Influence of barotropic shear on the poleward advection of upper-tropospheric air

Abstract

The characteristics of the poleward advection of upper-tropospheric air are investigated using meteorological analyses and idealized numerical models. Isentropic deformations of the tropopause during Northern Hemisphere winter are examined using maps of Ertel's potential vorticity together with contour advection calculations. Large poleward excursions of upper-tropospheric air are observed during Rossby wave breaking events. These "poleward" breaking events occur in regions of diffluence (over the eastern Atlantic Ocean-Europe region, and over the eastern Pacific Ocean-North America region), and the evolution of the tropospheric air depends on the local, meridional shear: in anticyclonic (or weak cyclonic) shear the tropospheric air tilts downstream, broadens, and wraps up anticyclonically, whereas in cyclonic shear the tropospheric air tilts upstream, thins, and is advected cyclonically. The role of ambient barotropic flow is further examined by considering the flow in two numerical models: a planar, equivalent-barotropic, contour dynamics model and a simplified general circulation model. In both models, the variation of the poleward wave breaking with the zonal and meridional shear is consistent with that in the analyses.