Properties and parameter regimes of cyclone--anticyclone vortex asymmetries remain unresolved problems in geophysical fluid dynamics. One example concerns the "balance" regime of small Rossby number where anticyclones appear to dominate for deep, barotropic, dynamics (e.g. shallow water and periodic balance equations) and cyclones appear to dominate for surface, baroclinic, dynamics such as near the tropopause. Here we test recent theoretical predictions for the latter case with an observational study documenting cyclone--anticyclone asymmetries near the tropopause.

Theoretical predictions are based on idealized numerical solutions of freely decaying turbulence in a model approximating the primitive equations for a homogeneous potential vorticity fluid containing a boundary, such as the tropopause (Hakim et al. 2002). An important asymmetry in these solutions is governed by the divergent-flow response to frontogenesis, which appears to promote the merger of anticyclones relative to cyclones, and conditions the spatial structure of the vortices.

Using a novel vortex-tracking algorithm, a database of cyclonic and anticyclonic vortices is constructed for the NCEP/NCAR reanalysis dataset. Composite divergence profiles for close-approach cyclones and anticyclones exhibit asymmetries consistent with theoretical predictions. Additional properties of the observed vortex populations suggest new tests and analyses to be applied to future theoretical research.