Inter-model spread in East Asian Summer Monsoon onset timing linked to the tropical Pacific and Indian oceans' mean state in CMIP6 models

A major challenge for Earth system climate models in simulating the onset of the East Asian summer monsoon (EASM) is the wide inter-model spread of outputs, which causes considerable uncertainty in projections. Based on 31 models of the Coupled Model Inter-comparison Project Phase 6 (CMIP6), this study investigated the inter -model spread of EASM onset biases and its physical connection with simulated atmospheric circulation and tropical Pacific mean state biases. The inter-model leading mode of EASM onset biases exhibits a meridional seesaw pattern, with inverse patterns for South China and the Yangtze-Yellow River basins. The leading mode is closely related to the simulated meridional displacement of the East Asian subtropical westerly jet (EASWJ) and the Northwest Pacific subtropical anticyclone (WNPAC). The northward EASWJ and WNPAC respectively cause ascending or descending motions with moisture convergence or divergence in the Yangtze-Yellow River basins of South China. The leading mode can be attributed to simulated biases in the zonal tripole sea surface temperature (SST) in the tropical Pacific Ocean. Statistical analysis and numerical experiments based on CAM5.0 indicate that the SST bias stimulates "double-loop" zonal vertical circulation in the tropical Pacific and Indian oceans, influencing convection precipitation in the tropical central-western Pacific. The altered convection then triggers a Pacific-Japan-like pattern anomaly, forming meridional seesaw circulation and precipitation anomalies in East Asia. Such meridional seesaw circulation modulates the meridional displacement of the EASWJ and WNPAC, thus affecting EASM onset. Our results provide potential indicators of inter-model uncertainty of EASM onset.