Historical Changes in the Japan Sea Polar Airmass Convergence Zone (JPCZ) and Associated Precipitation

Abstract The Japan sea polar airmass convergence zone (JPCZ) frequently produces short-duration heavy snowfall events over the coastal plains along the Sea of Japan. This study investigates long-term changes in the JPCZ using a 5-km dynamical downscaling dataset based on reanalysis data. The frequency of JPCZ events shows no significant long-term trend from 1959 to 2022 but shows pronounced decadal-scale variability. The JPCZ occurred frequently between 1959 and 1986 and after 2010. We mainly focus on two decadal periods (1959–71 and 2010–22) characterized by a higher frequency of JPCZ events. As compared with the former period (1959–71), JPCZ-induced precipitation increased across most regions in the latter period (2010–22), except to the south of the JPCZ. Snowfall has decreased over the Sea of Japan and adjacent coastal areas, while it has increased over inland regions of Japan. Meanwhile, rainfall has significantly increased over the Sea of Japan. The position of the JPCZ has shifted northeastward in the latter period, corresponding to the northward displacement of the upper-level trough and the jet stream. At the continental scale, the upper-level Rossby wave packets along the polar front jet and cold-air outbreaks enhance the synoptic-scale trough over the Sea of Japan during JPCZ events. They have weakened in the latter period. Our results also indicate that long-term climate change has already affected JPCZ-induced precipitation and the location of the JPCZ, while continental-scale atmospheric circulations exhibit different changes between the composites of JPCZ events and the climatological means. Significance Statement The purpose of this study is to clarify long-term changes in Japan sea polar airmass convergence zone (JPCZ) from 1959 to 2022 winter. The JPCZ frequently produces short-duration heavy snowfall events over the coastal plains along the Sea of Japan, including several major cities with population of more than 400 000. We found that JPCZ-induced snowfall decreased over the Sea of Japan and adjacent coastal areas in a recent decade (2010–22), while it increased over inland regions of Japan. The position of JPCZ shifted northeastward, corresponding to the northward displacement of the synoptic-scale upper-level trough and the jet stream in the recent decade. Both global warming and natural variability influences the position of JPCZ and JPCZ-induced precipitation amounts and types.