A Diagnostic Study of the Life Cycles of Persistent Flow Anomalies

The main objective of the thesis is to identify the primary mechanism(s) responsible for the formation of large-scale persistent flow anomalies. Towards this goal, we perform detailed synoptic and diagnostic analyses of the composite life cycles of persistent flow anomalies, focusing particularly on persistent flow anomaly (PA) development over the North Pacific and Atlantic Oceans during wintertime. The reproducibility of the results is demonstrated using an independent dataset, and a case study is presented in order to further illustrate the representativeness of the composite results. We first construct ensemble averages centered on PA onset and breakdown times to obtain a thorough observational description of the typical three-dimensional flow structures associated with PA life cycles. Negative PA's over the North Atlantic and Pacific are preceded by intensification of the upstream climatological-mean subtropical jet stream and by positive anomalies in upper-tropospheric potential vorticity over the upstream continent. The early portion of both Pacific negative (PN) and Atlantic negative (AN) development is characterized by synoptic-scale cyclogenesis and enhanced geostrophic frontogenesis over the western ocean. Analogous baroclinic ridge development occurs over the western North Pacific during Pacific positive (PP) formation. AN and Atlantic positive (AP) developments are both initially characterized by the westward retrogression of a large-scale flow anomaly towards the key region. During large-scale development, all four types exhibit zonally elongated and westward tilting perturbation structures in the jet exit regions, with associated fluxes of zonal momentum and heat directed down their respective time-mean gradients. The PN and AN cases also exhibit strong upward heat fluxes in the troposphere during large-scale growth, with weaker vertical heat fluxes for the positive cases. Specific diagnostic analyses are then applied in order to test particular hypotheses for PA development. Wave activity flux diagnostics indicate that the primary sources for PA development are local to mid-latitudes. Little evidence is found of anomalous tropical forcing. Except during PP developments, sources for development are also primarily local to the key regions. During PP developments, there is an anomalous wave activity flux from an upstream trough and a weak poleward flux emanating from the subtropical North Pacific. Nevertheless, budgets of energy and enstrophy suggest that, for all four PA types, local conversions associated with the composite perturbations are sufficient to account for eddy growth. The ratio of conversion to enstrophy change is largest during Pacific developments. Nonconservative processes and incoherent eddy forcing, calculated as a residual, oppose PA development. This effective dissipation is strongest during the Pacific developments. Potential vorticity analyses suggest that PN and AN