Linear inverse modeling is applied to the 10 day low-pass streamfunction field at 300, 500, and 850 mb for 38 Northern Hemisphere (NH) winter seasons of NCEP/NCAR Reanalysis data. We find that while all empirical normal modes of the system are decaying, increase in the streamfunction amplitude is possible through nonmodal growth. When the fastest growing initial perturbation is evolved forward in time, the most probable state of the atmosphere strongly resembles the Pacific-North American (PNA) teleconnection pattern. Comparison between the projection of the streamfunction field onto this pattern and one of the standard PNA indices strongly suggests this pattern does indeed represent the PNA. We also find that during the 38 NH winter seasons, 80% (75%) of the positive (negative) PNA events follow states that project strongly onto the fastest growing initial perturbation. These results suggest that the PNA may be treated as a linear response to white noise stochastic forcing, and that the PNA arises through nonmodal growth.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics