The synoptic climatology of polar-low outbreaks over the Gulf of Alaska and the Bering Sea

Abstract

The synoptic environments conducive to the development of polar-low outbreaks are investigated for the North Pacific region, including the Gulf of Alaska and the Bering Sea. Two case studies of polar low outbreaks are presented, using standard synoptic data, all available ship reports and satellite imagery. One case occurred over the Bering Sea, and the other over the Gulf of Alaska. The case studies show that the environments conducive to the development of strong polar lows include: a deep outflow of arctic air over open water and a cold-core, closed low aloft. Additionally, forcing from a small-scale vortex aloft is associated with the formation of strong polar lows. When synoptic conditions are favorable for the formation of polar lows, a series of them often develop in close proximity to each other. Furthermore, once favorable environmental conditions have developed, they often persist for several days and can result in several polar-low outbreaks.

To develop a climatology of the synoptic environments conducive to the formation of polar lows over the Gulf of Alaska, 500 mb height, temperature, 1000–500 mb thickness and surface pressure data were composited for days when mature polar lows were present. The composite studies reveal the presence of significant negative anomalies centered over the northern Gulf of Alaska in the 500 mb temperature, height and thickness fields. These results indicate the presence of enhanced positive vorticity and the potential for deep convection over the area.

The evolution of the negative-height anomaly indicates the development of a trough that results in a northerly component of the flow aloft over the Aleutian Islands and Alaska Peninsula as much as three days prior to the outbreak of polar lows. Strong northerly surface flow across open water indicated by the surface pressure composite results in small static stabilities at low levels and in the development of baroclinicity in the boundary layer through the modifying effects of surface fluxes of sensible and latent heat. Finally, a comparison is made between the climatology of polar lows over the Gulf of Alaska and over the Norwegian and Barents Seas.