Future Changes in Yamase: Focusing on the Meridional Pressure Gradient Over Northern Japan

Abstract The Yamase, a cold northeasterly flow, has frequently caused cool summers in northern Japan, leading to agricultural damage through persistent cloudiness and low temperatures. It remains unclear whether such cool summers will become less frequent under climate change. This study investigates projected changes in Yamase intensity and frequency using 25 CMIP6 models for 2081–2100. To address the mismatch between gridded model outputs and conventional station-based Yamase indices, we develop a grid-based index representing the meridional pressure gradient between northern and southern Japan, which we call the Okhotsk-Japan (OJ) index. The OJ index reproduces historical pressure patterns associated with Yamase days consistent with those derived from conventional indices. The mean OJ index during Yamase days increases by 2.1 hPa and 2.2 hPa in the SSP1-2.6 and SSP5-8.5 scenarios, respectively, accompanied by enhanced surface cooling of 0.2 °C and 0.3 °C. However, the magnitude of temperature decrease may remain comparable to that in the present climate. While the overall occurrence rate remains nearly unchanged at approximately 5%, strong Yamase events become more frequent and more intense. These results indicate that the risk of Yamase-related cool summers persists under future climate conditions.