Multi-scale successive dynamic processes leading to extraordinary snowfall on the Japan Sea side of central Japan

Abstract This study addresses the issue of why the Sea-of-Japan (SOJ) side of central Japan undergoes extremely heavy snowfall causing weather disasters, focusing on multi-scale successive dynamic processes. The East Asian winter monsoon regarding heavy snowfall is modulated by quasi-stationary Rossby wave packets along the northern Eurasian and South Asian waveguides, i.e., subpolar (SP) and subtropical (ST) teleconnection types. The SP (ST) type typically (rarely) induces a well-organized Japan-Sea polar-airmass convergence zone (JPCZ) over the SOJ, triggering intense precipitation over the coastal (mountainous) region on the SOJ side of central Japan. The SOJ coastal region experienced extreme snowfall events in December 2022 and December 2023 when the SP type and its associated mesoscale JPCZ prevailed. Simulations reveal that a coastal convergence line (CCL), which is much smaller than the horizontal scale of JPCZ, causes extraordinary local snowfall. The CCL is formed by two streams converging quasi-parallel to the SOJ coastal region: the northwesterly winds that directly intrude from the SOJ via the north of the Changbai Mountains (CM K ) north of the Korean Peninsula and the inland southwesterly flows. The inland flows originate from the northwesterlies detouring from the north and south of the CM K , merging over the SOJ, and finally approaching the CCL north of the Chubu Mountains (CM J ) of central Japan. We also find that the presence of CM K and CM J characterizes the mesoscale JPCZ and the associated local-scale CCL, respectively. Further understanding of these teleconnection-induced multi-scale processes could improve the prediction of severe snow disasters in wintertime Japan.