Characteristics of the North Pacific Oscillation in CMIP5 models in relation to atmospheric mean states

The North Pacific Oscillation (NPO), the second leading atmospheric mode in the North Pacific, is known to be responsible for climate variability and extremes in adjacent regions. The reproducibility of the NPO in climate models is thus a topic of interest for the more accurate prediction of climate extremes. By investigating the spatial characteristics of the NPO in Coupled Model Intercomparison Project Phase 5 (CMIP5) models, the present study reveals the intimate relationship with atmospheric mean states over the North Pacific simulated in the models. While the majority of the models reasonably capture the meridional contrast of pressure anomalies, the detailed horizontal characteristics of the NPO are found to differ between the models. Diagnostic analysis of 30 climate models and long-term observations suggest that systematic bias in the mean atmospheric baroclinicity over the North Pacific crucially affects the horizontal shape and zonal position of the NPO. In the model in which climatological continental trough over the western North Pacific is expanded to the east, the NPO tends to be simulated farther to the east, modulating the downstream climate more efficiently. In contrast, the climatological continental trough condition reduced in size towards the west over the western North Pacific hinders the normal growth of the NPO. This relationship can be understood via the altered available potential energy conversion resulting from the difference in the mean horizontal temperature gradient, which primarily contributes to the maintenance of the NPO. The present results provide further support for the importance of background baroclinicity in the structural modulation of the NPO via baroclinic energy conversion.