INFLUENCE OF BAROTROPIC SHEAR ON THE POLEWARD ADVECTION OF UPPER-TROPOSPHERIC AIR

The characteristics of the poleward advection of upper-tropospheric ai r are investigated using meteorological analyses and idealized numeric al models. Isentropic deformations of the tropopause during Northern H emisphere winter are examined using maps of Ertel's potential vorticit y together with contour advection calculations. Large poleward excursi ons of upper-tropospheric air are observed during Rossby wave breaking events. These ''pole-ward'' breaking events occur in regions of diffl uence (over the eastern Atlantic Ocean-Europe region, and over the eas tern Pacific Ocean-North America region), and the evolution of the tro pospheric air depends on the local, meridional shear: in anticyclonic (or weak cyclonic) shear the tropospheric air tilts downstream, broade ns, and wraps up anticyclonically, whereas in cyclonic shear the tropo spheric air tilts upstream, thins, and is advected cyclonically. The r ole of ambient barotropic flow is further examined by considering the how in two numerical models: a planar, equivalent-barotropic, contour dynamics model and a simplified general circulation model. In both mod els, the variation of the poleward wave breaking with the zonal and me ridional shear is consistent with that in the analyses.