CMIP6 Biases in Simulating Contrasting Indian Ocean Dipole-the Indian Ocean Basin Mode Relationships

The Indian Ocean Dipole (IOD) and Indian Ocean Basin Mode (IOBM) are two major modes of tropical Indian Ocean variability that exert important regional climate impacts. Observations show a marked contrast in their linkage: El Niño-Southern Oscillation (ENSO)-accompanied IOD (EIOD) events are significantly followed by IOBM in the subsequent boreal spring, whereas pure IOD (PIOD) events show little connection to the IOBM. Here, we assess whether CMIP6 models reproduce this observed contrast using 45 historical simulations and a consistent target-seasonal EOF framework for identifying ENSO, IOD, and IOBM variability. Thirty-four models realistically capture the spatial patterns of the three climate modes, but only 12 reproduce the observed contrast between EIOD–IOBM and PIOD–IOBM relationships. Another 12 models substantially overestimate the PIOD–IOBM relationship. The bias arises from overly strong climatological easterlies over the southeastern tropical Indian Ocean, which shoal the mean thermocline, strengthen Bjerknes feedback, amplify PIOD-related anomalies, and enhance westward-propagating Rossby waves. These processes favor persistent warming over the western Indian Ocean and lead to a spurious basin-wide warming in the following spring. The results identify mean-state biases in the tropical Indian Ocean as a key source of CMIP6 bias in simulating IOD–IOBM coupling.