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Keywords:

  • Intraseasonal variability;
  • Zonal index

Abstract

This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation.

The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ.

Both high-frequency (period <10 days) and low-frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low-frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society