A dynamical mechanism for tropopause sharpening

Observed composite profiles of buoyancy frequency squared N-2 in the extratropical tropopause region show characteristic features including a pronounced peak right above a very sharp tropopause. Earlier work suggested that these features are, at least partly, due to the impact of conservative balanced dynamics during the formation of anomalies from a large scale background atmosphere. This result was obtained using the twin principle of conservation and invertibility of ERTEL potential vorticity (PV). The interpretation remained difficult, since N-2 is mostly affected by the vertical wind, which is highly implicit in the PV based method used. The current work provides a mechanistic explanation of tropopause sharpening by explicitly focussing on the vertical wind and its impact on N-2. A balanced slab symmetric reference flow is forced by superimposing a barotropic deformation field. The resulting Sawyer-Eliassen secondary circulation is computed numerically. This secondary circulation implies vertical divergence and a material decrease of N-2 in the vortex center for cyclones, while it implies vertical convergence and a material increase of N-2 for anticyclones. The ensueings evolution leads to an asymmetry between cyclones and anticyclones, allowing a net effect to remain on average.