Intermodel Spread in Indian Ocean Basin Mode Simulation: Roles of Indian Ocean Winds and Pacific Cold Tongue

Abstract The Indian Ocean Basin Mode (IOBM) is a major component of interannual sea surface temperature (SST) variability in the tropical Indian Ocean and acts as an atmospheric–oceanic bridge extending El Niño–Southern Oscillation (ENSO)’s climate influence into the boreal summer of its decay phase—known as the capacitor effect. Despite advancements, state-of-the-art climate models continue to show systematic biases in simulating the IOBM during ENSO decay summers. Analyzing historical simulations from 56 models in phase 5 and 6 of the Coupled Models Intercomparison Project (CMIP5/6), this study identifies two dominant modes of intermodel spread in IOBM simulations: an Indian Ocean Dipole (IOD)-like mode linked to climatological easterly wind biases over the equatorial Indian Ocean, and an eastern Indian Ocean (EIO)-centered mode associated with equatorial Pacific cold tongue biases. The IOD-like spread pattern arises from easterly wind biases that weaken SST–thermocline coupling in the southwestern Indian Ocean, suppressing IOBM amplitude. The EIO-centered pattern stems from cold tongue biases that shift ENSO anomalies westward, altering EIO SSTs through Indo-Pacific interactions. Using statistical relationships between the spread modes and historical mean-state biases, we refine CMIP5/6 projections of the IOBM and its capacitor effect under warming, revealing a stronger and more consistent enhancement than previously estimated. These findings highlight the role of background climatological wind and SST biases in shaping model spread and underscore the importance of improving Indo-Pacific mean-state simulations for more reliable IOBM representation and future climate projections.