A numerical study of the Pacific polar low.

Abstract

A numerical study is performed to test the hypothesis that the Pacific polar low forms primarily through baroclinic instability. Polar lows are storms, which can be violent, occurring over the oceans primarily in the winter. Linear growth rates computed for a jet profile typical of conditions during the formation of Pacific polar lows yields a growth rate maximum at 1400 km. Although the horizontal scale of this disturbance agrees with observations of the Pacific polar low, this disturbance is shallower and forms further north relative to the jet than indicated by observations. Nonlinear simulations of the 1400 km disturbance indicate that the effects of convection and surface heat and moisture fluxes due to initially large air-sea temperature differences are needed for proper vertical structure, storm position and intensification, Energy budgets of model output indicate that the wave grows through baroclinic instability and that the diabatic effects enhance the baroclinic conversion within the storm.