Dynamical diagnosis of Pacific/North American variability pattern

A comprehensive dynamical diagnosis of the Pacific/North American (PNA) variability pattern was undertaken through observational analysis and modeling studies. As a prominent wintertime low-frequency variability mode in the Northern Hemisphere, the PNA pattern can impact the North American winter climate substantially. However, the notion that the PNA pattern is the primary atmospheric response to ENSO events appears to be untenable in view of the results analyzed.; The combined rotated principal component analysis (RPCA) of the 200 hPa geopotential and SST anomalies clearly separated the PNA variability from ENSO's extratropical impact. The choice of the vertical level is found to be important in this regard. We analyzed geopotential variability at the 200 hPa for the best identification of tropical-extratropical interactions. In contrast, 500 hPa would not be a reasonable choice as ENSO anomalies in the subtropics are weakly expressed at this level.; Through regressions against the PNA mode's principal component, it was found that there existed coherent structure of PNA divergent circulation over the Pacific basin and the striking in-phase relationship of the upper-level divergence and diabatic heating. It was conceived that the PNA upper-level divergence and diabatic heating anomalies could be ultimately generated by the related Pacific stormtrack displacements.; The PNA pattern in RPCA exhibited both intraseasonal as well as interdecadal variations. The lag and lead regression analysis helps to establish the OLR anomalies (or Madden-Julian Oscillation phase) at various stages of PNA evolution. It was also found that the North Pacific SST mode offers a basis for seasonal persistence by providing background forcing for PNA variability.; A major objective of the thesis was also to model PNA variability, in all its aspects. The diagnostic model provided a striking simulation of PNA variability including its rotational and divergent circulations. This modeling analysis revealed that the zonal/eddy interactions played rather significant role in generating the mature-phase PNA. This analysis also showed an insignificant role of PNA diabatic heating anomalies, but a considerable one for the submonthly vorticity transient anomalies, which generated a response in near-quadrature with the response to zonal/eddy forcing.