Accurately representing the variability of the tropical Indo-Pacific Ocean in climate models is important in both understanding and trusting future projections that climate models make, with regards to changes in variability and the associated impact. Using the historical simulation from 32 models in the Coupled Model Intercomparison Project phase 5 (CMIP5), we assess the fidelity of the simulated Indian Ocean Dipole (IOD) and El Nino Southern Oscillation (ENSO). We also evaluate the relationship between the IOD and ENSO, and how climatological biases impact on the strength, timing, and frequency of these climate events across the CMIP5 models.

Research has shown that ENSO impacts on both the evolution and strength of the IOD – usually increasing IOD magnitude or initiating IOD events. Realistic simulation of ENSO characteristics (e.g., pattern, timing, frequency, amplitude) is hampered by persistent model biases which vary in extent across CMIP5 models, in particular, in relation to the cold tongue being too cold, and the equatorial easterly wind being too strong. The Indian Ocean also features its own bias, especially in association with the Bjerknes feedback being too strong, which causes many  CMIP5 models to simulate overly strong IOD amplitude. However, there has been relatively little research into the impact of Pacific Ocean and ENSO biases on the IOD simulation across CMIP5 models.

This study aims to investigate the impact of mean state biases in both Indian and Pacific Oceans, and how these biases impact ENSO representation and in turn on the IOD. We have found that the Pacific Ocean cold tongue bias is significantly correlated with IOD strength. Across CMIP5 models we found no significant relationship between IOD event timing and ENSO strength or timing. However, mean state biases in zonal winds and sea surface temperature  in both ocean basins impact on the IOD.