Why Do Baroclinic Waves Tilt Poleward with Height?*

Theoretical and modeling studies of nongeostrophic effects in baroclinic waves predict that baroclinic waves should tilt poleward with height, with a larger tilt in total meridional wind than in geostrophic quantities. Regression analysis of NCEP–NCAR reanalysis 6-hourly data demonstrates that observed baroclinic waves do indeed tilt poleward with height, although the observed tilt is smaller than predicted by previous studies. The meridional ageostrophic wind enhances the poleward tilt of meridional wind perturbations, despite being smaller in amplitude than the meridional geostrophic wind by a factor of 5. An improved estimate of the structure of the meridional ageostrophic wind in baroclinic waves is calculated assuming force balance. Several important terms in this estimate have been left out of previous estimates of the meridional ageostrophic wind. Three terms in the improved estimate produce nearly all of the poleward tilt of the meridional wind: 1) the advection of geostrophic zonal wind perturbations by the mean zonal wind, 2) the convergence of the eddy momentum flux, and 3) the effect of friction. The poleward tilt with height of baroclinic waves explains why upper-level storm tracks tend to occur poleward of near-surface baroclinic regions, and may play a role in the midwinter suppression of the Pacific storm track.