Impacts of Historical Atmospheric and Oceanic Warming on Heavy Snowfall in December 2020 in Japan.

Strong cold air outbreaks caused heavy snowfall over the inland areas of the Sea of Japan coast of Japan in mid‐December 2020. The 48‐hr accumulated snowfall during the term broke a snowfall record at some observational stations of the Japan Meteorological Agency (JMA). We investigated the impacts of atmospheric and oceanic temperature changes due to historical global warming since the pre‐industrial period on this heavy snowfall event. Hindcast and pseudo non‐warming experiments were conducted using the nonhydrostatic model developed by the JMA. The historical climatic differences were obtained from the database for policy decision‐making for future climate change (d4PDF). Our experiments indicated that historical global warming enhanced precipitation over the whole region and enhanced heavy snowfall over the inland areas on the Sea of Japan side. In this event, the contribution of conversion from snowfall to rainfall due to warming was limited over coastal areas. The enhancement of precipitation was mainly caused by oceanic warming, which increased latent heat flux over the Sea of Japan and made the low‐level atmosphere less stable. Historical atmospheric warming suppressed precipitation due to atmospheric stabilization. Our results indicated that the heavy snowfall was caused by the cold air outbreaks under the warm sea surface temperatures in December and enhanced by historical global warming. Plain Language Summary: Heavy snowfall occurs over the inland areas of the Sea of Japan coast in mid‐December 2020. The 48‐hr accumulated snowfall broke a snowfall record at some observational stations. The numerical simulations indicate that historical global warming since the pre‐industrial period enhanced the heavy snowfall over the inland areas on the Sea of Japan side. The oceanic warming is more important for the enhancement of snowfall than atmospheric warming only, because oceanic warming supplies more moisture to the atmosphere and makes the low‐level atmosphere less stable. Since the sea surface temperature in December is warmer than that in other month in winter, it widely enhances snowfall over the inland areas along the Sea of Japan. Key Points: Historical global warming enhanced heavy snowfall occurring over inland areas along the Sea of Japan in central Japan in mid‐December 2020Sensitivity experiments indicate that warming of the Sea of Japan mainly contributed to the enhancement of heavy snowfallOceanic warming supplies more moisture to the atmosphere and makes the low‐level atmosphere less stable, which enhances precipitation [ABSTRACT FROM AUTHOR]