Regional Extreme Precipitation Events in Wintertime Japan Facilitated by East-Asian Large-Scale Flow Patterns

The statistical and dynamical relationships between regional extreme precipitation events (EPEs) during wintertime in five Japanese regions and East-Asian synoptic weather patterns are addressed. Two of the five weather patterns, the southerly flow (SF) and low pressure (LP), are associated with about 50% of EPEs in all the regions. A regional dependency is found, with SF being more likely to cause extreme precipitation in two regions in the south of Japan and LP in the other regions, respectively. The large-scale dynamics leading to EPEs in each region are assessed by a combined Lagrangian and Eulerian analysis. In the two southern regions, EPEs are predominantly associated with direct moisture supply from the subtropical oceans. This is modulated by the large-scale flow pattern of SF. In contrast, EPEs in the northern coastal areas of the Sea of Japan and the Pacific Ocean are influenced by anomalous moisture supply from the cyclone-induced moisture convergence modulated by LP. The eastern coastal region of the Sea of Japan shows a mixture of both these moisture supply mechanisms. The strong link between EPEs and synoptic patterns might help to improve predictions of extreme events, even on the sub-seasonal forecast skill horizon.