The peculiar behavior of baroclinic waves during the midwinter suppression of the Pacific storm track

Abstract

The midwinter suppression of the Pacific storm track is described in detail based on NCEP Reanalysis data from 1979 to 2001. The midwinter suppression is characterized by an equatorward shift and overall decrease in storm track intensity, measured in terms of eddy total energy (TE), that occurs primarily above 500 mb over the western and central Pacific. The suppression is accompanied by an equatorward shift and overall strengthening of the Pacific jet and a lowering of the tropopause poleward of the jet; it occurs despite the midwinter maximum in baroclinicity.An eddy energy budget analysis is performed to examine the changes in the flow of eddy energy in baroclinic waves during midwinter. Changes in baroclinic wave structure reduce the efficiency of baroclinic generation of eddy available potential energy (APE); decreased moisture and increased static stability reduce the efficiency of baroclinic conversion of eddy APE to eddy kinetic energy (KE); and shallower baroclinic conversion reduces the fraction of eddy KE that persists at upper levels. These effects overcome the increase in baroclinicity in midwinter to reduce the eddy TE at upper levels relative to fall and spring.Lag regression analysis reveals that, in midwinter, temperature perturbations are shallower because of increased upper level static stability and have a larger eastward tilt with height because of the stronger Pacific jet. Shallowing of the temperature perturbations decreases the fraction of eddy KE that persists, and the shallowing and larger tilt both decrease the efficiency of baroclinic generation. The lag regression analysis also shows that baroclinic waves tilt poleward with height in all seasons because of their ageostrophic wind structure.The combination of the effects of increased upper level static stability associated with the lowering of the tropopause causes most of the midwinter suppression of the Pacific storm track. Increased jet speed and decreased moisture make additional small contributions to the suppression. The poleward tilt with height of baroclinic waves places the maximum upper level eddy energy on the poleward side of the Pacific jet, where it is more efficiently suppressed by the lowering of the tropopause in midwinter.