A new simple and accurate measure of baroclinicity

Abstract. Baroclinicity is the measure of baroclinic growth rate in the midlatitude storm tracks. Spatially, it defines the genesis region of the storm track and, temporally, it sets the activity of the storm track. Baroclinicity can be quantified with the Eady growth rate, which is proportional to the vertical shear of the zonal wind. This can alternatively be expressed as a meridional gradient of dry entropy via thermal wind balance. Both expressions of the Eady growth rate are equally valid given their derivation under the quasi-geostrophic approximations in which thermal wind balance is assumed. With a focus on the North Atlantic winter storm track, we demonstrate that variability in lower-tropospheric baroclinicity, averaged over a region encompassing the storm track, is determined almost entirely by lower-tropospheric dry entropy to the north of the storm track, with decreases in northward entropy corresponding to increases in baroclinicity. We use physical arguments as well as a linear regression to relate storm track baroclinicity to the mean dry entropy to the north of the storm track, demonstrating that variability in area mean baroclinicity of the storm track is mainly determined by processes on its poleward side, rather than by local-scale internal processes within the storm track itself. It also provides a much simplified physical picture to accurately describe the dynamics of the N. Atlantic storm track.