Improved Response of δ 18 O sw in the Pacific Ocean to Atmosphere‐ocean Interaction and ENSO Using the Isotope‐Enabled Fully Coupled Model MIROC6‐Iso

Abstract Stable water isotopes serve as tracers of climate processes and are useful for identifying drivers of regional and global hydrological variability. To get a precise picture of spatiotemporal changes in the water cycle, it is necessary to describe the mechanisms in a coherent way among the involved climatic reservoirs (atmosphere, land, and ocean). For example, in the Pacific Ocean, the oxygen isotopic composition (denoted δ 18 O) of surface seawater recorded in corals is widely used to reconstruct the El Niño–Southern Oscillation (ENSO). However, the influences of atmosphere–ocean feedback and ocean circulation on the δ 18 O–ENSO relationship are not fully understood. Describing water isotopes in the full climate system is one way to tackle this issue. Therefore, this study introduces the fully coupled isotopic version of the Model for Interdisciplinary Research on Climate version 6 (MIROC6‐iso). MIROC6‐iso exhibits good performance in reproducing spatial isotopic variations in precipitation, water vapor, and ocean water, and the relationships of δ 18 O with temperature and salinity against observations. We also find that the atmosphere‐ocean Coupled General Circulation Model (CGCM) captures δ 18 O sw (seawater) variations significantly better than a configuration where an Atmospheric General Circulation Model is coupled to a 1D slab ocean model in Pacific Ocean in climatological condition and different ENSO phases. Budget analysis indicates that the better performance of the CGCM configuration is due to fully resolved oceanic vertical mixing and horizontal advection. This study shows that MIROC6‐iso is useful to reconstruct past climate changes and examine the recent changes in the water cycle.