Although Rossby waves near the extratropical tropopause are fairly well understood, considerably less is known about coherent vortices in this region, even with regard to basic facts about their frequency of occurrence, longevity and structure. This study addresses these issues through an objective census of observed tropopause vortices. We also test an hypothesis regarding vortex-merger asymmetry, where cyclone pairs are repelled and anticyclone pairs are attracted by divergent flow due to frontogenesis. Emphasis is placed on arctic vortices, where jet-stream influences are weaker, in order to facilitate comparisons with earlier idealized numerical simulations.

Results show that arctic cyclones are more numerous, persistent, and stronger than arctic anticyclones. An average of 15 cyclonic vortices and 11 anticyclonic vortices are observed per month, with maximum frequency of occurrence for cyclones (anticyclones) during winter (summer). There are are about 47% more cyclones than anticyclones that survive at least four days, and for longer lifetimes, one-day survival probabilities are nearly constant at 65% for cyclones, and 55% for anticyclones. Mean tropopause potential-temperature amplitude is 13 K for cyclones and 11 K for anticyclones, with cyclones exhibiting a greater tail toward larger values. Amplitude asymmetry is most evident in tropopause pressure, with mean values of 140 hPa for cyclones and 70 hPa for anticyclones.

An analysis of close-proximity vortex pairs reveals divergence between cyclones and convergence between anticyclones. This result agrees qualitatively with previous idealized numerical simulations, although it is unclear to what extent the divergent circulations regulate vortex asymmetries.

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