CMIP6 model performance in simulating three types of positive Indian Ocean Dipole and their future changes

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This study evaluates the performance of 40 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the diversity of positive Indian Ocean Dipole (pIOD) events and their projected changes under global warming. Based on the relative amplitudes of sea surface temperature anomalies between the western and eastern tropical Indian Ocean (TIO), pIOD events are classified into three categories: west-dominated (Type-W), east-dominated (Type-E), and comparable (Type-C). In historical simulations, CMIP6 models systematically overestimate the cooling magnitude in the southeastern TIO, resulting in a pronounced bias toward excessive Type-E events and underrepresentation of Type-W and Type-C events. Under the SSP5-8.5 scenario, the frequencies of both Type-E and Type-C pIODs are projected to increase significantly by the end of this century, with Type-E events showing the most pronounced increase (80%). This projected intensification is closely linked to a “pIOD-like” warming pattern across the TIO, where the western basin warms faster than the southeastern basin, thereby providing a more favorable background state for the development of eastern cooling anomalies.