Spatiotemporal Dynamics and Future Projections of the 400 mm Isohyet in China Based on Self-Organizing Maps

The 400 mm isohyet is an important geographical boundary in China. Owing to climate change, the spatial position of this precipitation line is shifting, which has significant impacts on agricultural production, population distribution, and ecosystems in China. First, precipitation data from 1950 to 2022 were obtained from the fifth-generation ECMWF atmospheric reanalysis (ERA5). The 400 mm isohyet was extracted from the gridded precipitation field and then resampled using an improved uniform parameterization method. Subsequently, the Self-Organizing Map (SOM) method was employed to classify the 400 mm isohyet. Then, the spatiotemporal trends in precipitation corresponding to different isohyet categories were analyzed. Furthermore, the mechanisms driving the spatiotemporal changes in the isohyet were explored from the perspectives of water vapor transport and atmospheric circulation. Finally, based on the optimized outputs from nine models of the Coupled Model Intercomparison Project Phase 6 (CMIP6), the future trends in the migration of the 400 mm isohyet in China during the carbon-neutrality period were simulated. The results show that the 400 mm isohyets from 1950 to 2022 can be classified into three types using the SOM method: the Northeast China wet precipitation pattern (NEWP), the Southwest China dry precipitation pattern (SWDP), and the Northeast dry–Southwest wet precipitation pattern (NED-SWWP). From 1950 to 2022, the annual precipitation in China significantly decreased. The multiyear average precipitation was 808.86 mm, and the three precipitation types ranked as NEWP (829.08 mm) > SWDP (808.70 mm) > NED-SWWP (781.47 mm). In terms of spatial distribution, precipitation decreased in Northeast China, North China, Yunnan, and the southern Qinghai–Tibet Plateau. Meanwhile, precipitation increased significantly in South China and the eastern and northwestern sides of the Qinghai–Tibet Plateau. The 400 mm isohyet types show a clear interdecadal shift around the year 2000, with the dominant pattern changing from the NEWP to NED-SWWP. Changes in water-vapor transport intensity between the two periods were a major factor contributing to the interdecadal shift. In addition, an anomalous anticyclone over East Asia weakened precipitation in Northeast China, and anomalous southwesterlies from the Arabian Sea, together with anomalous southeasterlies converging over the northern Qinghai–Tibet Plateau, increased precipitation in the southwestern segment of China. The multimodel ensemble (MME) method improved the simulation accuracy for the isohyet during the carbon-neutral period. The projections indicate that precipitation in China will increase, and the 400 mm isohyet will continue to migrate westward and northward during this period in China. All annual isohyets during this period are projected to be classified as NEWP. These findings are of practical significance for deepening the understanding of the spatiotemporal distribution and evolution of precipitation in China, and for improving the ability to respond to meteorological disaster risks.