Influence of ENSO Decadal Variations on the PDO and Indian Ocean SST in Observations and CMIP Models

The first mode of sea-surface-temperature (SST) variability in the north Pacific Ocean (north of 20N) is often referred as 'Pacific Decadal Osciallation' (PDO), owing to its prominent energy level at decadal time scales. This mode of variability with its spatial extension over the tropical Pacific (referred as Interdecadal Pacific Oscillation – IPO) is indeed the dominant mode of decadal variability in the Pacific Ocean. The PDO can be understood in observations as a direct response to ENSO and atmospheric white noise in the north Pacific, reddened by the reemergence of north Pacific SST from winter to winter. In this study, we characterize PDO simulated by 32 coupled models from the CMIP (Coupled Model Intercomparison Project) database, with a specific focus on the representation of tropical/extratropical teleconnections in these models. The analysis shows that the PDO, in a majority of the CMIP models, exhibits a clear tropical Pacific signature and agrees well with the hypothesis that (along with atmospheric white noise) ENSO is a major driver of PDO at both interannual and decadal timescales. One third of the models however show a PDO confined to the north Pacific, with weak tropical signals and ENSO influence. In the CMIP models, the amplitude of Indian Ocean response to PDO is found to be closely related to the amplitude of PDO signature over the tropical Pacific. The observational analysis suggests Indian Ocean teleconnection pattern associated with the tropical Pacific variability differ between interannual and decadal timescales, with a warming largely confined to the southwestern Indian Ocean at decadal timescales. In contrast, models show very similar signatures at both timescales, suggesting a possible misrepresentation of the teleconnections to the Indian Ocean at decadal timescales.