Fundamental mechanisms of the growth and decay of the PNA teleconnection pattern

This investigation performs diagnostic analyses on NCEP/NCAR re-analysis data, and also does forced, nonlinear, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the PaciŽ c/North American teleconnection pattern (PNA). The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the diagnostic analyses and model calculations reveal a PNA life cycle that is complete within approximately 2 weeks and is dominated by linear processes. The growth of the two upstream PNA anomaly centres is found to be by barotropic conversion from the zonally asymmetric climatological  ow, and the two downstream PNA anomaly centres by linear dispersion. The PNA anomaly growth eventually ceases because of changes in the spatial structure of the anomaly. An analysis of the role of Ekman pumping is performed with a very simple model. The results, although qualitative, suggest that the decay of the PNA may be through Ekman pumping. An examination of the role of transient eddy vorticity  uxes indicates that they play an important role during some stages of the PNA life cycle. Lastly, the model calculations also reveal a crucial role played by the divergence term in maintaining the PNA anomaly in a quasi-Ž xed position.