The synoptic climatology of polar low outbreaks

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Abstract
The evolution of the 500 mb height field has been analysed during outbreaks of well-developed polar lows over the Norwegian and Barents Seas using a “superposed epoch method” to composite the data. 42 cases were selected on the basis of synoptic and surface data. In addition, 10 independent cases were chosen on the basis of infrared satellite imagery. To further describe the synoptic climatology of polar low outbreaks, 500 mb temperature, 1000–500 mb thickness, and surface pressure data were each composited for the combined 52 cases of the superposed epoch studies. In order to provide a background from which to evaluate the results of the composite studies, a brief case study of a typical polar low development over the Barents Sea is given. The results of the composite studies reveal the presence of significant negative anomalies in both the temperature and height fields at the 500 mb level over the Norwegian and Barents Seas, indicating strong positive vorticity and very low static stability over the area on the days when mature polar lows were present. The evolution of the height anomaly pattern shows ridging along the west coast of Greenland, and the development of a trough north of Norway, resulting in a northerly component of the flow aloft and at the surface over the Norwegian Sea, as much as 3 days prior to the outbreak of polar lows. The northerly surface flow results in the development of low-level baroclinicity. Evidence suggests that a rapid deepening, characteristic of polar lows, occurs coincident with the outbreak of deep convection. It is suggested that the outbreak of convection is organized by the baroclinicity. The evolution of the negative height anomaly just prior to polar low outbreaks is suggestive of forcing by a migratory short wave aloft. The 1000–500 mb thickness composite supports this view, with the presence of some deeper baroclinicity across the Norwegian Sea. However, comparison of the surface and thickness composites reveals only slight asymmetries in the respective contours, indicating that the atmosphere was nearly equivalent barotropic on the days when strong polar lows were present over the region. The in-situ evolution, and wavelength of the height anomaly pattern suggest that topographical forcing by the high Greenland Plateau, juxtaposed with the warm water of the Norwegian Sea, may enhance the development of polar low outbreaks off the Norwegian coast. DOI: 10.1111/j.1600-0870.1985.tb00441.x