The Structure and Energetics of Midlatitude Disturbances Accompanying Cold-Air Outbreaks over East Asia

Abstract

The onset dates for 11 individual cold-air outbreaks over the East Asian seaboard during the Winter Monsoon Experiment (1 December 1978–28 February 1979) are used for constructing composite synoptic charts. The three-dimensional structure and energetics of disturbances with time males shorter than ∼5 days are distinguished from the corresponding properties of more slowly varying fluctuations by using time-filtering techniques. It is seen that the high-frequency disturbances accompanying the cold surges experience systematic structural changes as they migrate along a well-defined storm track from East Asia to the Gulf of Alaska. The typical life cycle of such extratropical storms is characterized by a barolinic growth phase coinciding with the polar outbreaks, and a decay phase in which barotropic processes play an active role. The propagation of low-frequency fluctuations is oriented toward lower latitudes, with new vorticity centers developing downstream and equatorward of the primary disturbances associated with the outbreaks. The shapes of the disturbances appearing in the composite charts indicate that a strong degree of anisotropy exists in both the high-frequency and low-frequency disturbances. The fluctuations with short time scales are elongated in the meridional direction, whereas those with long time scales are elongated in the zonal direction. The findings of this composite study are seen to be consistent with circulation statistics derived from continuous climatological records. The behavior of the fluctuations with short and long time scales is also reminiscent of the characteristics of baroclinically unstable waves and Rossby-wave trains, respectively, appearing in model experiments.