Characteristics of baroclinic wave packets over the North Atlantic during strong and weak polar vortex events

Variations in stratospheric polar vortex strength have been observed to precede shifts in the location of mid-latitude storm tracks. Recent numerical and theoretical explanations suggest that baroclinic waves respond to anomalous stratospheric baroclinic shear associated with stratospheric polar vortex anomalies and produce like-signed tropospheric vortex anomalies through baroclinic wave-mean flow interactions. Motivated by these predictions, a wave activity diagnostic is applied to the NCEP-NCAR reanalysis dataset to observe changes in the characteristics of baroclinic waves during the evolution of weak and strong stratospheric polar vortices.

A comparison is made between observations and theoretical predictions by examining baroclinic wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients in terms of the counter-propagating Rossby wave perspective. Changes in stratospheric baroclinic wind shear alone cannot explain observed changes in wavelength, phase speed, and meridional propagation. Changes in lower stratospheric zonal wind speed and the vertical structure of baroclinic shear together account for observed changes in baroclinic wave propagation during strong and weak vortex events.