The Dynamics of Large-Scale Cyclogenesis over the North Pacific Ocean

Abstract

Earlier studies of persistent large-scale flow anomalies have been extended, with the aim of identifying the primary mechanisms for persistent anomaly development. In this study the focus is on wintertime cases of persistent cyclonic flow anomalies over the North Pacific. These cases are typically manifested by an abnormally intense cyclonic circulation extending over the North Pacific basin, an unusually strong and eastward-extended East Asian jet, and a well-defined Pacific-North American teleconnection pattern. We have conducted extensive diagnostic analyses in order to determine the mechanisms responsible for development. In particular, these diagnostics examine the processes influencing the time evolution of eddy potential enstrophy and potential vorticity anomalies. The cases are preceded by a buildup of anomalously high potential vorticity air at upper levels over eastern Asia. This high potential vorticity air is initially advected eastward in association with synoptic-scale cyclogenesis over the western North Pacific. As the disturbance propagates eastward into the central Pacific, it evolves toward a more zonally elongated and equivalent barotropic structure. Large-scale cyclogenesis ensues as the low becomes quasi-stationary near the Aleutians. In conjunction with large-scale development, the disturbance reacquires an upshear tilt with height. Diagnostic analyses of wave activity fluxes indicate that the primary source region for the developments is over the extratropical North Pacific. Potential enstrophy analyses show that eddy enstrophy increases result mainly from downgradient potential vorticity fluxes by the large-scale eddy. The conversions are primarily baroclinic in nature, although barotropic processes also provide positive contributions. Anomalous nonconservative and nonlinear processes are relatively small and oppose the observed enstrophy changes. Potential vorticity (PV) inversions are then performed to further clarify the dynamical mechanisms for large-scale development. A few days prior to large-scale development, anomalous upper-level northwesterly winds, associated with low-level thermal anomalies over the western North Pacific region, advect high PV air south-eastward from Asia into the western Pacific. As the PV maximum reaches the central Pacific, its associated circulation penetrates to the surface, resulting in a thermal advection pattern that produces a warm surface anomaly and associated surface cyclone downshear of the upper-level center. This is followed by strong baroclinic intensification. In several respects this behavior resembles a classical Petterssen Type B development, but occurs on a scale that is much larger than for typical synoptic-scale cyclogenesis. The results indicate that the primary mechanism for the developments is a large-scale instability of (or initial value development upon) the three-dimensional time-mean flow, and suggest that nonmodal transient growth plays a significant role during development.