Day-to-day atmospheric circulation modulated the impact of marine heatwaves on heatstroke risk in Hokkaido in August 2023

Abstract

Abstract Extreme high temperature and humidity threaten high-latitude regions that are less adapted to such conditions. While marine heatwaves (MHWs) can contribute to hot and humid conditions over land, the modulation of these impacts by the day-to-day atmospheric circulation remains poorly understood. This study focused on Hokkaido, the northernmost prefecture in Japan, to investigate how the daily atmospheric circulation regulated the distribution and intensity of MHW-related heatstroke risk during August 2023. Regional atmospheric model simulations indicated that the MHWs that developed south of Hokkaido increased the Wet Bulb Globe Temperature (WBGT) by approximately 2°C in southern coastal areas. Backward trajectory analysis showed that the intensity of MHW-induced heatstroke risk was governed by the daily atmospheric circulation. On days when MHWs substantially impacted the WBGT in the southeastern coastal region, air parcels passed over regions with high sea surface temperatures (SSTs). Conversely, when trajectories bypassed these MHW areas, the impact remained low. These results demonstrate that the daily atmospheric circulation can determine the magnitude of heatstroke risk over land. Our findings suggest that the relative positioning of airflow pathways and the SST distribution should be considered to improve heatstroke risk prediction in coastal regions.