Influence of the Eurasian Wave Trains on Summer Rainfall Extremes Along East Asian Quasi-Stationary Rainy Fronts from 1991 to 2024

During the early summer monsoon in East Asia, quasi-stationary frontal precipitation in July accounted for approximately 41% of Korea’s total summer rainfall from 1991 to 2024. Despite the inclusion of the global warming hiatus within this period, interannual trends in frontal rainfall were not statistically significant over Korea or most of East Asia. We found that changes in the meridional temperature gradient, resulting from pronounced warming or cooling north of 40°N, contributed to shifts in both the location and intensity of the rainy front. In extreme positive precipitation years in Korea (EPP-K), a sequence of alternating near-surface air temperature anomalies across the Eurasian continent intensified a wave-3 Eurasian wave train, forcing wave activity into the mid-latitude regions. Blocking highs over eastern Siberia, induced by this wave train, triggered mid-Siberian cooling intrusions north of East Asia, thereby enhancing the meridional temperature gradient near the quasi-stationary rainy front over the Yellow Sea–Korea (YK) region. The enhanced southwesterly moisture convergence promoted frequent mesoscale convective clouds and associated EPP-K events. Under these conditions, increased convergence of anthropogenic aerosols via southwesterly flows facilitated aerosol–cloud interactions, promoting cloud droplet formation and increasing rainfall efficiency by 24.9%. Conversely, during extreme negative precipitation years (ENP-K), broad near-surface warming across the high- to mid-latitudes of the Eurasian continent strengthened a wave-5 Eurasian wave train, resulting in weakened wave flux compared with EPP-K events and further weakening the meridional temperature gradient near the rainy front over the YK region. The reduced southwesterly moisture convergence limited the development of mesoscale convective clouds, resulting in ENP-K events. Although anthropogenic aerosols—primarily emitted from eastern China—continued to be advected toward the YK region even during the rainy season, the warming-induced stable atmospheric conditions inhibited cloud development and further suppressed precipitation by 7.5%. These findings underscore the critical roles of the surface warming in the Eurasian continent and aerosol–cloud microphysical processes in shaping extreme wet and dry anomalies of East Asian summer frontal rainfall.