Life Cycles of Persistent Anomalies. Part II: The Development of Persistent Negative Height Anomalies over the North Pacific Ocean

Abstract

The present study extends our previous work on the life cycles of persistent anomalies by providing more comprehensive analyses of the synoptic and dynamical characteristics associated with the developments of the anomalies. We focus here on the developments of major cases of persistent negative height anomalies over the extratropical central North Pacific (PAC) region during wintertime. These ewes are generally manifested at the surface by an anomalously intense and eastward-displaced Aleutian low and, at upper levels, by an abnormally strong zonal jet that extends across most of the western and central Pacific at midlatitudes. The associated flow anomalies usually resemble particularly strong realizations of the Pacific-North American (PNA) teleconnection pattern. The large-scale flow anomalies are typically preceded by a buildup of anomalously cold air over Asia and an intensification of the upper-level jet over southeastern Asia and the fox western Pacific. A few days prior to the larger-scale developments, a synoptic-scale disturbance intensifies over the northwest Pacific in a region of pronounced baroclinity on the cyclonic-shear side of the upper-level jet. As this disturbance propagates eastward into the mid-Pacific, it acquires a more zonally elongated, equivalent barotropic structure. During this period, the upper-level zonal wind anomalies initially over the western Pacific also extend eastward to the central Pacific. The large-scale anomaly pattern that subsequently develops over the Pacific and North America resembles the most rapidly growing normal mode associated with barotropic instability of the climatological-mean wintertime flow. Diagnostic analyses confirm that, particularly at later stages in the developments, barotropic conversions from the time-mean flow contribute positively to the growth of the anomalies. These results support the idea that barotropic instability of the time-mean flow provides one mechanism for the developments. Nevertheless, baroclinic processes also appear to play a significant role, particularly during the early stages of the developments. Temperature advection patterns associated with the growing disturbance tend to concentrate temperature gradients along the axis of the intensifying jet. Net eddy heat fluxes during the developments are both downgradient and upward, although most consistently so during the early stages of the developments. Net heat fluxes at later stages continue to have a substantial downgradient component although they also display a strong rotational (nondivergent) component, consistent with the more equivalent barotropic structure of the disturbance observed at later times. The overall impression that emerges is of initial baroclinic development at long synoptic scales, followed by increasing barotropic contributions and decreasing baroclinic contributions to the growth of the anomalies after the disturbance reaches the jet exit region over the central Pacific. Additional baroclinic contributions to the developments may also occur at later stages. The observed characteristics are consistent with the hypothesis that the large-scale flow anomalies in these cases develop primarily as a result of an instability of the three-dimensional wintertime mean flow.