Climatology of Coherent Structures on the Extratropical Tropopause

Abstract

This paper poses and tests the hypothesis that some of the synoptic-scale and mesoscale tropopause-based disturbances that produce organized vertical motion and induce surface cyclones in the extratropical troposphere are vortexlike coherent structures. Based on the theory of nonlinear waves and vortices, tests are constructed and applied to observations of relative vorticity maxima for a 33-winter climatology at 500 hPa and three-dimensional composites for a single winter season. The method is designed to determine the following disturbance properties: nonlinearity, quasigeostrophic potential vorticity?streamfunction relationship, speed, and trapping of fluid particles. These properties are determined for four disturbance-amplitude categories, defined here in terms of 500-hPa relative vorticity. The results show that, on average, 500-hPa relative vorticity maxima are localized monopolar vortices with length scales (radii) of approximately 500?800 km; there is a slight increase in length scale with disturbance amplitude. Nonlinearity is O(1) or greater for all amplitude categories, approaching O(10) for the strongest disturbances. Trapping of fluid particles, estimated by the presence of closed contours of potential vorticity on isentropic surfaces near the tropopause, requires greater than O(1) nonlinearity; the threshold disturbance amplitude is approximately 8 ? 10?5 s?1 in the vertical component of 500-hPa relative vorticity and ?8 K in anomaly tropopause potential temperature. The vortices move westward with respect to the background flow, with a slight northward drift. The observational evidence does not support an interpretation of these features in terms of modons or solitary waves.