The North Pacific Oscillation (NPO), the second leading atmospheric mode in the North Pacific Ocean, 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 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), this study reveals the intimate relationship between the NPO structure and the atmospheric mean states over the North Pacific. The majority of the models reasonably capture the meridional contrast of pressure anomalies, but the detailed horizontal characteristics of the NPO are found to differ among 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 models in which the climatological continental trough over the western North Pacific extends farther to the east, the NPO tends to be simulated farther to the east, strengthening its impact on the downstream climate. In contrast, when the climatological continental trough is reduced in size toward the west, the growth of the NPO is limited to the west, and its influence is weakened downstream. This relationship can be understood via the altered available potential and kinetic energy conversions that feed the total energy of the NPO, primarily stemming from the difference in the mean horizontal temperature gradient and stretching deformation of the mean horizontal wind.
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Acknowledgments. We thank three anonymous reviewers for their constructive comments. This research was supported by National Research Foundation of Korea (NRF) grants funded by the South Korean government (MSIT) (NRF-2018R1A5A1024958 and 2019 R1C1C1003161). Author Sung was supported by NRF-2018R1D1A1B07044112. SWY was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2018-03211. Author Kosaka is supported by the Japan Society for Promoting Science (Grant 18H01278) and Japan Ministry of Education, Culture, Sports, Science and Technology through ‘‘Arctic Challenge for Sustainability’’ and ‘‘The Integrated Research Program for Advancing Climate Models.’’
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All Science Journal Classification (ASJC) codes
- Atmospheric Science