The assessment of simulated thermocline depth over southwestern tropical Indian Ocean in CMIP models

Abstract The southwestern tropical Indian Ocean (SWTIO) is a key region that influences both regional and global climate systems. In this study, we examine the simulated thermocline depth in the SWTIO using models from the Coupled Model Intercomparison Project Phases 3 (CMIP3), 5 (CMIP5), and 6 (CMIP6). The results reveal that the multi-model ensemble (MME) means from CMIP3, CMIP5, and CMIP6 differ in their climatological representations of thermocline depth. Notably, all MMEs simulate a deeper thermocline than observed, with CMIP6 showing the greatest spatial deviation. For the long-term trend during 1981–1999, the MME means from all three CMIP generations simulate a significant increasing trend in thermocline depth, which contrasts with the observed decreasing trend. Among the CMIP3, CMIP5, and CMIP6 models analyzed in this study, only a subset successfully captures the observed shoaling trend, and notable discrepancies persist between the simulations and observational data. Moreover, the performance of some models that are common across the three CMIP generations has deteriorated in simulating thermocline depth. Overall, the findings suggest that the ability of CMIP models to accurately simulate thermocline depth has not substantially improved from CMIP3 to CMIP6.