An Investigation of Small Synoptic-Scale Cyclones in Polar Air Streams

Abstract

The large-scale environment of small synoptic-scale cyclones that occur in polar air streams over the wintertime North Pacific behind or poleward of major frontal bands is objectively documented by the compositing of meridional (latitude-height) cross sections for 22 cases. The cold air mass cyclones are found to be associated with deep baroclinity throughout the troposphere and are located on the low-pressure side of well-developed jet streams in regions of strong cyclonic wind shear. The lower troposphere is conditionally unstable and in the early stage of development is strongly heated from below by the warmer ocean. Based on the results of the composites, and theoretical considerations, it is concluded that the oceanic polar air cyclones are a baroclinic instability phenomenon whose small scales relate to low values of the Richardson number near the surface and whose large upper level amplitudes relate to the effects of latent heat release on baroclinic wave development. Barotropic instability cannot be ruled as a possible contributing factor in their formation.