Satellite climatological aspects of the “polar low” and “instant occlusion”



Middle resolution DMSP (Defense Meterological Satellite Program) visible and infrared imagery are analyzed in conjunction with surface and upper-air synoptic observations for two years' (1977, 1978/79) mid-season months (January, April, July, October) to derive synoptic climatological information on the “polar low” and “instant occlusion” phenomena. Polar-air vortices occur most frequently over the oceans in winter. Regional differences in the dominance of cloud signature sub-types confirm variations in the dynamics involved in polar-air cyclogenesis, as noted by previous workers. The “comma cloud” appears to occur more frequently in the North Pacific, while the spiraliform vortex predominates in the North Atlantic. The latter seems to be at least partly attributable to the effect of the eastern Arctic sea-ice boundary, as demonstrated by an analysis of imagery for two winters (D. J. F). each representative of an opposing mode of the North Atlantic Oscillation (1974/75. 1976/77).

Composite synoptic “models” of polar lows reveal surface-pressure departures similar to those for cyclogenesis of the frontal wave type. However, a cold core and little vertical tilt distinguish the polar low in the mean. The “instant occlusion” exhibits surface-pressure departures that are deeper and comparable to the mature/dissipating stages of wave cyclone evolution. Spatial patterns of the upper-air anomalies associated with instant occlusions show two distinct vortices: one apparently derived from the polar low, the other from the incipient frontal wave with which it has merged. The seasonal and latitude-dependent cyclone intensity statistics (all types) are applied to the cloud vortex inventory of the North Atlantic-Europe sector for January 1979, The results support the rôle of the polar low as a synoptic indicator of anomalous winter circulation in the extratropics.