent with EASM, and its changes favor water vapor transported by EASM to ES after the late 1990s.