Abstract
A climatology of polar low cyclogenesis in the North Pacific Ocean for seven winter seasons (November–March) is presented. The data are collected by the subjective interpretation of Defense Meteorological Satellite Program (DMSP) infrared imagery. The results demonstrate that, on average, the most active polar low cyclogenesis takes place in the western extratropical North Pacific, with the greatest rates of formation found just to the east of northern Japan; few polar lows form east of the date line. Agreeing with theory and earlier observations, comma clouds are by far the most common form of polar low in the North Pacific and spiraliforms are relatively rare. Intra-annual variability is explained by the seasonal cycle of the large scale ocean-atmosphere system, with early winter polar lows forming off the land or ice edge in the more northerly parts of the basin, and midseason cyclones forming southward and increasingly away from land as the circumpolar vortex expands. Late in the season, the frequency of polar low cyclogenesis drops as the circulation weakens and the circumpolar vortex breaks down. Despite this underlying control by the seasonal cycle, however, in any given winter the regions of peak cylogenesis can be located nearly anywhere in the extratropical portions of the basin and are associated with interannual variations in the large scale ocean-atmosphere environment.
