Mechanistic Insights Into Regional Air‐Sea Coupling Effects on East Asian Summer Climate: A Comparative Modeling Study With a New Regional Earth System Model

Abstract Regional air‐sea coupling critically influences East Asian summer climate, yet its representation in current modeling systems remains challenging. This study enhances mechanistic understanding of this coupling by comparing two long‐term simulations (1991–2014) using the new Regional Integrated Earth Model System (RIEMS) with air‐sea coupling and its standalone atmospheric model (WRF). Results show both RIEMS and WRF demonstrate advantages and added values in reproducing summer air temperature (T2mean and T2max) and precipitation compared to the ensemble mean of global climate models (ENS). Meanwhile, RIEMS better reproduces T2mean and T2max compared to WRF, particularly over East China. RIEMS reduces the T2max warm bias in WRF by 0.7°C on average over land. Additionally, RIEMS offers more accurate simulations of surface sensible and latent heat fluxes while WRF overestimates sensible heat flux by nearly 7.0 W m −2 over land. Mechanistic analyses indicate that WRF reproduces less cloud cover and increased incoming shortwave radiation, resulting in higher sensible heat flux and warmer air temperature. The climatological decrease in cloud cover in WRF can be attributed to its warmer air temperature. Meanwhile, WRF simulated less outgoing shortwave radiation and trapped more longwave radiation, leading to warmer surface air temperature. Furthermore, without air‐sea coupling, WRF strengthened water and energy transportation from ocean to land through a stronger southwesterly airflow, which is more conducive to simulating higher surface air temperatures in WRF over a large area. These findings provide mechanistic evidence that regional air‐sea coupling is essential for credible climate simulations and projections over East Asia.